Tool
From LoveToKnow 1911
TOOL (O. Eng. 161, generally referred to a root seen in the Goth. taujan, to make, or in the English word " taw," to work or dress leather), an implement or appliance used by a worker in the treatment of the substances used in his handicraft, whether in the preliminary operations of setting out and measuring the materials, in reducing his work to the required form by cutting or otherwise, in gauging it and testing its accuracy, or in duly securing it while thus being treated.
For the tools of prehistoric man see such articles as Archaeology; Flint Implements; and Egypt, § Art and Archaeology. In beginning a survey of tools it is necessary to draw the distinction between hand and machine tools. The former class includes any tool which is held and operated by the unaided hands, as a chisel, plane or saw. Attach one of these to some piece of operating mechanism, and it, with the environment of which it is the central essential object, becomes a machine tool. A very simple example is the common power-driven hack saw for metal, or the small high-speed drill, or the wood-boring auger held in a frame and turned by a winch handle and bevel-gears. The difference between these and a big frame-saw cutting down a dozen boards simultaneously, or the immense machine boring the cylinders of an ocean liner, or the great gun lathe, or the hydraulic press, is so vast that the relationship is hardly apparent. Often the tool itself is absolutely dwarfed by the machine, of which nevertheless it is the central object and around which the machine is designed and built. A milling machine weighing several tons will often be seen rotating a tool of but two or three dozen pounds' weight. Yet the machine is fitted with elaborate slides and self-acting movements, and provision for taking up wear, and is worth some hundreds of pounds sterling, while the tool may not be worth two pounds. Such apparent anomalies are in constant evidence. We propose, therefore, first to take a survey of the principles that underlie the forms of tools, and then pursue the subject of their embodiment in machine tools.
Hand Tools The most casual observation reveals the fact that tools admit of certain broad classifications. It is apparent that by far the larger number owe their value to their capacity for cutting or removing portions of material by an incisive or wedge-like action, leaving a smooth surface behind. An analysis of the essential methods of operation gives a broad grouping as follows: I. The chisel group.. Typified by the chisel of the woodworker.
II. The shearing group. „ „ scissors.
III. The scrapers. „ „ cabinet-maker's scrape.
IV. The percussive and „ hammer and the punch. detrusive group V. The moulding group. „ trowel.
The first three are generally all regarded as cutting tools, notwithstanding that those in II. and III. do not operate as wedges, and therefore are not true chisels. But many occupy a border-line where the results obtained are practically those due to cutting, as in some of the shears, saws, milling cutters, files and grinding wheels, where, if the action is not directly wedge-like, it is certainly more or less incisive in character.
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Cutting Tools
The cutting edge of a tool is the practical outcome of several conditions. Keenness of edge, equivalent to a small degree of angle between the tool faces, would appear at first sight to be the prime element in cutting, as indeed it is in the case of a razor, or in that of a chisel for soft wood. But that is not the prime condition in a tool for cutting iron or steel. Strength is of far greater importance, and to it some keenness of edge must be sacrificed. All cutting tools are wedges; but a razor or a chisel edge, included between angles of 15° or 20°, would be turned over at once if presented to iron or steel, for which angles of from 60° to 75° are required. Further, much greater rigidity in the latter, to resist spring and fracture, is necessary than in the former, because the resistance to cutting is much greater. A workman can operate a turning tool by hand, even on heavy pieces of metal-work. Formerly all turning, no matter how large, was done by hand-operated tools, and after great muscular exertion a few pounds of metal might be removed in an hour. But coerce a similarly formed tool in a rigid guide or rest, and drive it by the power of ten or twenty men, and it becomes possible to remove say a hundredweight of chips in an hour. Or, increase the size of the tool and its capacity for endurance, and drive by the power of 40 or 60 horses, and half a ton of chips may be removed in an hour.
All machine tools of which the chisel is the type operate by cutting;. that is, they act on the same principle and by the same essential method as the knife, razor or chisel, and not by that of the grindstone. A single tool, however, may act as a cutting instrument at one time and as a scrape at another. The butcher's knife will afford a familiar illustration. It is used as a cutting tool when severing a steak, but it becomes a scrape when used to clean the block. The difference is not therefore due to the form of the knife, but to the method of its application, a distinction which holds good in reference to the tools used by engineers. There is a very old hand tool once much used in the engineer's turnery, termed a " graver." This was employedfor cutting and for scraping indiscriminately, simply by varying the angle of its presentation. At that time the question of the best cutting angles was seldom raised or discussed, because the manipulative instinct of the turner settled it as the work proceeded, and as the material operated on varied in texture and degree of hardness. But since the use of the slide rest holding tools rigidly fixed has become general, the question of the most suitable tool formation has been the subject of much experiment and discussion. The almost unconscious experimenting which goes on every day in every workshop in the world proves that there may be a difference of several degrees of angle in tools doing similar work, without having any appreciable effect upon results. So long as certain broad principles and reasonable limits are observed, that is sufficient for practical purposes.
Clearly, in order that a tool shall cut, it must possess an incisive form. In fig. I, A might be thrust over the surface of the plate of metal, but no cutting action could take place. It would simply grind and polish the surface. If it were formed like B, the grinding action would give place to scraping, by which some material would be removed. Many tools are formed thus, but there is still no incisive or knife-like action, and the tool is simply a scrape and not a cutting tool. But C is a cutting tool, possessing penetrative capacity. If now B were tilted backwards as at D, it would at once become a cutting tool. But its bevelled face would rub and grind on the surface of the work, producing friction and heat, and interfering with the penetrative action of the cutting edge. On the other hand, if C were tilted forwards as at E its action would approximate to that of a scrape for the time being. But the high angle of the hinder bevelled face would not afford adequate support to the cutting edge, and the latter would therefore become worn off almost instantly, precisely as that of a razor or wood-working chisel would crumble away if operated on hard metal. It is,obvious FIG. I.
A, Tool which would burnish F, G, H, Presentations of tools only. for planing, turning and B, Scrape. boring respectively.
C, Cutting tool. J, K, L, Approximate angles of D and E, Scraping and cutting tools; a, clearance angle, or tools improperly presented. bottom rake; b, front or top rake; c, tool angle.
therefore that the correct form for a cutting tool must depend upon a due balance being maintained between the angle of the front and of the bottom faces - " front " or " top rake," and " bottom rake " or " clearance " - considered in regard to their method of presentation to the work. Since, too, all tools used in machines are held rigidly in one position, differing in this respect from handoperated tools, it follows that a constant angle should be given to instruments which are used for operating on a given kind of metal or alloy. It does not matter whether a tool is driven in a lathe, or a planing machine, or a sharper or a slotter; whether it is cutting on external or internal surfaces, it is always maintained in a direction perpendicularly to the point of application as in fig. 1, F, G, H, planing, turning and boring respectively. It is consistent with reason and with fact that the softer and more fibrous the metal, the keener must be the formation of the tool, and that, conversely, the harder and more crystalline the metal the more obtuse must be the cutting angles, as in the extremes of the razor and the tools for cutting iron and steel already instanced. The three figures J, K, L show tools suitably formed for wrought iron and mild steel, for cast iron and cast steel, and for brass respectively. Cast iron and cast steel could not be cut properly with the first, nor wrought iron and fibrous steel with the second, nor either with the third. The angles given are those which accord best with general practice, but they are not constant, being varied by conditions, especially by lubrication and rigidity of fastenings. The profiles of the first and second tools are given mainly with the view of having material for grinding away, without the need for frequent reforging. But there are many tools which are formed quite differently when used in tool-holders and in turrets, though the same essential principles of angle are observed.
The angle of clearance, or relief, a, in fig. i, is an important detail of a cutting tool. It is of greater importance than an exact angle of top rake. But, given some sufficient angle of clearance, its exact amount is not of much moment. Neither need it be uniform for a given cutting edge. It may vary from say 3° to 10°, or even 20 °, and under good conditions little or no practical differences will result. Actually it need never var y much from 5° to 7°. The object in giving a clearance angle is simply to prevent friction between the non-cutting face immediately adjacent to the edge and the surface of the work. The limit to this clearance is that at which insufficient support is afforded to the cutting edge. These are the two facts, which if fulfilled permit of a considerable range in clearance angle. The softer the metal being cut the greater can be the clearance; the harder the material the less clearance is permissible because the edge requires greater support.
The front, or top rake, b in fig. i, is the angle or slope of the front, or top face, of the tool; it is varied mainly according as materials are crystalline or fibrous. In the turnings and cuttings taken off the more crystalline metals and alloys, the broken appearance of the chips is distinguished from the shavings removed from the fibrous materials. This is a feature which always distinguishes cast iron and unannealed cast steel from mild steel, high carbon steel from that low in carbon, and cast iron from wrought iron. It indicates too that extra work is put on the tool in breaking up the chips, following immediately on their severance, and when the comminutions are very small they indicate insufficient top rake. This is a result that turners try to avoid when possible, or at least to minimize. Now the greater the slope of the top rake the more easily will the cuttings come away, with the minimum of break in the crystalline materials and absolutely unbroken over lengths of many feet in the fibrous ones. The breaking up, or the continuity of the cuttings, therefore affords an indication of the suitability of the amount of top rake to its work. But compromise often has to be made between the ideal and the actual. The amount of top rake has to be limited in the harder metals and alloys in order to secure a strong tool angle, without which tools would lack the endurance required to sustain them through several hours without regrinding.
