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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 See also:wedge-like See also:action, leaving a smooth See also:surface behind. An See also:analysis of the essential methods of operation gives a broad grouping as follows: I. The See also:chisel See also:group . . Typified by the chisel of the woodworker. II. The shearing group IV. The percussive and detrusive group V. The moulding group . „ See also: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 See also:chisels. But many occupy a border-See also:line where the results obtained are practically those due to cutting, as in some of the See also: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 See also:character. Cutting Tools.—The cutting edge of a See also:tool is the See also:practical outcome of several conditions. Keenness of edge, See also:equivalent to a small degree of See also:angle between the tool faces, would appear at first sight to be the See also:prime See also:element in cutting, as indeed it is in the See also:case of a See also:razor, or in that of a chisel for soft See also:wood. But that is not the prime See also:condition in a tool for cutting See also:iron or See also: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 6o° to 75° are required. Further, much greater rigidity in the latter, to resist See also: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 See also:metal-See also:work. Formerly all turning, no See also:matter how large, was done by hand-operated tools, and after See also:great See also:muscular exertion a few pounds of metal might be removed in an See also:hour. But coerce a similarly formed tool in a rigid See also:guide or See also:rest, and drive it by the See also:power of ten or twenty men, and it becomes possible to remove say a hundredweight of chips in an hour. Or, increase the See also:size of the tool and its capacity for endurance, and drive by the power of 40 or 6o horses, and See also:half a ton of chips may be removed in an hour.
All See also:machine tools of which the chisel is the type operate by cutting; that is, they See also:act on the same principle and by the same essential method as the See also:knife, razor or chisel, and not by that of the grind-See also: The See also:butcher's knife will afford a See also:familiar See also:illustration. It is used as a cutting tool when severing a See also:steak, but it becomes a scrape when used to clean the See also:block. The difference is not therefore due to the See also:form of the knife, but to the method of its application, a distinction which holds See also:good in reference to the tools used by See also:engineers. There is a very old hand tool once much used in the engineer's turnery, termed a " graver.” This was employed for 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 See also:instinct of the See also: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 See also:general, the question of the most suitable tool formation has been the subj6ct of much experiment and discussion. The almost unconscious experimenting which goes on every See also:day in every workshop in the See also: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 See also:long as certain broad principles and reasonable limits are observed, that is sufficient for practical purposes. Clearly, in See also:order that a tool shall cut, it must possess an incisive form. In fig. 1, A might be thrust over the surface of the See also:plate of metal, but no cutting action could take See also:place. It would simply grind and See also: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 If „ See also:scissors. See also:cabinet-maker's scrape. „ See also:hammer and the See also:punch. once become a cutting tool. But its bevelled See also:face would rub and grind on the surface of the work, producing See also:friction and See also: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 A, Tool which would burnish F, G, H, Presentations of tools only. for planing, turning and B, Scrape. See also: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 See also:rake; b, front or See also:top rake; c, tool angle. therefore that the correct form for a cutting tool must depend upon a due See also: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 See also:machines are held rigidly in one position, differing in this respect from hand-operated tools, it follows that a See also:constant angle should be given to See also:instruments which are used for operating on a given See also:kind of metal or alloy. It does not matter whether a tool is driven in a See also:lathe, or a planing machine, or a sharper or a slotter; whether it is cutting on See also:external or See also:internal surfaces, it is always maintained in a direction perpendicularly to the point of application as in fig. i, F, G, H, planing, turning and boring respectively. It is consistent with See also: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 See also:cast iron and cast steel, and for See also: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 See also:accord best with general practice, but they are not constant, being varied by conditions, especially by See also: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 See also: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 See also: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 See also:differences will result. Actually it need never vary much from 5' to 70. The See also: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, See also:bin 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 See also:alloys, the broken See also: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 See also:carbon steel from J M I Fie. 2.—Metal-turning Tools. E, See also:Diamond or angular-edge tool for cutting all metals. F, See also:Plan of See also:finishing tool. G, Spring tool for finishing. H, See also:Side or knife tool. J, Parting or cutting-off tool. K, L, See also:Round-See also:nose tools. M, See also:Radius tool. Fin. 3.—Group af Planer Tools. A, Planer type of tool, cranked E, Parting or cutting-off or to avoid digging into the grooving tool. metal. F, V tool for grooves. B, Face view of roughing tool. G, Right- and See also:left-hand tools for C, Face view of finishing tool. V-slots. D, Right- and left-hand knife or H, See also:Ditto for T-slots. side tools. J, Radius tool held in holder. that See also: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 See also: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 See also: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 See also:groups in the cutting See also:category; they are on the border-line between cutting and scraping instruments. Typical Tools.