Online Encyclopedia

MACHINE

Online Encyclopedia
Originally appearing in Volume V27, Page 33 of the 1911 Encyclopedia Britannica.
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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. 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. 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 pre-ceding paragraphs. XI. Appliances.—There is a very large number of articles which are neither tools nor machine tools, but which are in-dispensable 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. I.-LATHES1 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 $ in. in the bench lathes to 9 or to ft. in gun lathes, and weights will range from say 5o 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 for-ward, 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. 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 sup-ported, 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 head-stock, 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 " self-acting 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 Hendey-Norton 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 screw-thread, 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 alsothis is an economical system, but in others not. It cannot be considered so when bolts, screws and allied forms are of small dimensions. 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 head-stock 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 i 2 in. diameter, the hole in the mandrel would be made 18 in. Then the bar, inserted from the rear-end, is gripped 3y 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 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 alternatively 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 caseslathe. (Webster Bennett, Ltd., Coventry.) H, Vertical slide, carrying turret J. K, Screw feeding F across. L, Splined shaft connecting to H 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 wire 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 Turret Lathes.—The turret or capstan (fig. 32) is a device for grippsoduced. Out of the demands for this more complicated work, ping as many separate tools as there are distinct operations to be 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. 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 RlKLANMUt Wa nm. _FIG. 31.—Turret, Lathe. °... Webster & Bennett, Ltd., Coventry.) A, Bed. N, Bearing to feed the work through mandrel (constituting the B, Waste oil tray. wire or bar feed). A collar is clamped on the work, and is C, Headstock. pushed by the bearing N at each time of feeding. Hollow 0, Cross-slide. mandrel. D, P, Hand-wheel operating screw to travel O. E, Cones keyed to D. F, Split tapered close-in chuck, actuated by tube G. Q, Turret-slide. H, Toggle dogs which push G. R, Cross-handle moving Q to and fro. J, Coned collar acting on H. S, Turret or capstan. K, Handle to slide J through sleeve on bar L. T, U, Sets of fast and loose pulleys, for open and crossed belts. M, Rack slid on release of chuck, moving bearing N forward. V, Cone belted down to E on lathe. 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 perpendicularly 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 cross-slide, 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. C 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. 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 a, a, a, Cams for actuating chuck movements through pins b, b. The cam which re-turns 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 operating 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 n 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. 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 6o°, and the points may enter the work to depths ranging from as little as in. in very 116 light pieces to i in., a 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- D FIG. 34. A, Centring and driving ;a, point B, Face-plate driver or catch-centre; b, carrier; c, driver plate; a, centre; b, driver. fixed in slot in body of point C, Common heart-shaped carrier. centre; d, back centre; e, D, Clement doubledriver ; a, f ace- work. 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. A, Travelling steady with adjust- slotted 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 A, Main body. B, Waste oil tray. C, Headstock. • Wire-feed tube. • Slide for closing chuck. • Shaft for ditto. • Feed-slide. • Piece of work. • Turret with box tools. • Turret slide. • Sadule for ditto, adjustable along bed. Screw for locating adjustable slide. • Cut-off and forming cross-slide. • 0, Back and front tool-holders on slide. • Cam shaft. Cam drum chuck. • Cam drum turret. • Cam disk for actuating cross-slide. for operating for operating 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 ; in, to 1 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 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 face-plate 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 face-plate. In all these types the radial setting is tentative, that is, 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 back-ward 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 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 A, Body. b, Square heads of screws for a, Recess to receive face-plate. key, B, Jaws or dogs. c, Tee-grooves for bolts. C, Screws for operating jaws. A a a - ----- -------- C A, Face-plate screwed to man- E, Jaws in chuck face, having drel nose. sectional scroll teeth en- B, Back of chuck screwed to gaging with scroll a, and A. moved inwards or outwards C, Knurled chuck body with by the scroll when C is scroll a on face. turned. D, Chuck face. b, Tommy or lever hole in C. F, Piece of work outlined. Scroll Chuck. A, Back plate; a, recess for face-plate. 