The tool angle, c, is the angle included between top and bottom faces, and its amount, or thickness expressed in degrees, is a measure of the strength and endurance of any tool. At extremes it varies from about 15° to 85°. It is traceable in all kinds of tools, having very diverse forms. It is difficult to place some groups in the cutting category; they are on the border-line between cutting and scraping instruments.
Typical Tools
A bare enumeration of the diverse forms in which tools of the chisel type occur is not even possible here. The grouped illustrations (figs. 2 to 6) show some of the types, but it will be understood that each is varied in dimensions, angles and outlines to suit all the varied kinds of metals and alloys and conditions of operation. For, as every tool has to be gripped in a holder of some kind, as a slide-rest, tool-box, turret, tool-holder, box, cross-slide, &c., this often determines the choice of some one form in preference to another. A broad division is that into roughing and finishing H FIG. 2. - Metal-turning Tools.
E, Diamond or angular-edge tool for cutting all metals.
F, Plan of finishing tool.
G, Spring tool for finishing.
H, Side or knife tool.
J, Parting or cutting-off tool.
K, L, Round-nose tools. Al, Radius tool.
FIG. 3. - Group A, Planer type of tool, cranked to avoid digging into the metal.
B, Face view of roughing tool.
C, Face view of finishing tool.
D, Rightand left-hand knife or side tools.
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C E 1 G Shape of tool used for scraping brass.
Straightforward tool for turning all metals.
Rightand left-hand tools for all metals.
A better form of same.
of Planer Tools.
E, Parting or cutting-off or grooving tool.
F, V tool for grooves.
G, Rightand left-hand tools for V-slots.
H, Ditto for T-slots.
J, Radius tool held in holder.
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tools. Generally though not invariably the edge of the first is narrow, of the second broad, corresponding with the deep cutting and fine traverse of the first and the shallow cutting and broad.
FIG. 4. - Group of Slotter Tools.
A, Common roughing tool. B, Parting-off or grooving tool. C, Roughing or finishing tool in a holder. D, Double-edged tool for cutting opposite sides of a slot.
- ?11 bar of steel. This is costly when the best tool steel is used, hence large numbers of tools comprise points only, which are gripped in permanent holders in which they interchange. Tool steel usually ranges from about z in. to 4 in. square; most engineers' work is done with bars of from 2 in. to i a in. square. It is in the smaller and medium sizes of tools that holders prove of most value. Solid tools, varying from 22 in. to 4 in. square, are used for the heaviest cutting done in the planing machine. Tool-holders are not employed for very heavy work, because the heat generated would not get away fast enough from small tool points. There are scores of holders; perhaps a dozen good approved types are in common use. They are divisible into three great groups: those in which the top rake of the tool point is embodied in the holder, and is constant; those in which the clearance is similarly embodied; and those in which neither is provided for, but in which the tool point is ground to any angle. Charles Babbage designed the first tool-holder, and the essential type survives in several modern forms. The best-known holders now are the Tangye, the Smith & Coventry, the Armstrong, some by Mr C. Taylor, and the Bent. The Smith & Coventry (fig. 5), used more perhaps than any other single design, includes two forms. In one E the tool is a bit of round steel set at an angle which gives front rake, and having the top end ground to an angle of top rake. In the other A the tool has the section of a truncated wedge, set for constant top rake, or cutting angle, and having bottom rake or clearance angle ground. The Smith & Coventry round tool is not applicable for all classes of work. It will turn plain work, and plane level faces, but will not turn or plane into corners or angles. Hence the invention of the tool of V-section, and the swivel toolholder. The round tool-holders are made rightand left-handed, the swivel tool-holder has a universal movement. The amount of projection of the round tool points is very limited, which impairs their utility when some overhanging of the tool is necessary. The V-tools can be slid out in their holders to operate on faces and edges situated to some considerable distance inwards from the end of the tool-holder.
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Box Tools
In one feature the box tools of the turret lathes resemble tool-holders. The small pieces of steel used for tool points are gripped in the boxes, as in tool-holders, and all the advantages which are derived from this arrangement of separating the point from its holder are thus secured (fig. 7). But in all other FIG. 5. - Group of Tool-holders.
A, Smith & Coventry swivelling holder. B, Holder for square steel. C, D, rightand left-hand forms of same. E, Holder for round steel. F, Holder for narrow parting-off tool.
traverse of the second. The following are some of the principal forms. The round-nosed roughing tool (fig. 2) B is of straightforward type, used for turning, planing and shaping. As the correct tool angle can only occur on the middle plane of the tool, it is usual to employ cranked tools, C, D, E, rightand left-handed, for heavy and moderately heavy duty, the direction of the cranking corresponding with that in which the tool is required to traverse. Tools for boring are cranked and many for planing (fig. 3). The slotting tools (fig. 4) embody the same principle, but their shanks are in line with the direction of cutting. Many roughing and finishing tools are of knife type H. Finishing tools a have broad edges, F, G, H. They occur in straightforward and FIG. 6. - Group of Chisels. rightand left-hand types.
FIG. 7. - Box Tool for Turret Lathe. (Alfred Herbert, Ltd., Coventry.) A, Cutting tool. B, Screw for adjusting radius of cut. C C, V-steadies supporting the work in opposition to A. D, Diameter of work. E, Body of holder. F, Stem which fits in the turret.
respects the two are dissimilar. Two or three tool-holders of different sizes take all the tool points used in a lathe, but a new box has to be devised in the case of almost every new job, with the exception of those the principal formation of which is the turning down of plain bars. The explanation is that, instead of a single point, several are commonly carried in a box. As complexity increases with the number of tools, new designs and dimensions of boxes become necessary, even though there may be family resemblances in groups. A result is that there is not, nor can there be, anything like finality in these designs. Turret work has become one of the most highly specialized departments of machine-shop practice, and the design of these boxes is already the work of specialists. More and more of the work of the common lathe is being constantly appropriated by the semiand full-automatic machines, a result to which the magazine feeds for castings and forgings that cannot pass through a hollow spindle have contributed greatly. New work is constantly being attacked in the automatic machines that was deemed impracticable a short time before; some of the commoner jobs are produced with greater economy, while heavier castings and forgings, longer and larger bars, are tooled in the turret lathes. A great deal of the efficiency of the box tools is due to the support which is afforded to the cutting edges in opposition to the stress of cutting. V-blocks are introduced in most cases as in fig. 7, and these not only resist the stress of the cutting, but gauge the diameter exactly.
Shearing Action
In many tools a shearing operation takes place, by which the stress of cutting is lessened. Though not very apparent, it is present in the round-nosed roughing tools, in the knife tools, in most milling cutters, as well as in all the shearing tools proper - the scissors, shears, &c.
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Planes
We pass by the familiar great chisel group, used by woodworkers, with a brief notice. Generally the tool angles of these lie between 15° and 25°. They include the chisels proper, and the gouges in numerous shapes and proportions, used by carpenters, These as a rule remove less than in. in depth, while the roughing tools may cut an inch or more into the metal. ' But the traverse of the first often exceeds an inch, while in that of the second $ in. is a very coarse amount of feed. Spring tools, G, used less now than formerly, are only of value for imparting a smooth finish to a surface. They are finishing tools only. Some spring tools are formed with considerable top rake, but generally they act by scraping only.
Solid Tools v. Tool-holders. - It will be observed that the foregoing are solid tools; that is, the cutting portion is forged from a solid Paring chisel.
Socket chisel for heavy duty. Common chipping chisel. Narrow cross-cut or cape chisel. Cow-mouth chisel, or gouge. Straight chisel or sett.
Hollow chisel or sett.
cabinet-makers, turners, stone-masons and allied tradesmen. These are mostly thrust by hand to their work, without any mechanical control. Other chisels are used percussively, as the stout mortise chisels, some of the gouges, the axes, adzes and stone-mason's tools. The large family of planes embody chisels coerced by the mechanical control of the wooden (fig. 8) or metal stock. These also differ FIG. 8. - Section through Plane.
A, Cutting iron. B, Top or back iron. C, Clamping screw. D, Wedge. E, Broken shaving. F, Mouth.
from the chisels proper in the fact that the face of the cutting iron does not coincide with the face of the material being cut, but lies at an angle therewith, the stock of the plane exercising the necessary coercion. We also meet with the function of the top or non-cutting A B C H FIG. 9. - Group of Wood-boring Bits.
A, Spoon bit. B, Centre-bit. C, Expanding centre-bit. D, Gilpin or Gedge auger. E, Jennings auger. F, Irwin auger.
FIG. 10. - Group of Drills for Metal.
A, Common flat drill. B, Twist drill. C, Straight fluted drill. D, Pin drill for flat countersinking. E, Arboring or facing tool. F, Tool for boring sheet-metal.
iron in breaking the shaving and conferring rigidity upon the cutting iron. This rigidity is of similar value in cutting wood as in cutting metal though in a less marked degree.
Drilling and Boring Tools
Metal and timber are bored with equal facility; the tools (figs. 9 and to) embody similar differences to the cutting tools already instanced for wood and metal. All the wood-working bits are true cutting tools, and their angles, if analysed, will be found not to differ much from those of the razor and common chisel. The drills for metal furnish examples both of scrapers and cutting tools. The common drill is only a scraper, but all the twist drills cut with good incisive action. An advantage possessed by all drills is that the cutting forces are balanced on each side of the centre of rotation. The same action is embodied in the best woodboring bits and augers, as the Jennings, the Gilpin and the Irwin - much improved forms of the old centre-bit. But the balance is impaired if the lips are not absolutely symmetrical about the centre. This explains the necessity for the substitution of machine grinding for hand grinding of the lips, and great developments of twist drill grinding machines. Allied to the drills are the D-bits, and the reamers (fig. I I). The first-named both initiate and finish a hole; FIG. I I.