—A See also:bare enumeration of the diverse forms in which tools of the chisel type occur is not even possible here. The grouped illustrations (See also: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-See also:box, See also:turret, tool-holder, box, See also:cross-slide, &c., this often determines the choice of some one form in preference to another. A broad See also:division is that into roughing and finishing \ C cd$' B E~ 0 L It F A, Shape of tool used for scraping brass. B, Straightforward tool for turning all metals. C, Right- and left-hand tools for all metals. D, A better form of same. tools. Generally though not invariably the edge of the first is narrow, of the second broad, corresponding with the deep cutting and See also:fine See also:traverse of the first and the shallow cutting and broad A, See also:Common roughing tool. B, Parting-off or grooving tool. C, Roughing or finishing tool in a holder. D, See also:Double-edged tool for cutting opposite sides of a slot. I See also:bar of steel. This is costly when the best tool steel is used, hence large See also:numbers of tools comprise points only, which are gripped in permanent holders in which they interchange. Tool steel usually ranges from about a in. to 4 in. square; most engineers' work is done with bars of from z in. to i % in. square. It is in the smaller and See also: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. See also: The amount of See also: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. 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 A es- C R S. w d1' A, Smith & Coventry swivelling holder. B, Holder for square steel. C, D, right- and 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 See also:principal forms. The round-nosed roughing tool (fig. 2) B is of straight-forward type, used for turning, planing and shaping. As the correct tool angle can only occur on the See also:middle plane of the tool, it is usual to employ cranked tools, C, D, E, right- and left-handed, for heavy and moderately heavy See also: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 planin (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 G have broad edges, F, G, H. They occur in straightforward and FIG. 6.—Group of Chisels. right- and left-hand types. A, Paring chisel. These as a See also:rule remove less than B, Socket chisel for heavy duty. ;g in. in See also:depth, while the rough- C, Common chipping chisel. See also:ing tools may cut an See also:inch or D, Narrow cross-cut er cape chisel. more into the metal. But the E, Cow-mouth chisel, or See also:gouge. traverse of the first often exceeds F, Straight chisel or sett. an inch, while in that of the G, Hollow chisel or sett. second i 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. See also:Tod-holders.—It will be observed that the fore-going are solid tools ; that is, the cutting portion is forged from a solid 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 See also: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 See also: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-See also: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 semi- and full-automatic machines, a result to which the See also: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 See also:short time before; some of the commoner jobs are produced with greater See also:economy, while heavier castings and forgings, longer and larger bars, are tooled in the turret lathes. A great See also: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 See also:gauge the See also: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 See also: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. Planes.—We pass by the familiar great chisel group, used by wood-workers, with a brief See also:notice. Generally the tool angles of these See also:lie between 15° and 25°. They include the chisels proper, and the gouges in numerous shapes and proportions, used by carpenters, cabinet-makers, tuners, stone-masons and allied tradesmen. These are mostly thrust by hand to their work, without any See also:mechanical See also:control. Other chisels are used percussively, as the stout See also:mortise chisels, some of the gouges, the axes, adzes and stone-See also: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 A, Cutting iron. B, Top or back iron. C, Clamping See also: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 See also:coercion. We also meet with the See also:function of the top or non-cutting LiFc A, See also:Spoon bit. B, Centre-bit. C, Expanding centre-bit. D, See also:Gilpin or Gedge See also:auger. E, Jennings auger. F, Irwin auger. A, Common See also:flat See also:drill. B, Twist drill. C, Straight fluted drill. D, See also:Pin drill for flat countersinking. E, Arboring or facing tool. F Tool for boring See also: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 See also:timber are bored with equal facility; the tools (figs. 9 and io) 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 See also: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 wood-boring 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 See also: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. II). The first-named both initiate and finish a hole; A, D-bit. B, Solid reamer. C, Adjustable reamer, having six flat See also:blades forced outward by the tapered plug. Two See also:lock-nuts at the end See also:fix the blades firmly after See also: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 See also: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 See also: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 See also:bore with more than an approximation to concentricity throughout. Boring tools (fig. 12) held in the slide-rest will spring and See also:bend and chatter, and unless the lathe is true, or careful See also: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 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. See also: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. 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 See also:main varieties: in one the cutters are fixed directly in the bar (fig. 13, A to D), in the other in a See also:head fitted on the See also:bat (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 traversed by the mechanism of the lathe See also: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 See also: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 See also: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 See also: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 See also:band saws are common in the smithy and the See also:boiler and machine shops for cutting off bars, forgings 7 and rolled sections. But the tooth shapes are not those used for timber, nor is the cutting See also:speed the same. In the individual saw-See also:teeth both cutting and scraping actions are illustrated (fig. 14). Saws which cut timber continuously with the See also: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 cross-cutting teeth scrape only, the teeth being either of triangular or of M-form, variously modified. Teeth for metal cutting also act strictly by scraping. The pitching of the teeth is 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 See also: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 See also: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 See also:escape freely. Milling Cutlers.—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 See also:cylinder with teeth lying across its periphery, or parallel with its See also:axis—the edge See also: 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 See also:province of the single-edged tools of the planer, shaper and slotter. Of these two typical forms the
F
A, Narrow edge mill, with D, Cutter having teeth like C. straight teeth. E, Flat teeth held in with screws
B, Wide edge mill with See also:spiral and wedges.
teeth. F, Large inserted tooth mill ; with
C, Teeth on face and edges. See also:taper pins secure cutters.