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. A, Back. B, Body. C, Spiral plate with teeth engaging in jaws D. E, Bevel pinions gearing with teeth on back of C. (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 con-centrically, 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 C1 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 V 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 up-right members to suit work of varying depths. These up-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 imposed by the stress of cutting. a 44 S -a 44 b 44 GO -~ R! U N a a 3 w nv 0 N o a • ss 'le SID ''5 n op x E- in a N _- ':N cv oa 3 u . X .0 v 64Coal / ro N o ° o P. ~. [° txO w 0.4 Zs, > b.O C3r °= chi', > buo 44 ae u cub 3s. ° ao v a d 008. Zm 7 +dam d • N m o o to [ v • w P.P. a i .--• ;. ~No 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. To such a design there are objections, which, though their importance has often been exaggerated, are yet real. First, the cross-rail 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, Bed. G, Tool-box on travelling arm H, travelled by fast and loose B, B, Feet. pulleys J for cutting, and by pulleys K for quick return. C, C, Work tables adjustable vertically on the faces D, D, by L, Feed-rod with adjustable dogs a, a, for effecting reversals through means of screws E, E, from handles F, F, through bevel the belt forks b, b. gears. M. Brickwork pit to receive deep objects. (G. Richards & Co., Ltd., Manchester.) FIG. 43.—20-in. Side Planing Machine. c B 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 angft. b, Crank handle spindle for operating ditto. c, Handle for actuating down feed of tool. J, 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. k, Feed disk driven by small gears from cone pulley. j, Pawl driven from disk through levers at various rates, and con-trolling the amount of rotation of the feed screw F. K, 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-powerthere 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 non-cutting 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 right-angled 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 wifh 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- 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. Slotting Machines.—In the slotting machine (fig. 45) the cutting takes place vertically and there is a lost return stroke. All the necessary movements save the simple reciprocating stroke are imparted to the compound table on which the work is carried. These include two linear movements at right angles with each other and a circular motion capable of making a complete circle. Frequently a tilting adjustment is included to permit of slotting at an angle. The slotting machine has the disadvantage of an arm unsupported beyond the guides in which it moves. But the compound movements of the table permit of the production of shapes which cannot be done on planers and shapers, as circular parts and circular arcs, in combination with straight portions. Narrow key grooves in the bores of wheels are also readily cut, the wheels lying on the horizontal table, which would only be possible on planer and shaper by the use of awkward angle brackets, and of specially projecting tools. Quick return in planers is accomplished by having two distinct sets of gearing— a slow set for cutting and a quick train for return, each operated from the same group of driving pulleys. The return travel is thus accomplished usually three, often four, times more quickly than the forward rate; sometimes even higher rates are arranged for. In the shaper and slotter such acceleration is not practicable, a rate of two to one being about the limit, and this is obtained not by gears, but by the slotted crank, the Whitworth return, on shapers and slotters, or by elliptical toothed wheels on slotters. The small machines are generally unprovided with this acceleration. The double-cutting device 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 18o° 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 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. 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 3o 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 determinethe 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 worm-shaft 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 pieces of work being introduced, resting on the floor or on blocking. Wall Machines.