A, D-bit. B, Solid reamer. C, Adjustable reamer, having six flat blades forced outward by the tapered plug. Two lock-nuts at the end fix the blades firmly after adjustment.
the second are used only for smoothing and enlarging drilled holes, and for correcting holes which pass through adjacent castings or plates. The reamers remove only a mere film, and their action is that of scraping. The foregoing are examples of tools operated from one end and unsupported at the other, except in so far as they receive support within the work. One of the objectionable features of tools operated in this way is that they tend to " follow the hole," and if this is cored, or rough-drilled out of truth, there is risk of the boring tools following it to some extent at least. With the one exception of the D-bit there is no tool which can be relied on to take out a long bore with more than an approximation to concentricity throughout. Boring tools (fig. 12) held in the slide-rest will spring and bend and chatter, and unless the lathe is true, or careful compensation is made for its want of truth, they will bore bigger at one end than the other. Boring tools thrust by the back centre are liable to wabble, and though they are variously coerced to prevent them from turning round, that does not check the to-and-fro wabbly gft s FIG. 12. - Group of Boring Tools.
A, Round boring tool held in V-blocks on slide-rest. B, C, Square and V-pointed boring tools. D, Boring bar with removable cutters, held straight, or angularly.
motion from following the core, or rough bore. In a purely reaming tool this is permitted, but it is not good in tools that have to initiate the hole.
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This brings us to the large class of boring tools which are supported at each end by being held in bars carried between centres. There are two main varieties: in one the cutters are fixed directly in the bar (fig. 13, A to D), in the other in a head fitted on the bar F ?-- - -- ?^ A (fig. 13, E), hence termed a " boring head." As lathe heads are fixed, the traverse cannot be imparted to the bars as in boring machines. The boring heads can be traversed, or the work can be FIG. 13. - Group of Supported Boring Tools.
A, Single-ended cutter in boring D, Flat double-ended finishing bar. cutter.
B, Double-ended ditto. E, Boring head with three cutters C, Flat single-ended finishing and three steady blocks. cutter.
traversed by the mechanism of the lathe saddle. The latter must be done when cutters are fixed in bars. A great deal of difference exists in the details of the fittings both of bars and heads, but they are not so arbitrary as they might seem at first sight. The principal differences are those due to the number of cutters used, their shapes, and their method of fastening. Bars receiving their cutters direct include one, two or four, cutting on opposite sides, and therefore balanced. Four give better balance than two, the cutters being set at right angles. If a rough hole runs out of truth, a single cutter is better than a double-ended one, provided a tool of the roughing shape is used. The shape of the tools varies from roughing to finishing, and their method of attachment is by screws, wedges or nuts, but we cannot illustrate the numerous differences that are met with.
Saws
The saws are a natural connecting link between the chisels and the milling cutters. Saws are used for wood, metal and stone. Slabs of steel several inches in thickness are sawn through as readily as, though more slowly than, timber planks. Circular and band saws are common in the smithy and the boiler and machine shops for cutting off bars, forgings and rolled sections. But the tooth shapes are not those used for timber, nor is the cutting speed the same. In the individual saw-teeth both cutting and scraping actions are illustrated (fig. 14). Saws which cut timber continuously with the grain, as rip, hand, band, circular, have incisive teeth. For though many are destitute of front rake, the method of sharpening at an angle imparts a true shearing cut. But all crosscutting teeth scrape only, the teeth being either of A, Teeth of band and ripping saws. triangular or of M-form, B, Teeth of circular saw for hard wood; variously modified. Teeth shows set. for metal cutting also act C, Ditto for soft wood. strictly by scraping. The D, Teeth of cross-cut saw. pitching of the teeth is E, M-teeth for ditto. related to the nature of the material and the direction of cutting. It is coarser for timber than for metal, coarser for ripping or sawing with the grain than for cross cutting, coarser for soft than for hard woods. The setting of teeth, or the bending over to right and left, by which the clearance is provided for the blade of the saw, is subject to similar variations. It is greatest for soft woods and least for metals, where in fact the clearance is often secured without set, by merely thinning the blade backwards. But it is greater for cross cutting than for ripping timber. Gulleting follows similar rules. The softer the. timber, the greater the gulleting, to permit the dust to escape freely. Milling Cutters. - Between a circular saw for cutting metal and a thin milling cutter there is no essential difference. Increase the thickness as if to produce a very wide saw, and the essential plain edge milling cutter for metal results. In its simplest form the milling cutter is a cylinder with teeth lying across its periphery, or parallel with its axis - the edge mill (fig. 15), or else a disk with teeth radiating on its face, or at right angles with its axis - the end mill (fig. 16). Each is used indifferently for producing flat faces and edges, and for cutting grooves which are rectangular in cross-section. These milling cutters invade the province of the single-edged tools of the planer, shaper and slotter. Of these two typical forms the FIG. 16. - Group of End Mills.
A, End mill with straight teeth. B, Ditto with spiral teeth. C, Showing method of holding shell cutter on arbor, with screw and key. D, T-slot cutter.
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A, Narrow edge mill, with straight teeth.
B, Wide edge mill with spiral teeth.
C, Teeth on face and edges.
D, Cutter having teeth like C.
E, Flat teeth held in with screws and wedges.
F, Large inserted tooth mill; with taper pins secure cutters.
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F FIG. 15. - Group of Milling Cutters.
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FIG. 14. - Typical Saw Teeth.
changes are rung in great variety, ranging from the narrow slitting tools which saw off bars, to the broad cutters of 24 in. or more in width, used on piano-millers.
When more than about an inch in width, surfacing cylindrical cutters are formed with spiral teeth (fig. 15, B), a device which is FIG. 18. - Group of Angular Mills.
A, Cutter with single slope.
B, Ditto, producing teeth in another cutter.
C, Double Slope Mill, with unequal angles.
essential to sweetness of operation, the action being that of shearing. These have their teeth cut on universal machines, using the dividing and spiral head and suitable change wheels, and after hardening they are sharpened on universal grinders. When cutters exceed about 6 in. in length the difficulties of hardening and grinding render the " gang " arrangement more suitable. Thus, two, three or more similar edge mills are set end to end on an arbor, with the spiral teeth running in reverse directions, giving a broad face with balanced endlong cutting forces. From these are built up the numerous gang mills, comprising plane faces at right angles with each other, of which the straddle mills are the best known (fig. 17, A). A common element in these combinations is the key seat type B having teeth on the periphery and on both faces as in fig. 15, C, D. By these combinations half a dozen faces or more can be tooled simultaneously, and all alike, as long as the mills retain their edge. The advantages over the work of the planer in this class of work are seen in tooling the faces and edges of machine tables, beds and slides, in shaping the faces and edges of caps to fit their bearing blocks. In a single cutter of the face type, but having teeth on back and edge also, T-slots are readily milled (fig. 16, D); this if done on the planer would require re-settings of awkwardly cranked tools, and more measurement and testing with templets than is required on a milling machine.
When angles, curves and profile sections are introduced, the capacity of the milling cutter is infinitely increased. The making of the cutters is also more difficult. Angular cutters (fig. 18) are used for producing the teeth of the mills themselves, for shaping the teeth of ratchet wheels, and, in combination with straight cutters in gangs, for angular sections. With curves, or angles and curves in combination, taps, reamers and drills can be fluted or grooved, the teeth of wheels shaped, and in fact any outlines imparted (fig. 19). Here the work of the fitter, as well as that of the planing and allied machines, is invaded, for much of this work if prepared on these machines would have to be finished laboriously by the file.
There are two ways in which milling cutters are used, by which their value is extended; one is to transfer some of their work proper to the lathe and boring machine, the other is by duplication. A "good many light circular sections, as wheel rims, hitherto done in lathes, are regularly prepared in the milling machine, gang mills being used for tooling the periphery and edges at once, and the wheel blank being rotated. Similarly, holes are bored by a rotating mill of the cylindrical type. Internal screw threads are done similarly. Duplication occurs when milling sprocket wheels in line, or side by side, in milling nuts on an arbor, in milling a number of narrow faces arranged side by side, in cutting the teeth of several spur-wheels on one arbor and in milling the teeth of racks several at a time.
One of the greatest advances in the practice of milling was that of making backed-off cutters. The sectional shape behind the tooth face is continued identical in form with the profile of the edge, the outline being carried back as a curve equal in radius to that of the cutting edge (fig. 20). The result is that the cutter may be sharpened on the front faces of the teeth without interfering with the shape which will be milled, because the periphery is always constant in outline. After repeated sharpenings the teeth would assume the form indicated by the shaded portion on two of the teeth. The limit of grinding is reached when the tooth becomes too thin and weak to stand up to its work. But such cutters will endure weeks or months of constant service before becoming useless. The A E F FIG. 21. - Group of Scrapes.
A, Metal-worker's scrape, pushed D, Diamond point used by straightforward. wood-turners.
B, Ditto, operated laterally. E, F, Cabinet-makers' scrapes.
C, Round-nosed tool used by wood-turners.
chief advantage of backing-off or relieving is in its application to cutters of intricate curves, which would be difficult or impossible to sharpen along their edges. Such cutters, moreover, if made with N 0 Q R S T U FIG. 22. - Cross-sectional Shapes of Files.
A, Warding. J, Topping. P, Round.
B, Mill. K, Reaper. Q, Pit-saw or C, Flat. L, Knife. frame-saw.