B ;t•~ ii FIG. 16.—Group of End See also:Mills.
A, End mill with straight teeth. B, Ditto with spiral teeth. C, Showing method of holding See also:shell cutter on arbor, with screw and See also: 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. F ~E A, Teeth of band and ripping saws. B, Teeth of circular saw for hard wood; shows set. C. Ditto for soft wood. D, Teeth of cross-cut saw. E, M-teeth for ditto. I D changes are See also: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 See also:device which is A B FIG. 17. A, Straddle Mill, cutting faces and edges. B, Set of three mills cutting grooves. A 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 See also: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 See also:running in See also: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 See also: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 See also: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 See also: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 See also:file. There are two ways in which milling cutters are used, by which their value is extended; one is to See also:transfer some of their work proper to the lathe and boring machine, the other is by duplication. A good many See also:light circular sections, as See also: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 See also: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 See also: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 toothface is continued identical in form with the profile of the edge, the outline being carried back as a See also: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 indi- cated by the shaded portion on two of the teeth. The FIG. 20.-Relieved Teeth of Milling limit of grinding is reached Cutter. when the tooth becomes too thin and weak to stand up to its work. But such cutters will endure See also:weeks or months of constant service before becoming useless. The A 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. See also: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 U A B C Q R S T U A, Warding. J, Topping. P, Round. B, Mill. K, Reaper. Q, See also:Pit-saw or C, Flat. L, Knife. See also:frame-saw. D, See also:Pillar. M, Three-square. R, Half-round. E, Square. N, Cant. S, T, Cabinet. F, G, Swaged reapers. 0, S I Slitting or U, See also:Tumbler. H, Mill. See also:feather-edge. V, See also:Crossing. See also: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 See also:special lathes, employing a profile tool which cuts the surface A, Parallel or See also:blunt. F, Tapered triangular. K, Tapered half- B, Taper bellied. G, Parallel round. round. C, Knife reaper. H, Taper or See also:rat-tail. L, Riffler. D, Tapered square. J, Parallel half- E, Parallel triangular. round. A, See also:Convex Cutter. B, See also:Concave Cutter. C, Profile Cutter. j p ' ////d B 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 See also:objects, was due to See also:Sir See also:Joseph See also: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 work-See also:man 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 See also: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 See also:float cut, or double cut (fig. 24). The rasps are another group. Degrees of coarseness are designated as rough, middle cut, See also: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 See also: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 a, a, Blades. b, Plate being sheared. severe detrusive effort; for the punch, with its bolster (fig. 26), forms a pair of cylindrical shears. Hence a shorn or punched edge is always rough, ragged, and covered with See also: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 a in. diameter, so that homogeneous metal is left throughout the entire unpunched section. The same results follow reamering both in iron and steel. See also: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 See also:influence as reamering and annealing punched holes. Hammers.—These form an immense group, termed percussive, from the manner of their use (fig. 27). Every See also:trade has its owrl See also:peculiar shapes, the-See also: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 See also:jar the hands. The fullering tools, and flatters, and setts, though not hammers strictly, are actuated by A, See also:Exeter type. B, Joiner's hammer. C, See also:Canterbury claw hammer (these are wood-workers' hammers). D, Engineer's hammer, See also:ball See also:pane. E, Ditto, cross-pane. Moulding Tools.—This is a group of tools which, actuated either by See also:simple pressure or percussively, See also:mould, shape and See also:model forms in the See also:sand of the moulder, in the metal of the smith, and in See also: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 H A, Square trowel. E, Flange See also:bead. J, See also:Button sleeker. B, See also:Heart trowel. F, Hollow bead. K, See also: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 See also:counter-See also:part of the dies, or of some portion of them. The practice of See also:die See also:forging consists almost wholly of moulding processes. 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 See also: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 See also:tungsten, of which it contains from about 5 to 8 %. It is termed self-hardening, because it is cooled in See also:air instead of being quenched in See also:water. Its value consists in its endurance at high temperatures, even at a low red heat. Until the See also: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 See also: 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 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. A, Float cut. B, Double cut. C, Rasp cut. F, Ditto, straight pane. G, Sledge hammer, straight pane. H, Ditto, double-faced. J, K, L, M, Boiler makers' hammers. N, Scaling hammer. tools adaptable to cutting various classes of metals and alloys. Tungsten is the principal controlling element, but See also:chromium is essential, and See also:molybdenum and See also:vanadium are often found of value. The steels are forged at a yellow tint, equal to about 185o° F. They are raised to a white heat for hardening, and cooled in an air blast to a See also:bright red. They are then often quenched in a See also:bath of oil. The first public demonstration of the capacities of high speed steels was made at the See also:Paris See also: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 See also:loo 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 See also:horse-power absorbed is proportionately large. Ordinary heavy lathes will take from 40 to 6o 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 See also: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. 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