—One group of these machines resembles the bench machines in general design, but they are made to bolt to a wall instead of on a bench. Their value lies in the facilities which they afford for drilling large pieces of work lying on the floor o on blocking, which could not go on the tables of the bench machines. Some-times a compound work-table is fastened to the floor beneath; and several machines also are ranged in line, by means of which long plates, angles, boilers or castings may be brought under the simultaneous action of the group of machines. Another type is the radial arm machine, 'with or without a table beneath. In each case 'FIG. 46.—Pillar Radial Drilling Machine, 5 ft. radius. Base-plate. Pillar. • Radial arm. • Spindle carriage. • Drill spindle. • Main driving cones driving through mitre-gears H. • Spur-wheels, driving from C to vertical shaft K. • Mitre-wheels, driving from K to horizontal shaft M, having its bearings in the radial arm. • Nest of mitre-wheels driving the wheel spindle E from M. • Feed-gears to drill spindle, actuated by wheel P or worm-gears Q. hand- G vertical shaft an advantage gained is that a supporting pillar or standard is not required, its place being taken by the wall. Self-contained Pillar Machines include a large number having the above-named feature in common. In the older and less valuable types the framework is rigid, and the driving and feeding are by belt cones. But the machines being mostly of larger capacities than those just noted, back-gears similar to those of lathes are generally introduced. The spindles also are usually counterbalanced. The machine framing is bolted to a bed-plate. A circular work-table may or may not be included. When it is, provision is made for elevating the table by gears, and also for swinging it aside when deep work has to be put on the base-plate. Radial Arm Machines.—In these (fig. 46) the drilling mechanism is carried on a radial arm which is pivoted to the pillar with the object of moving the drill over the work, when the latter is too massive to permit of convenient adjustment under the drill. The driving takes place through shafts at right angles, from a horizontal shaft carrying the cones and back-geared to a vertical one, thence to a horizontal one along the radial arm, whence the vertical drilling A, Bed. B, B, Legs. C, Upright. D, Spindle or arbor. E, Headstock, carrying bearings for spindle D. F, Tailstock, carrying point centre for tail end of spindle. G, Hand-wheel for effecting adjustment in height of headstock, through bevel-gears H and screw J. K, Cross-bar connecting head- and tail-stocks, and ensuring equal vertical adjustment of the spindle bearings from the screw J. spindle is driven. The latter has its bearings in a carriage which can be traversed along the arm for adjustment of radius. The spindle is counterbalanced. Hand as well as power adjustments are included. In the work-tables of radial and rigid machines there is a great diversity, so that work can be set on top, or at the sides, or at an angle, or on compound tables, so covering all the requirements of practice. Sensitive Machines have developed greatly and have superseded many of the older, slower designs. The occasion for their use lies in the drilling of small holes, ranging up to about an inch in diameter. They are belt-driven, without back-gears, and usually without bevel-gears to change the direction of motion. The feed is by lever moving a rack sleeve. A slender pillar with a foot supports the entire mechanism, and the work-table, with a range of vertical adjustment. Multiple Spindle Machines.—Many of the sensitive machines are fitted with two, three or more spindles operated in unison with a belt common to all. In other machines the multiple spindles are capable of adjustment for centres, as in the machines used by boilermakers and platers. In others the spindles are adjustable in circles of varying radii, as in those employed for drilling the bolt holes in pipe flanges. In many of these the spindles are horizontal. Some very special multiple-spindle machines have the spindles at different angles, horizontal and vertical, or at angles. Universal Machines are a particular form of the pillar type in which the spindle is horizontal, moving with its carriage on a pillar capable of traversing horizontally along a bed; the carriage has vertical adjustment on its pillar and so commands the whole of the face of a large piece of work bolted to a low bed-plate adjacent to the machine. The term " universal " signifies that the machine combines provision for drilling, boring, tapping screws and inserting screw studs, facing and in some cases milling. The power required for boring is obtained by double and treble gears. These machines are used largely in marine engine works, where very massive castings and forgings must be operated on with their faces set vertically. Boring Machines.