D, Pillar. M, Three-square. R, Half-round.
E, Square. N, Cant. S, T, Cabinet.
F, G, Swaged reapers. 0, Slitting or U, Tumbler.
H, Mill. feather-edge. I T, Crossing. ordinary teeth would soon be worn down, and be much weaker than the strong form of teeth represented in fig. 20. The relieving is usually done in special lathes, employing a profile tool which cuts the surface FIG. 23. - Longitudinal Shapes of Files.
A, Parallel or blunt. F, Tapered triangular. K, Tapered half B, Taper bellied. G, Parallel round. round.
C, Knife reaper. H, Taper or rat-tail. L, Riffler.
D, Tapered square. J, Parallel half E, Parallel triangular. round.
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'/i. ' A B FIG. 17.
A, Straddle Mill, cutting faces and edges.
B, Set of three mills cutting grooves.
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| C |
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A A A B D E F G FI K L ii FIG. 19.
A, Convex Cutter.
B, Concave Cutter.
C, Profile Cutter.
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FIG. 20. - Relieved Teeth of Milling Cutter.
of the teeth back at the required radius. Relieved cutters can of course be strung together on a single arbor to form gang mills, by which very complicated profiles may be tooled, beyond the capacity of a single solid mill.
Scrapes
The tools which operate by scraping (fig. 21) include many of the broad finishing tools of the turner in wood and metal (cf. fig. 2), and the scrape of the wood worker and the fitter. The practice of scraping surfaces true, applied to surface plates, machine slides and similar objects, was due to Sir Joseph Whitworth. It superseded the older and less accurate practice of grinding to a mutual fit. Now, with machines of precision, the practice of grinding has to a large extent displaced the more costly scraping. Scraping is, however, the only method available when the most perfect contact is desired. Its advantage lies in the fact that the efforts of the workman can be localized over the smallest areas, and nearly infinitesimal amounts removed, a mere fine dust in the last stages.
Files
These must in strictness be classed with scrapes, for, although the points are keen, there is never any front rake. Collectively there is a shearing action because the rows of teeth are cut diagonally. The sectional forms (fig. 22) and the longitudinal forms (fig. 23) of the files are numerous, to adapt them to all classes of work. In addition, the method of cutting, and the degrees of coarseness of the teeth, vary, being single, or float cut, or double cut (fig. 24). The rasps are another group. Degrees of coarseness are designated as rough, middle cut, bastard cut, second cut, smooth, double dead smooth; the first named is the coarsest, the last the finest. The terms are relative, since the larger a file is the coarser are its teeth, though of the same name as the teeth in a shorter file, which are finer.
Screwing Tools
The forms of these will be found discussed under Screw. They can scarcely be ranked among cutting tools, yet the best kinds remove metal with ease. This is due in great measure to the good clearance allowed, and to the narrowness of the cutting portions. Front rake is generally absent, though in some of the best screwing dies there is a slight amount.
Shears and Punches
These may be of cutting or non-cutting types. Shears (fig. 25) have no front rake, but only a slight clearance. They generally give a slight shearing cut, because the blades do not lie parallel but the cutting begins at one end and continues in detail to the other. But strictly the shears, like the punches, act by a FIG. 25. - Shear Blades. FIG. 26. - Punching.
a, a, Blades. a, Punch. b, Bolster.
b, Plate being sheared. c, Plate being punched.
severe detrusive effort; for the punch, with its bolster (fig. 26), forms a pair of cylindrical shears. Hence a shorn or punched edge A I always rough, ragged, and covered with minute, shallow cracks. Both processes are therefore dangerous to iron and steel. The metal being unequally stressed, fracture starts in the annulus of metal. Hence the advantage of the practice of reamering out this annulus, which is completely removed by enlargement by about an sin. diameter, so that homogeneous metal is left throughout the entire unpunched section. The same results follow reamering both in iron and steel. Annealing, according to many experiments, has the same effect as reamering, due to the rearrangement of the molecules of metal. The perfect practice with punched plates is to punch, reamer, and finally to anneal. The effect of shearing is practically identical with that of punching, and planing and annealing shorn edges has the same influence as reamering and annealing punched holes.
Hammers
These form an immense group, termed percussive, from the manner of their use (fig. 27). Every trade has its own peculiar shapes, the total of which number many scores, each with its own appropriate name, and ranging in size from the minute forms of the jeweler to the sledges of the smith and boiler maker and the planishing hammers of the coppersmith. Wooden hammers are termed mallets, their purpose being to avoid bruising tools or the surfaces of work. Most trades use mallets of some form or another. Hammer handles are rigid in all cases except certain percussive tools of the smithy, which are handled with withy rods, or iron rods flexibly attached to the tools, so that when struck by the sledge they shall not jar the hands. The fullering tools, and flatters, and setts, though not hammers strictly, are actuated by percussion. The dies of the die forgers are actuated percussively' being closed by powerful hammers. The action of caulking tools is percussive, and so is that of moulders' rammers.
FIG. 27. - Hammers.
A, Exeter type.
B, Joiner's hammer. G, Sledge hammer, straight F, Ditto, straight pane.
C, Canterbury claw hammer pane.
H, Ditto, double-faced.
(these are wood-workers' J, K, L, M, Boiler makers' hamhammers).
D, Engineer's hammer, ball pane. mers.
N, Scaling hammer.
E, Ditto, cross-pane.
Moulding Tools
This is a group of tools which, actuated either by simple pressure or percussively, mould, shape and model forms in the sand of the moulder, in the metal of the smith, and in press work. All the tools of the moulder (fig. 28) with the exception of the rammers and vent wires act by moulding the sand into shapes FIG. 28. - Moulding Tools.
A, Square trowel. E, Flange bead. J, Button sleeker.
B, Heart trowel. F, Hollow bead. K, Pipe smoother.
C, D, Cleaners. G, H, Square corner sleekers.
by pressure. Their contours correspond with the plane and curved surfaces of moulds, and with the requirements of shallow and deep work. They are made in iron and brass. The fullers, swages and flatters of the smith, and the dies used with hammer and presses, all mould by percussion or by pressure, the work taking the counterpart of the dies, or of some portion of them. The practice of die forging consists almost wholly of moulding processes.
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Tool Steels
These now include three kinds. The common steel, the controlling element in which is carbon, requires to be hardened and tempered, and must not be overheated, about 500° F. being the highest temperature permissible - the critical temperature. Actually this is seldom allowed to be reached. The disadvantage of this steel is that its capabilities are limited, because the heat generated by heavy cutting soon spoils the tools. The second is the Mushet steel, invented by R. F. Mushet in 1868, a carbon steel, in which the controlling element is tungsten, of which it contains from about 5 to 8%. It is termed self-hardening, because it is cooled in air instead of being quenched in water. Its value consists in its endurance at high temperatures, even at a low red heat. Until the advent of the high-speed steels, Mushet steel was reserved for all heavy cutting, and for tooling hard tough steels. It is made in six different tempers suitable for various kinds of duty. Tools of Mushet steel must not be forged below a red heat. It is hardened by reheating the end to a white heat, and blowing cold in an air blast. The third kind of steel is termed high-speed, because much higher cutting speeds are practicable with these than with other steels. Tools made of them are hardened in a blast of cold air. The controlling elements are numerous and vary in the practice of different manufacturers, to render the L H AA. ...: .
FIG. 24. - File Teeth.
A, Float cut.
B, Double cut.
C, Rasp cut.
tools adaptable to cutting various classes of metals and alloys. Tungsten is the principal controlling element, but chromium is essential, and molybdenum and vanadium are often found of value. The steels are forged at a yellow tint, equal to about 1850° F. They are raised to a white heat for hardening, and cooled in an air blast to a bright red. They are then often quenched in a bath of oil.
The first public demonstration of the capacities of high speed steels was made at the Paris Exhibition of 1900. Since that time great advances have been made. It has been found that the section of the shaving limits the practicable speeds, so that, although cutting speeds of 300 and 400 ft. a minute are practicable with light cuts, it is more economical to limit speeds to less than 100 ft. per minute with much heavier cuts. The use of water is not absolutely essential as in using tools of carbon steel. The new steels show to much greater advantage on mild steel than on cast iron. They are more useful for roughing down than for finishing. The removal of 20 lb of cuttings per minute with a single tool is common, and that amount is often exceeded, so that a lathe soon becomes half buried in turnings unless they are carted away. The horse-power absorbed is proportionately large. Ordinary heavy lathes will take from 40 to 60 h.p. to drive them, or from four to six times more than is required by lathes of the same centres using carbon steel tools. Many remarkable records have been given of the capacities of the new steels. Not only turning and planing tools but drills and milling cutters are now regularly made of them. It is a revelation to see these drills in their rapid descent through metal. A drill of i in. in diameter will easily go through 5 in. thickness of steel in one minute.
Machine Tools The machine tools employed in modern engineering factories number many hundreds of well-defined and separate types. Besides these, there are hundreds more designed for special functions, and adapted only to the work of firms who handle specialities. Most of the first named and many of the latter admit of grouping in classes. The following is a natural classification: I. Turning Lathes. - These, by common consent, stand as a class alone. The cardinal feature by which they are distinguished is that the work being operated on rotates against a tool which is held in a rigid fixture - the rest. The axis of rotation may be horizontal or vertical.
II. Reciprocating Machines
The feature by which these are characterized is that the relative movements of tool and work take place in straight lines, to and fro. The reciprocations may occur in horizontal or vertical planes.
III. Machines which Drill and Bore Holes
These have some features in common with the lathes, inasmuch as drilling and boring are often done in the lathes, and some facing and turning in the drilling and boring machines, but they have become highly differentiated. In the foregoing groups tools having either single or double cutting edges are used.
IV. Milling Machines
This group uses cutters having teeth arranged equidistantly round a cylindrical body, and may therefore be likened to saws of considerable thickness. The cutters rotate over or against work, between which and the cutters a relative movement of travel takes place, and they may therefore be likened to reciprocating machines, in which a revolving cutter takes the place of a single-edged one.
V. Machines for Cutting the Teeth of Gear-wheels
These comprise two sub-groups, the older type in which rotary milling cutters are used, and the later type in which reciprocating single-edged tools are employed. Sub-classes are designed for one kind of gear only, as spur-wheels, bevels, worms, racks, &c.
VI. Grinding Machinery
This is a large and constantly extending group, largely the development of recent years. Though emery grinding has been practised in crude fashion for a century, the difference in the old and the new methods lies in the embodiment of the grinding wheel in machines of high precision, and in the rivalry of the wheels of corundum, carborundum and alundum, prepared in the electric furnace with those of emery.
VII. Sawing Machines
In modern practice these take an important part in cutting iron, steel and brass. Few shops are without them, and they are numbered by dozens in some establishments. They include circular saws for hot and cold metal, band saws and hack saws.
VIII. Shearing and Punching Machines
These occupy a border line between the cutting and non-cutting tools. Some must be classed with the first, others with the second. The detrusive action also is an important element, more especially in the punches.
IX. Hammers and Presses
Here there is a percussive action in the hammers, and a purely squeezing one in the presses. Both are made capable of exerting immense pressures, but the latter are far more powerful than the former.
X. Portable Tools
This large group can best be classified by the common feature of being readily removable for operation on large pieces of erection that cannot be taken to the regular machines. Hence they are all comparatively small and light. Broadly they include diverse tools, capable of performing nearly the whole of the operations summarized in the preceding paragraphs.
XI. Appliances
There is a very large number of articles which are neither tools nor machine tools, but which are indispensable to the work of these; that is, they do not cut, or shape, or mould, but they hold, or grip, or control, or aid in some way or other the carrying through of the work. Thus a screw wrench, an angle plate, a wedge, a piece of packing, a bolt, are appliances. In modern practice the appliance in the form of a templet or jig is one of the principal elements in the interchangeable system.
XII. Wood-working Machines
This group does for the conversion of timber what the foregoing accomplish for metal. There is therefore much underlying similarity in many machines for wood and metal, but still greater differences, due to the conditions imposed on the one hand by the very soft, and on the other by the intensely hard, materials operated on in the two great groups.
XIII. Measurement
To the scientific engineer, equally with the astronomer, the need for accurate measurement is of paramount importance. Neither good fitting nor interchangeability of parts is possible without a system of measurement, at once accurate and of ready and rapid application. Great advances have been made in this direction lately.
I
Lathes The popular conception of a lathe, derived from the familiar machine of the wood turner, would not give a correct idea of the lathe which has been developed as the engineer's machine tool. This has become differentiated into nearly fifty well-marked,types, until in some cases even the term lathe has been dropped for more precise definitions, as vertical boring machine, automatic machine, while in others prefixes are necessary, as axle lathe, chucking lathe, cutting-off lathe, wheel lathe, and so on. With regard to size and mass the height of centres may range from 3 in. in the bench lathes to 9 or io ft. in gun lathes, and weights will range from say 50 lb to 200 tons, or more in exceptional cases. While in some the mechanism is the simplest possible, in others it is so complicated that only the specialist is able to grasp its details.
Early Lathes
Space will not permit us to trace the evolution of the lathe from the ancient bow and card lathe and the pole lathe, in each of which the rotary movement was alternately forward, for cutting, and backward. The curious thing is that the wheel-driven lathe was a novelty so late as the 14th and 15th centuries, and had not wholly displaced the ancient forms even in the West in the 19th century, and the cord lathe still survives in the East. Another thing is that all the old lathes were of dead centre, instead of running mandrel type; and not until 1794 did the use of metal begin to take the place of wood in lathe construction. Henry Maudslay (1771-1831) did more than any other man to develop the engineer's self-acting lathe in regard to its essential mechanism, but it was, like its immediate successors for fifty years after, a skeleton-like, inefficient weakling by comparison with the lathes of the present time.
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Broad Types
A ready appreciation of the broad differences in lathe types may be obtained by considering the differences in the great groups of work on which lathes are designed to operate. Castings and forgings that are turned in lathes vary not only in size, but also in relative dimensions. Thus a long piece of driving shafting, or a railway axle, is very differently proportioned in length and diameter from a railway wheel or a wheel tire. Further, while the shaft has to be turned only, the wheel or the tire has to be turned and bored. Here then we have the first cardinal distinction between lathes, viz. those admitting work between centres (fig. 29) and face and boring lathes. In the first the piece of work is pivoted and driven between the centres of head-stock and tail-stock or loose poppet; in the second, it is held and gripped only by the dogs or jaws of a face-plate, on the head-stock spindle, the loose poppet being omitted.
These, however, are broad types only, since proportions of length to diameter differ, and with them lathe designs are modified whenever there is a sufficient amount of work of one class to justify the laying down of a special machine or machines to deal with it. Then further, we have duplicate designs, in which, for example, provision is made in one lathe for turning two or three long shafts simultaneously, or for turning and boring two wheels or tires at once. Further, the position of the axis of a face lathe need not be horizontal, as is necessary when the turning of long pieces has to be done between centres. There are obvious advantages in arranging it vertically, the principal being that castings and forgings can be more easily set and secured to a horizontal chuck than to one the face of which lies vertically. The chuck is also better supported, and higher rates of turning are practicable. In recent years these vertical lathes or vertical turning and boring mills (fig. 30) have been greatly increasing in numbers; they also occur in several designs to suit either general or special duties, some of them being used for boring only, as chucking lathes. Some are of immense size, capable of boring the field magnets of electric generators 40 ft. in diameter.
Standard Lathes
But for doing what is termed the general work of the engineer's turnery, the standard lathes (fig. 29) predominate, i.e. self-acting, sliding and surfacing lathes with headstock, loose poppet and slide-rest, centres, face plates and chucks, and an equipment by which long pieces are turned, either between centres or on the face chucks, and bored. One of the greatest objections to the employment of these standard types of lathes for indiscriminate duty is due to the limited height of the centres or axis of the headstock, above the face of the bed. This is met generally by providing a gap or deep recess in the bed next the fast headstock, deep enough to take face work of large diameter. The device is very old and very common, but when the volume of work warrants the employment of separate lathes for face-work and for that done between centres it is better to have them.
Screw-cutting
A most important section of the work of the engineer's turnery is that of cutting screws (see Screw). This has resulted in differentiation fully as great as that existing between centres and face-work. The slide-rest was designed with this object, though it is also used for plain turning. The standard " selfacting sliding, surfacing and screw-cutting lathe " is essentially the standard turning lathe, with the addition of the screw-cutting mechanism. This includes a master screw - the lead or guide screw, which is gripped with a clasp nut, fastened to the travelling carriage of the slide-rest. The lead-screw is connected to the headstock spindle by change wheels, which are the variables through which the relative rates of movement of the spindle and the lead-screw, and therefore of the screw-cutting tool, held and traversed in the slide-rest, are effected. By this beautiful piece of mechanism a guide screw, the pitch of which is permanent, is made to cut screw-threads of an almost infinite number of possible pitches, both in whole and fractional numbers, by virtue of rearrangements of the variables, the change wheels. The objection to this method is that the trains of change wheels have to be recalculated and rearranged as often as a screw of a different pitch has to be cut, an operation which takes some little time. To avoid this, the nest or cluster system of gears has been largely adopted, its most successful embodiment being in the HendeyNorton lathe. Here all the change wheels are arranged in a series permanently on one shaft underneath the headstock, and any one of them is put into engagement by a sliding pinion operated by the simple movement of a lever. Thus the lead-screw is driven at different rates without removing any wheel from its spindle. This has been extensively applied to both small and large lathes. But a moment's thought will show that even this device is too cumbrous when large numbers of small screws are required. There is, for example, little in common between the screw, say of 5 or 6 ft. in length, for a massive penstock or valve, and 2-in. bolts, or the small screws required in thousands for electrical fittings. Clearly while the self-acting screw-cutting lathe is the best possible machine to use for the first, it is unsuitable for the last. So here at once, from the point of view of screw cutting only, an important divergence takes place, and one which has ultimately led to very high specialization.
Small Screws
When small screws and bolts are cut in ?
? = -' ' large quantities, the guide-screw and change wheels give place to other devices, one of which involves the use of a separate master-screw for every different pitch, the other that of encircling cutting instruments or dies. The first are represented by the chasing lathe, the second by the screwing lathes and automatics. Though the principles of operation are thus stated in brief, the details in design are most extensive and varied.
In a chasing lathe the master-screw or hob, which may be either at the rear of the headstock or in front of the slide-rest, receives a hollow clasp-nut or a half-nut, or a star-nut containing several pitches, which, partaking of the traverse movement of the screwthread, imparts the same horizontal movement to the cutting tool. The latter is sometimes carried in a hinged holder, sometimes in a common slide-rest. The attendant throws it into engagement at the beginning of a traverse, and out when completed, and also this is an economical system, but in others not. It cannot be considered so when bolts, screws and allied forms are of small dimensions.