—Many machines are classified as suitable for drilling and boring. That simply means that provision is made on (John Holroyd & Co., Ltd., Milnrow.) L, Speed cones for driving spindle, through pinion M and wheel N. 0, Frame, carrying the bearings for the cone pulley L, and pivoted to the bed at a, and to the headstock E. This device keeps the gears M and N in engagement in all variations in the height of the spindle D. P, Q, Cones for driving the table R through worm-gears S, T, and spurs U, V, to the table screw. TV, Stop for automatic knock-off to feed. X, Hand-wheel for turning the same screw through worm-gears Y, Z. a drilling machine for boring holes of moderate size, say up to 8 or to in., by double and treble back-gears. But the real boring machine is of a different type. In the horizontal machines a splined bar actuated by suitable gears carries a boring head which holds the cutters, which head is both rotated with, and traversed or fed along the bar. The work to be bored is fixed on a table which has pro-vision for vertical adjustment to suit work of different dimensions. .The boring-bar is supported at both ends. In the case of the largest work the boring-bar is preferably set with its axis vertically, and the framing of the machine is arch-like. The bar is carried in a bearing at the crown of the arch and driven and fed there by suit-able gears, while the other end of the bar rotates in the table which forms the base of the machine. Some boring machines for small engine cylinders and pump barrels have no bar proper, but a long boring spindle carrying cutters at the further end is supported along its entire length in a long stiff boss projecting from the headstock of the machine—the snout machine. The work is bolted on a carriage which slides along a bed similar to a lathe bed. Many of these machines have two bars for boring two cylinders simultaneously. IV.—MILLING MACHINES In milling machines rotary saw-like cutters are employed. To a certain extent these and some gear-cutting machines overlap because they have points in common. Many gear-wheel teeth are produced by rotary cutters on milling machines. In many machines designed for gear cutting only, rotary cutters alone are used. For this reason the two classes of machines are conveniently and naturally grouped together, notwithstanding that a large and increasing group of gear-cutting machines operate with reciprocating tools. The French engineer, Jacques de Vaucanson (1709-1782), is credited with having made the first milling cutter. The first very crude milling machine was made in 1818 at a gun factory in Connecticut. To-day the practice of milling ranks as of equal economic value with that of any other department of the machine shop, and the varieties of milling machines made are as highly differentiated as are those of any other group. An apparent incongruity which is rather striking is the relative disproportion between the mass of these machines and the small dimensions of the cutters. The failures of many of the early machines were largely due to a lack of appreciation of the intensity of the stresses involved in milling. A single-edged cutting tool has generally a very narrow edge in operation. Milling cutters are as a rule very wide by comparison, and several teeth in deep cuts are often in simultaneous operation. The result is that the machine spindle and the arbor or tool mandrel are subjected to severe stress, the cutter tends to spring away from the surface being cut, and if the framings are of light proportions they vibrate, and in-accuracy and chatter result. Even with the very stiff machines now made it is not possible to produce such accurate results on wide surfaces as with the planer using a narrow-edged tool. Because of this great resistance and stress, cutters of over about an inch in width are always made with the teeth arranged spirally, and wide cutters which are intended for roughing down to compete with the planer always have either inserted cutters or staggered teeth. Hence the rotary cutter type of machine has not been able to displace the planing machine in wide work when great accuracy is essential. Its place lies in other spheres, in some of which its position is unassailable. Nearly all pieces of small and medium dimensions are machined as well by milling as by single-edged tools. All pieces which have more than one face to be operated on are done better in the milling machine than elsewhere. All pieces which have profiled outlines involving combinations of curves and plane faces can generally only be produced economically by milling. Nearly all work that involves equal divisions, or pitchings, as in the manufacture of the cutters themselves, or spiral cutting, or the teeth of gear-wheels when produced by rotary cutters, must be done in milling machines. Beyond these a large quantity of work lies on the border-line, where the choice between milling and planing, shaping, slotting, &c., is a matter for individual judgment and experience. It is a matter for some surprise that round the little milling cutter so many designs of machines have been built, varying from each other in the position of the tool spindles, in their number, and in the means adopted for actuating them and the tables which carry the work. A very early type of milling machine, which remains extremely popular, was the Lincoln. It was designed, as were all the early machines, for the small arms factories in the United States. The necessity for all the similar parts of pistols and rifles being inter-changeable, has had the paramount influence in the development of the milling machine. In the Lincoln machine as now made (fig. 47) the work is attached to a table, or to a vice