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Hollow Mandrel Lathes
It has been the growing practice since the last decade of the 19th century to produce short articles, required in large quantities, from a long bar. This involves making the lathe with a hollow mandrel; that is, the mandrel of the headstock has a hole drilled right through it, large enough to permit of the passage through it of the largest bar which the class of work requires. Thus, if the largest section of the finished pieces should require a bar of 12 in. diameter, the hole in the mandrel would be made 18 in. Then the bar, inserted from the rear-end, is gripped by a chuck or collet at the front, the operations of turning, screwing and cutting off done, and the bar then thrust farther through to the exact length for the next set of identical operations to be FIG. 30. - Boring and Turning Mill, vertica A, Table, running with stem in vertical bearing.
B, Frame of machine.
C, Driving cones.
D, Handle giving the choice of two rates, through concealed sliding gears, shown dotted.
E, Bevel-gears driving up to pinion gearing with ring of teeth on the table.
F, Saddle moved on cross-rail G. changes the hobs for threads of different sections. The screwed stays of locomotive fire-boxes are almost invariably cut on chasing lathes of this class.
In the screwing machines the thread is cut with dies, which encircle the rotating bar; or alternativel y the dies rotate round a fixed pipe, and generally the angular lead, or advance of the thread draws the dies along. These dies differ in no essentials from similar tools operated by a hand lever at the bench. There are many modifications of these lathes, because the work is so highly specialized that they are seldom used for anything except the work of cutting screws varying but little in dimensions. Such being the case they can hardly be classed as lathes, and are often termed screwing machines, because no provision exists for preliminary turning work, which is then done elsewhere, the task of turning and threading being divided between two lathes. In some cases lathe. (Webster Bennett, Ltd., Coventry.) H, Vertical slide, carrying turret J. K, Screw feeding F across.
L, Splined shaft connecting to II for feeding the latter up or down.
M, M, Worm-gears throwing out clutches N, N at predetermined points.
0, Cone pulley belted up to P, for driving the feeds of saddle and down-slide.
performed, and so on. This mechanism is termed a w ire feed, because the first lathes which were built of this type only operated on large wires; the heavy bar lathes have been subsequently developed from it. In the more advanced types of lathes this feeding through the hollow spindle does not require the intervention of the attendant, but is performed automatically.
The amount of preliminary work which has to be done upon a portion of a bar before it is ready for screwing varies. The simplest object is a stud, which is a parallel piece screwed up from each end. A bolt is a screw with a head of hexagonal, square or circular form, and the production of this involves turning the shank and shoulder and imparting convexity to the end, as well as screwing. But screw-threads have often to be cut on objects which are not primarily bolts, but which are spindles of various kinds used on mechanisms and machine tools, and in which reductions in the form of steps have to be made, and recesses, or flanges, or other features produced. Out of the demands for this more complicated work, as well as for plain bolts and studs, has arisen the great group of turret or capstan lathes (fig. 31) and the automatics or automatic screw machines which are a high development of the turret lathes.
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Turret Lathes
The turret or capstan (fig. 32) is a device for gripping as many separate tools as there are distinct operations to be performed on a piece of work; the number ranges from four to as many as twenty in some highly elaborated machines, but five or six is the usual number of holes. These tools are brought round FIG. 31. - Turret 1 Lathe.
A, Bed.
B, Waste oil tray.
C, Headstock.
D, Hollow mandrel.
E, Cones keyed to D. F, Split tapered close-in chuck, actuated by tube G. H, Toggle dogs which push G. J, Coned collar acting on H. K, Handle to slide J through sleeve on bar L. M, Rack slid on release of chuck, moving bearing N forward.
Webster & Bennett, Ltd., Coventry.) N, Bearing to feed the work through mandrel (constituting the wire or bar feed). A collar is clamped on the work. and is pushed by the bearing N at each time of feeding.
0, Cross-slide.
P, Hand-wheel operating screw to travel 0. Q, Turret-slide.
R, Cross-handle moving Q to and fro.
S, Turret or capstan.
T, U, Sets of fast and loose pulleys, for open and crossed belts.1.
V, Cone belted down to E on lathe.
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FIG. 32. - Plan of Set of Turret Tools. (A. Herbert, Ltd.) A, Turret.
B, Tool for first operation or chucking.
C, Cutting tools for second operation, starting or pointing.
D, Box tool carrying two cutters for third operation, rough turning.
E, Similar tool for fourth operation, finish turning.
F, Screwing tools in head for final operation of screwing.
in due succession, each one doing its little share of work, until the cycle of operations required to produce the object is complete, the cycle including such operations as turning and screwing, roughing and finishing cuts, drilling and boring. Severance of the finished piece is generally done by a tool or tools held by a cross-slide between the headstock and turret, so termed because its movements take place at right angles with the axis of the machine. This also often performs the duty of " forming," by which is meant the shaping of the exterior portion of an object of irregular outline, by a tool the edge of which is an exact counterpart of the profile required. The exterior of a cycle hub is shaped thus, as also are numerous handles and other objects involving various curves and shoulders, &c. The tool is fed p erpendicularly to the axis of the rotating work and completes outlines at once; if this were done in ordinary lathes much tedious manipulation of separate tools would be involved, Automatics. - But the marvel of the modern automatics (fig. 33) lies in the mechanism by which the cycle of operations is rendered absolutely independent of attendance, beyond the first adjustments and the insertion of a fresh bar as often as the previous one becomes used up. The movements of the rotating turret and of the crossslide, and the feeding of the bar through the hollow spindle, take place within a second, at the conclusion of the operation preceding. These movements are effected by a set of mechanism independent of that by which the headstock spindle is rotated, viz. by cams or cam drums on a horizontal cam shaft, or other equivalent device, differing much in arrangement, but not principle. Movements are hastened or retarded, or pauses of some moments may ensue, according to the cam arrangements devised, which of course have to be varied for pieces of different proportions and dimensions. But when the machines with their tools are once set up, they will run for days or weeks, repeating precisely the same cycle of operations; they are self-lubricating, and only require to be fed with fresh lengths of bar and to have their tools resharpened occasionally. Of these automatics alone there are something like a dozen distinct types, some with their turrets vertical, others horizontal. Not only so but the use of a single spindle is not always deemed sufficiently economical, and some of these designs now have two, three and four separate work spindles grouped in one head.
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Specialized Lathes. - Outside of these main types of lathes there are a large number which do not admit of group classification. They are designed for special duties, and only a representative list can be given. Lathes for turning tapered work form a limited This of course is an extremely comprehensive classification, because chucks of the same name differ vastly when used in small and large lathes. The chucks, again, used in turret work, though they grip the work by one end only, differ entirely in design from the face chucks proper.
Chucking between Centres
The simplest and by far the commonest method adopted is to drill countersunk centres at the ends of the work to be turned, in the centre or longitudinal axis (fig. 34, A), and support these on the point centres of headstock and poppet. The angle included by the centres is usually 60°, and the points may enter the work to depths ranging from as little as 7 1 6 in. in very light pieces to 2 in., 4 in. or i in. in the heaviest. Obviously a piece centred thus cannot be rotated by the mere revolution of the lathe, but it has to be driven by some other agent making con FIG. 33. - Automatic Lathe or Screw Machine. (A. Herbert, Ltd.) a, a, a, Cams for actuating chuck movements through pins b, b. The cam which returns D is adjustable but is not in view.
c, Feeding cam for turret. d,d,Return cams for turret.
e, e,Cams on cam disk for oper ating the lever f, which actuates the cut-off and forming slide.
T, Worm-wheel which drives cam shaft by a worm on the same shaft as the feed-pulley U. V, Handwheel on worm shaft for making first adjustments.
W, Change feed disk.
g, g,Change feed dogs adjustable round disk.
X, Change feed lever.
Y, Oil tube and spreader for lubricating tools and work.
Z, Tray for tools, &c.
number, and they include the usual provisions for ordinary turning. In some designs change wheels are made use of for imparting a definite movement of cross traverse to the tool, which being compounded with the parallel sliding movements produces the taper, In others an upper bed carrying the heads and work swivels on a lower bed, which carries the slide rest. More often tapers are turned by a cross adjustment of the loose poppet, or by a taper attachment at the rear of the lathe, which coerces the movement of the top or tool-carrying slide of the rest. Or, as in short tapers, the slide-rest is set to the required angle on its carriage. Balls are sometimes turned by a spherical attachment to the slide-rest of an ordinary lathe. Copying lathes are those in which an object is reproduced from a pattern precisely like the objects required. The commonest example is that in which gun-stocks and the spokes of wheels are turned, but these are used for timber, and the engineer's copying lathe uses a form or cam and a milling cutter. The form milling machine is the copying machine for metal-work. The manufacture of boilers has given birth to two kinds of lathes, one for turning the boiler ends, the other the boiler flue flanges, the edges of which have to be caulked. Shaft pulleys have appropriated a special lathe containing provision for turning the convexity of the faces. Lathes are duplicated in two or three ways. Two, four, six or eight tools sometimes operate simultaneously on a piece of work. Two lathes are mounted on one bed. A tool will be boring a hole while another is turning the edges of the same wheel. One will be boring, another turning a wheel tire, and so on. The rolls for iron and steel mills have special lathes for trueing them up. The thin sheet metal-work produced by spinning has given rise to a special kind of spinning lathe where pressure, and not cutting, is the method adopted.
Methods of Holding and Rotating Work. Chucks
The term chuck signifies an appliance used in the lathe to hold and rotate work. As the dimensions and shapes of the latter vary extensively, so also do those of the chucks. Broadly, however, the latter correspond with the two principal classes of work done in the lathe, that between centres, and that held at one end only or face work. A, Centring and driving; a, point centre; b, carrier; c, driver centre; d, back centre; e, fixed in slot in body of point work. B, Face-plate driver or catch D, Clement double driver; a, face C, Common heart-shaped carrier.
plate; a, centre; b, driver.
plate; b, b, drivers; c, loose plate carrying drivers.
nexion between it and the mandrel. The wood turner uses a forked or prong centre to obtain the necessary leverage at the headstock end, but that would be useless in metal. A driver is therefore used, of which there are several forms (fig. 34), the essential element being a short stiff prong of metal set away from the centre, and rotating the work directly, or against a carrier which encircles and pinches the work. As this method of driving sets up an unbalanced force, the " Clement " or double driver (fig. 34, D), was invented, and is frequently made use of, though not nearly so much as the common single driver. In large and heavy work it is frequently the practice to drive in another way, by the dogs of the face-plate.
Steadies
Pieces of work which are rigid enough to withstand the stress of cutting do not require any support except the centres.
FIG. 35.
A, Travelling steady with adjustslotted bolt holes a, a; b, b, able studs a, a; b, work; brass or steel facings.
c, tool; d, slide-rest. C, Fixed steady with hinged top B, Steady with horizontal and and three setting pieces. vertical adjustment through But long and comparatively slender pieces have to be steadied at intermediate points (fig. 35). Of devices for this purpose there are many designs; some are fixed or bolted to the bed and are shifted when necessary to new positions, and others are bolted to the carriage of the slide-rest and move along with it - travelling Main body.
Waste oil tray.
Headstock.
Wire-feed tube.
Slide for closing chuck.
Shaft for ditto.
Feed-slide.
Piece of work.
Turret with box tools.
Turret slide.
Saddle for ditto, adjustable along bed.
Screw for locating adjustable slide.
Cut-off and forming crossslide.
0, Back and front tool-holders on slide.
Cam shaft.
Cam drum for operating chuck.
Cam drum for operating turret.
Cam disk for actuating cross-slide.
0, steadies. In some the work is steadied in a vee, or a right angle, in others adjustable pins or arms are brought into contact with it. As the pressure of the cut would cause an upward as well as backward yielding of the work, these two movements are invariably provided against, no matter in what ways the details of the steadies are worked out. Before a steady can be used, a light cut has to be taken in the locality where the steady has to take its bearing, to render the work true in that place. The travelling steady follows immediately behind the tool, coming in contact therefore with finished work continually.
Mandrels
Some kinds of work are carried between centres indirectly, upon mandrels or arbors (fig. 36). This is the method J FIG. 36. - Mandrels.
A, Plain mandrel. B, Stepped mandrel. C, Expanding mandrel. adopted when wheels, pulleys, bushes and similar articles are bored first and turned afterwards, being chucked by the bore hole, which fits on a mandrel. The latter is then driven between point centres and the bore fits the mandrel sufficiently tightly to resist the stress of turning. The large number of bores possible involves stocking a considerable number of mandrels of different diameters. As it is not usual to turn a mandrel as often as a piece of work requires chucking, economy is studied by the use of stepped mandrels, which comprise several diameters, say from three to a dozen. A better device is the expanding mandrel, of which there are several forms. The essential principle in all is the capacity for slight adjustments in diameter, amounting to from 4 in. to i in., by the utilization of a long taper. A split, springy cylinder may be moved endwise over a tapered body, or separate single keys or blades may be similarly moved.
Face- Work. - That kind of work in which support is given at the headstock end only, the centre of the movable poppet not being required, is known as face-work. It includes pieces the length of which ranges from something less than the diameter to about three or four times the diameter, the essential condition being that the unsupported end shall be sufficiently steady to resist the stress of cutting. Work which has to be bored, even though long, cannot be steadied on the back centre, and if long is often supported on a cone plate. The typical appliance used for face-work is the common face-plate (fig. 37). It is a plain disk, screwed on the mandrel FIG. 37. - Face-plate.
A, Screwed hole to fit mandrel nose. B, Slots for common bolts. C, Tee-slots for tee-head bolts.
nose, and having slot holes in which bolts are inserted for the purpose of cramping pieces of work to its face. There are numerous forms of these clamps, and common bolts also are used. The faceplate may also serve to receive an intermediary, the angle-plate, against which work may be bolted when its shape is such as to render bolting directly to the plate inconvenient.
Jaw Chucks
When a face-plate has fitted to it permanent dogs or jaws it is termed a dog or jaw chuck (fig. 38). In the commonest form the jaws are moved radially and independently, each by its own screw, to grip work either externally or internally. In some cases the dogs are loosely fitted to the holes in a plain faceplate. In all these types the radial setting is tentative, that is, the jaws being independent, there is no self-centring capacity, and thus much time is lost. A large group, therefore, are rendered self-centring by the turning of a ring which actuates a face scroll FIG. 39. - Scroll Chuck, ungeared.
A, Face-plate screwed to man- E, Jaws in chuck face, having sectional scroll teeth engaging with scroll a, and moved inwards or outwards by the scroll when C is turned.
b, Tommy or lever hole in C. F, Piece of work outlined.
FIG. 40. - Combination Geared Scroll Chuck.
A, Back plate; a, recess for faceplate.
B, Pinions.
C, Circular rack with scroll b on face.
D, Chuck body.
E, Jaws fitting on intermediate pieces c that engage with the scroll b. d, Screws for operating jaws independently.
Toolo-28.jpg
a a FIG. 38. - Independent Jaw Chuck.
b, Square heads of screws for key.
c, Tee-grooves for bolts.
A, Body.
a, Recess to receive face-plate.
B, Jaws or dogs.
C, Screws for operating jaws.
Toolo-29.jpg
Toolo-30.jpg
FIG. 41. - Spiral Geared Chuck,. concentric movement. (C.Taylor, Birmingham.) A, Back.
B, Body.
C, Spiral plate with teeth engaging in jaws D. E, Bevel pinions gearing with teeth on back of C.
Toolo-31.jpg
Toolo-32.jpg
drel nose.
B, Back of chuck screwed to A. C, Knurled chuck body with scroll a on face.
D, Chuck face.
] (fig. 39) or a circular rack with pinions (fig. 40), turned with a key which operates all the jaws simultaneously inwards or outwards. But as some classes of jobs have to be adjusted eccentrically, many chucks are of the combination type (fig. 40), capable of being used independently or concentrically, hence termed universal chucks. The change from one to the other simply means throwing the ring of teeth out of or into engagement with the pinions by means of cams or equivalent devices. Each type of chuck occurs in a large range of dimensions to suit lathes of all centres, besides which every lathe includes several chucks, large and small, in its equipment. The range of 'diameters which can be taken by any one chuck is limited, though the jaws are made with steps, in addition to the range afforded by the operating screws. The " Taylor " spiral chucks (fig. 41) differ essentially from the scroll types in having the actuating threads set spirally on the sloping interior of a cone. The result is that the outward pressure of each jaw is received behind the body, because the spiral rises up at the back. In the ordinary scroll chucks the pressure is taken only at the bottom of each jaw, and the tendency to tilt and pull the teeth out of shape is very noticeable. The spiral, moreover, enables a stronger form of tooth to be used, together with a finer pitch of threads, so that the wearing area can be increased.
The foregoing may be termed the standard chucks. But in addition there are large numbers for dealing with special classes of work. Brass finishers have several. Most of the hollow spindle lathes and automatics have draw-in or push-out chucks, in which the jaws are operated simultaneously by the conical bore of the encircling nose, so that their action is instantaneous and self-centring. They are either operated by hand, as in fig. 31, or automatically, as in fig. 33. There is also a large group used for drills and reamers - the drill chucks employed in lathes as well as in drilling machines.
II. - Reciprocating Machine Tools This is the only convenient head under which to group three great classes of machine tools which possess the feature of reciprocation in common. It includes the planing, shaping and slotting machines. The feature of reciprocation is that the cutting tool is operative only in one direction; that is, it cuts during one stroke or movement and is idle during the return stroke. It is, therefore, in precisely the same condition as a hand tool such as a chisel, a carpenter's plane or a hand saw. We shall return again to this feature of an idle stroke and discuss the devices that exist to avoid it.
Planing Machines
In the standard planer for general shop purposes (fig. 42) the piece of work to be operated on is attached to a horizontal;y table moving to and fro on a rigid bed, and passing underneath the fixed cutting tool. The tool is gripped in a box having certain necessary adjustments and movements, so that the tool can be carried or fed transversely across the work, or at right angles with the direction of its travel, to take successive cuts, and also downwards or in a vertical direction. The tool-box is carried on a cross-slide which has capacity for several feet of vertical adjustment on upright members to suit work of varying depths. These up- j rights or housings are bolted to the sides of the bed, and the whole framing is so rigidly designed that no perceptible tremor or yielding takes place under the heaviest duty img posed by the stress of cutting.
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Toolo-33.jpg
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-6: a Ci - k a, 7j cd [[[Reciprocating Machines]] Moreover, after the required adjustments have been made and the machine started, the travel and the return of the work-table and the feeding of the tool across the surface are performed by self-acting mechanism actuated by the reciprocations of the table itself, the table being driven from the belt pulleys.
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To such a design there are objections, which, though their im- portance has often been exaggerated, are yet real. First, the crossrail and housings make a rigid enclosure over the table, which sometimes prevents the admission of a piece that is too large to pass under the cross-rail or between the housings. Out of this a FIG. 43.-20-in. Side Planing Machine.
A, Bed.