on the table, which has horizontal and cross traverse movements on a bed, but no capacity for vertical adjustment. The cutter is held and rotated on an arbor driven from a adjustment. pulley, and supported on a tail- stock centre at the other end, with capacity for a good range of vertical adjustment. This is necessary both to admit pieces of work of different depths or thicknesses between the table and the cutter, and to regulate the depth of cutting (vertical feed). Around this general design numerous machines small and large, with many variations in detail, are built. But the essential feature is the vertical movement of the spindle and cutter, the support of the arbor (cutter spindle) at both ends, and the rigidity afforded by the bed which supports head- and tail-stock and table. The pillar and knee machines form another group which divides favour about equally with the Lincoln, the design being nearly of an opposite character. The vertical movements for setting and feed are imparted to the work, which in this case is carried on a bracket or knee that slides on the face of the pillar which supports the headstock. Travelling and transverse movements are imparted to the table slides. The cutter arbor may or may not be supported away from the headstock by an arched overhanging arm. None of these machines is of large dimensions. They are made in two leading designs—the plain and the universal. The first embodies rectangular relations only, the second is a marvellous instrument both in its range of movements and fine degree of precision. The first machine of this kind was exhibited at Paris in 1867. The design permits the cutting of spiral grooves, the angle of which is embodied In the adjustment of a swivelling table and of a headstock thereon (universal or spiral head). The latter embodies change-gears likea screw-cutting lathe and worm-gear for turning the head, in combination with an index or dividing plate having several circles of holes, which by the insertion of an index peg permit of the work spindle being locked during a cut. The combinations possible with the division plate and worm-gear number hundreds. The head also has angular adjustments in the vertical direction, so that tapered work can be done as well as parallel. The result is that there is nothing in the range of spiral or parallel milling, or tapered work or spur or bevel-gear cutting, or cutter making, that cannot be done on this type of machine, and the accuracy of the results of equal divisions of pitch and angle of spiral do not depend on the human element, but are embodied in the mechanism. A, Main framing. B, Knee. C, Spindle, having its vertical position capable of adjustment by the sliding of D on A. E, Driving cone, belt driving over guide pulleys F to spindle pulley G. H, Enclosed gears for driving spindle by back gear. J, Hand-wheel for adjusting spindle vertically. K, K, Pulleys over which spindle is counterbalanced. L, Feed pulley, driven from counter shaft. M, Vertical feed shaft, driven from L through mitre-gears. N, Change gear box. 0, Horizontal feed shaft, operating longitudinal and transverse feed of table through spiral and spur-gears. P, P, Handles for operating changes in feed speeds, nine in number. Q, Handle for reversing direction of motion of table R. S, Hand-wheel for longitudinal movement of table. T, Hand-wheel for effecting cross adjustments. V, Spiral gears indicated for effecting self-acting rotation of circular table W. X, Hand-wheel for rotation of table. Y, Hand-wheel for vertical movements of knee B on screw Z. Machines with vertical spindles (fig. 48) form another great group, the general construction of which resembles that either of the common drilling machine or of the slotting machine. In many cases the horizontal position is preferable for tooling, in others the vertical, but often the matter is indifferent. For general purposes, the heavier class of work excepted, the vertical is more convenient. But apart from the fitting of a special brace to the lower end of the spindle which carries the cutter, the spindle is unsupported there and is thus liable to spring. But a brace can only be used with a milling cutter that operates by its edges, while one advantage of the vertical spindle machine is that it permits of the use of end or face cutters. One of the greatest advantages incidental to the vertical position of the spindle is that it permits of profile milling being done. One of the most tedious operations in the machine shop is the production of outlines which are not those of the regular geometric figures, as rectangles and circles, or combinations of the same. There is only one way in which irregular forms can be produced cheaply and interchangeably, and that is by controlling the movements of the tool with an object of similar shape termed a " form" or " former," as in the well-known copying lathes, in the cam grinding machine, and in the forming adjuncts fitted to vertical spindle milling machines, so converting those into profiling machines. The principle and its application are alike simple. An object (the form) is made in hardened steel, having the same outlines as the object to be milled, and the slide which carries the