B, B, Feet.
C, C, Work tables adjustable vertically on the faces D, D, by means of screws E, E, from handles F, F, through bevel gears.
(G. Richards & Co., Ltd., Manchester.) G, Tool-box on travelling arm H, travelled by fast and loose pulleys J for cutting, and by pulleys K for quick return.
L, Feed-rod with adjustable dogs a, a, for effecting reversals through the belt forks b, b. M, Brickwork pit to receive deep objects.
Toolo-36.jpg
n?iiiinn FIG. 44.-8-in. Shaping Machine. (Cunliffe & Croom, Ltd., Manchester.) k, A, Base.
B, Work-table, having vertical movement on carriage C, which has horizontal movement along the face of A. D, Screw for effecting vertical movement, by handle E, and bevel gears.
F, Screw for operating longitudinal movement with feed by hand or power.
G, Tool ram.
H, Tool-box.
a, Worm-gear for setting tool-holder at an angle.
b, Crank handle spindle for operating ditto.
c, Handle for actuating down feed of tool.
Driving cone pulley actuating pinion d, disk wheel e, with slotted disk, and adjustable nut moving in the slot of the crank f, which actuates the lever g, connected to the tool ram G, the motion constituting the Whitworth quick return; g is pivoted to a block which is adjustable along a slot in G, and the clamping of this block in the slot regulates the position of the ram G, to suit the position of the work on the table.
Feed disk driven by small gears from cone pulley.
Pawl driven from disk through levers at various rates, and controlling the amount of rotation of the feed screw F. Conical mandrel for circular shaping, driven by worm and wheel 1. ] objection has arisen a new design, the side planer (fig. 43), in which the tool-box is carried by an arm movable along a fixed bed or base, and overhanging the work, which is fastened to the side of the base, or on angle brackets, or in a deep pit alongside. Here the important difference is that the work is not traversed under the tool as in the ordinary planer, but the tool moves over the work. But an evil results, due to the overhang of the tool arm, which being a cantilever supported at one end only is not so rigid when cutting as the cross-rail of the ordinary machine, supported at both ends on housings. The same idea is embodied in machines built in other respects on the reciprocating table model. Sometimes one housing is omitted, and the tool arm is carried on the other, being therefore unsupported at one end. Sometimes a housing is made to be removable at pleasure, to be temporarily taken away only when a piece of work of unusual dimensions has to be fixed on the table.
Another objection to the common planer is this. It seems unmechanical in this machine to reciprocate a heavy table and piece of work which often weighs several tons, and let the tool and its holder of a few hundredweights only remain stationary. The mere reversal of the table absorbs much greater horse-power there is no limitation whatever to the length of the work, since it may extend to any distance beyond the base-plate.
Shaping Machines
The shaping machine (fig. 44) does for comparatively small pieces that which the planer does for long ones. It came later in time than the planer, being one of James Nasmyth's inventions, and beyond the fact that it has a reciprocating noncutting return stroke it bears no resemblance to the older machine. Its design is briefly as follows: The piece of work to be shaped is attached to the top, or one of the vertical side faces, of a rightangled bracket or brackets. These are carried upon the face of a main standard and are adjustable thereon in horizontal and vertical directions. In small machines the ram or reciprocating arm (see fig. 44, G) slides in fixed guides on the top of the pillar, and the necessary side traverse is imparted to the work table B. To the top of the main standard, in one design, a carriage is fitted with horizontal traverse to cover the whole breadth, within the capacity of the machine, of any work to be operated on. In the largest machines two standards support a long bed, on which the carriage, with its ram, traverses past the work. These machines are frequently made double-headed, that is carriages, rams and work tables are dupli FIG. 45.-12-in. Stroke Slotting Machine.
A, Main framing.
B, Driving cone.
C, D, Gears driven by cones.
E, Shaft of L. F, Tool ram driven from shaft E through disk G and rod H, with quick return mechanism D. J, Counter-balance lever to ram.
than the actual work of cutting. Hence a strong case is often stated for the abandonment of the common practice. But, on the other hand, the centre of gravity of the moving table and work lies low down, while when the cross-rail and housings with the cutting tool are travelled and reversed, their centre of gravity is high, and great precautions have to be taken to ensure steadiness of movement. Several planers are made thus, but they are nearly all of extremely massive type - the pit planers. The device is seldom applied to those of small and medium dimensions.
But there is a great group of planers in which the work is always fixed, the tools travelling. These are the wall planers, vertical planers or wall creepers, used chiefly by marine engine builders. They are necessary, because many of the castings and forgings are too massive to be put on the tables of the largest standard machines. They are therefore laid on the base-plate of the wall planer, and the tool-box travels up and down a tall pillar bolted to the wall or standing independently, and so makes vertical cutting strokes. In some designs horizontal strokes are provided for, or either vertical or horizontal as required. Here, as in the side planer, (Greenwood & Batley, Ltd., Leeds.) K, Flywheel.
L, Driving-disk.
M, N, Feed levers and shaft operated from disk, actuating linear movements of slides 0, P, and circular movement of table Q, through gears R. S, Hand-feed motions to table.
T, Countershaft.
cated, and the operator can set one piece of work while the other is being shaped. In all cases the movement of the reciprocating arm, to the outer end of which the tool is attached, takes place in a direction transversely to the direction of movement of the carriage, and the tool receives no support beyond that which it receives from the arm which overhangs the work. Hence the shaper labours under the same disadvantages as the side planer - it cannot operate over a great breadth. A shaper with a 24-in. stroke is one of large capacity, 16 in. being an average limit. Although the non-cutting stroke exists, as in the planer, the objection due to the mass of a reciprocating table does not exist, so that the problem does not assume the same magnitude as in the planer. The weak point in the shaper is the overhang of the arm, which renders it liable to spring, and renders heavy cutting difficult. Recently a novel design has been introduced to avoid this, the draw-cut shaper, in which the cutting is done on the inward or return stroke, instead of on the outward one.
| [DRILLING MACHINES |
Slotting Machines
In the slotting machine (fig. 45) the cutting takes place vertically and there is a lost return stroke. All the seems at first sight the best solution, and it is adopted on a number of machines, though still in a great minority. The pioneer device of this kind, the rotating tool-box of Whitworth, simply turns the tool round through an angle of 180° at the termination of each stroke, the movement being self-acting. In some later designs, instead of the box being rotated to reverse the tool. two tools are used set back to back, and the one that is not cutting is relieved for the time being, that is tilted to clear the work. Neither of these tools will plane up to a shoulder as will the ordinary ones.
Allied Machines
The reciprocation of the tool or the work, generally the former, is adopted in several machines besides the standard types named. The plate-edge planer is used by platers and boiler makers. It is a side planer, the plates being bolted to a bed, and the tool traversing and cutting on one or both strokes. Provision is often included for planing edges at o right angles. The key-seaters are a special type, designed mainly to remove the work of cutting key grooves in the bores of wheels and pulleys from the slotting machine. The work is fixed on a table and the keyway cutting tool is drawn downwards through the bore, with several resulting practical advantages. Many planing machines are portable so that they may be fixed upon very massive work. Several gear-wheel cutting machines embody the reciprocating tool.
III.-Drilling And Boring Machines The strict distinction between the operations of drilling and boring is that the first initiates a hole, while the second enlarges one already existing. But the terms are used with some latitude. A combined drilling and boring machine is one which has provision for both functions. But when holes are of large dimensions the drilling machine is useless because the proportions and gears are unsuitable. A 6-in. drill is unusually large, but holes are bored up to 30 ft. or more in diameter.
Types of Machines
The distinction between machines with vertical and horizontal spindles is not vital, but of convenience only The principal controlling element in design is the mass of the work, which often determines whether it or the machine shall be adjusted relatively to each other. Also the dimensions of a hole determine the speed of the tools, and this controls the design of the driving and feeding mechanism. Another important difference is that between drilling or boring one or more holes simultaneously. With few exceptions the tool rotates and the work is stationary. The notable exceptions are the vertical boring lathes already mentioned. Obviously the demands made upon drilling machines are nearly as varied as those on lathes. There is little in common between the machines which are serviceable for the odd jobs done in the general shop and those which are required for the repetitive work of the shops which handle specialities. Provision often has to be made for drilling simultaneously several holes at certain centres or holes at various angles or to definite depths, while the mass of the spindles of the heavier machines renders counter-balancing essential.
Bench Machines are the simplest and smallest of the group. They are operated either by hand or by power. In the power machines generally, except in the smallest, the drill is also fed downwards by power, by means of toothed gears. The upper part of the drilling R, R, Feed cones driving from shaft M to wormshaft S, for self-acting feed of drill.
T, Change-speed gears.
U, Hand-wheel for racking carriage D along radial arm C.
V, Clutch and lever for reversing direction of rotation of spindle.
W, Worm-gear for turning pillar B. d, Handle for turning worm.
X, Screw for adjusting the height of the radial arm.
Y, Gears for actuating ditto from shaft C.
Z, Rod with handle for operating elevating gear.
spindle being threaded is turned by an encircling spur-wheel, operated very slowly by a pinion and hand-wheel by the right hand of the attendant, the movement being made independent of the rotation of the spindle. A rack sleeve encircling the spindle is also common. In the power machines gears are also used, but a belt on small cone pulleys drives from the main cone shaft at variable speeds. From three to four drilling and feeding speeds are provided for by the respective cone pulleys. Work is held on or bolted to a circular table, which may have provision for vertical adjustment to suit pieces of work of different depths, and which can usually be swung aside out of the way to permit of deep p