cutter spindle has a hardened former pin or roller, which is pulled hard against the edges of the form by a suspended weight, so causing the tool to move and cut in the same path and in the same plane around the edges of the work. Here the milling machine holds a paramount place. No matter how many curves and straight portions may be combined in a piece, the machine reproduces them all faultlessly, and a hundred or a thousand others all precisely alike without any tentative corrections. Plano-millers, also termed slabbing machines, form a group that grows in value and in mass and capacity. They are a comparatively late development, becoming the chief rivals to the planing machines, for all the early milling was of a very light character. In general outlines the piano-millers closely resemble the planing machines, having bed, table, housings and cross-rail. The latter in the piano-miller carries the bearings for the cutter spindle or spindles under which the work travels and reciprocates. These spindles are vertical, but in some machines horizontal ones are fitted also, as in planers, so that three faces at right or other angles can be operated on simultaneously. The slabbing operations of the piano-millers do not indicate the full or even the principal utilities of these machines. To understand these it must be remembered that the cross-sections of very many parts which have to be tooled do not lie in single planes merely, but in combinations of plane surfaces, horizontal, vertical or angular. In working these on the planing machine separate settings of tools are required, and often successive settings. But milling cutters are built up in " gangs " to deal with such cases, and in this way the entire width of profile is milled at once. Horizontal faces, and vertical and angular edges and grooves, are tooled simultaneously, with much economy in time, and the cutter profile will be accurately reproduced on numbers of separate pieces. Allied to the piano-millers are the rotary planers. They derive their name from the design of the cutters. An iron disk is pierced with holes for the insertion of a large number of separate cutters, which by the rotation of the disk produce plane surfaces. These are milling cutters, though the tools are single-edged ones, hence termed " inserted tooth mills." These are used on other machines besides the rotary planers, but the latter are massive machines built on the planer model, with but one housing or upright to carry the carriage of the cutter spindle. These machines, varied considerably in design, do good service on a class of work in which a very high degree of accuracy is not essential, as column flanges, ends of girders, feet of castings, and such like. V.—GEAR-CUTTING MACHINES The practice of cutting the teeth of gear-wheels has grown but slowly. In the gears used by engineers, those of large dimensions are numerous, and the cost of cutting these is often prohibitive, though it is unnecessary in numbers of mechanisms for which cast wheels are as suitable as the more accurately cut ones. The smallest gears for machines of precision have long been produced by cutting, but of late years the practice has been extending to include those of medium and large dimensions, a movement which has been largely favoured by the growth of electric driving, the high speeds of which make great demands on reduction and trans-mission gears. Several new types of gear-cutting machines have been designed, and specialization is still growing, until the older machines, which would, after a fashion, cut all forms of gears, are being ousted from modern establishments. The teeth of gear-wheels are produced either by rotary milling cutters or by single-edged tools (fig. 49). The advantage of the first is that the cutter used has the same sectional form as the inter-tooth space, so that the act of tooth cutting imparts the shapes without assistance from external mechanism. But this holds good only in regard to spur-wheel teeth, that is, those in which the teeth lie parallel with the axis of the wheel. The teeth of bevel-wheels, though often produced by rotary cutters, can never be formed absolutely correctly, simply because a cutter of unalterable section is employed to form the shapes which are constantly changing in dimensions along the length of the teeth (the bevel-wheel being a frustum of a cone). Hence, though fair working teeth are obtained in this way, they result from the practice of varying the relative angles of the cutters and wheel and removing the material in several successive operations or traverses, often followed by a little correction with the file. Although this practice is still commonly followed in bevel-wheels of small dimensions, and was at one time the only method available, the practice has been changing in favour of shaping the teeth by a process of planing with a single-edged reciprocating tool. As, however, such a tool embodies no formative section as do the milling cutters, either it or the wheel blank, or both, have to be coerced and controlled by mechanism outside the tool itself. Around this method a number of very ingenious
End of Article: MACHINE
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