Fig. 465 represents an application of keys to a square shaft that has not been planed true. The wheel is hung upon the shaft and four temporary gib-headed keys are inserted in the spaces a, a, a, a, in Fig. 465. (It may be mentioned here that similar heads are generally forged upon keys to facilitate their withdrawal while fitting them to their seats, the heads being cut off after the key is finally driven home.) These sustain the wheel while the permanent keys, eight in number, as shown in the figure at b, b, b, b, b, b, b, b, are fitted, the wheel being rotated and tested for truth from a fixed point, the fitting of the keys being made subservient to making the wheel run true.
The proportions of sunk keys are thus given by the Manchester (England) rule. The key is square in cross section and its width or depth is obtained by subtracting 1⁄2 from the diameter of the shaft and dividing the sum thus obtained by 8, and then adding to the subtrahend 1⁄4.
Example.—A shaft is 6 inches in diameter, what should be the cross section dimensions of its key diameter of shaft?
6 - 1⁄2 = 51⁄2, 51⁄2 ÷ 8 = .687, and .687 + .25 = 937⁄1000 inch.
In general practice, however, the width of a key is made slightly greater than its depth, and one-half its depth should be sunk in the shaft.
Taper keys are tapered on their surfaces a and b in Fig. 466, and are usually given 1⁄8-inch taper per foot of length. There is a tendency either in a key or a set screw to force the hub out of true in the direction of the arrow. It therefore causes the hub bore to grip the shaft, and this gives a driving duty more efficient than the friction of the key itself. But the sides also of the key being a sliding fit they perform driving duty in the same manner as a feather which fits on the sides a, d in Fig. 467, but are clear either top or bottom. In the figure the feather is supposed to be fast in the hub and therefore free at c, but were it fast in the shaft it would be free on the top face.
Fig. 468 represents a shaft held by a single set screw, the strain being in the direction of the arrow, hence the driving duty is performed by the end of the set screw and the opposite half circumference of the bore and shaft. On account, however, of the small area of surface of the set screw point the metal of the shaft is apt, under heavy duty and when the direction of shaft rotation is periodically reversed, to compress (as will also the set screw point unless it is of steel and hardened), permitting the grip to become partly released no matter how tightly the set screw be screwed home. On this account a taper key will under a given amount of strain upon the hub perform more driving duty, because the increased area of contact prevents compression. Furthermore, the taper key will not become loose even though it suffer an equal amount of compression. Suppose, for example, that a key be driven lightly to a fair seating, then all the rest of the distance to which the key is driven home causes the hub to stretch as it were, and even though the metal of the key were to compress, the elasticity thus induced would take up the compression, preventing the key from coming loose. It is obvious, then, that set screws are suitable for light duty only, and keys for either heavy or light duty. It is advanced by some authorities that keys are more apt to cause a wheel or pulley to run out of true than a set screw, but such is not the case, because, as shown in Figs. 466 and 468, both of them tend to throw the wheel out of true in one direction; but a key may be made with proper fitting to cause a wheel to run true that would not run true if held by a set screw, as is explained in the directions for fitting keys given in examples in vice work.
If two set screws be used they should both be in the same line (parallel to the shaft axis) or else at a right angle one to the other as in Fig. 469, so that the shaft and bore may drive by frictional contact on the side opposite to the screws. Theoretically the contact of their surface will be at a point only, but on account of the elasticity of the metal the contact will spread around the bore in the arc of a circle, the length of the arc depending upon the closeness of fit between the pulley bore and the shaft. If the bore is a close fit to the shaft it is by reason of the elasticity of the metal relieved of contact pressure on the side on which the set screw or key is to an amount depending upon the closeness of the bore fit, but this will not in a bore or driving fit to the shaft be sufficient to set the wheel out of true.
If two set screws are placed diametrally opposite they will drive by the contact of their ends only, and not by reason of their inducing frictional contact between the bore and the shaft.
A very true method of securing a hub to a shaft is to bore it larger than the shaft and to a taper of one inch to the foot. A bushing is then bored to fit the shaft and turned to the same taper as the hub is turned, but left, say, 1⁄100 inch larger in diameter and 1⁄4 or 3⁄8 longer. The bush is then cut into three pieces and these pieces are driven in the same as keys, but care must be taken to drive them equally to keep the hub true.
Feathers are used under the following conditions:—When the wheel driven by a shaft requires to slide along the shaft during its rotation, in which case the feather is fast in the wheel and the shaft is provided with a keyway or spline (as it is termed when the sliding action takes place), of the necessary length, the sides of the feather being a close but sliding fit in the spline while fixed fast in the wheel.
It is obvious that the feather might extend along the shaft to the requisite distance and the spline or keyway be made in the wheel: but in this case the work is greater, because the shaft would still require grooving to receive the feather, and the feather instead of being the simple width of the wheel would require to be the width of the wheel longer than the traverse of the wheel on the shaft. Nor would this method be any more durable, because the keyway’s bearing length would be equal to the width of the wheel only.
When a feather is used to enable the easy movement of a wheel from one position to another a set screw may be used to fix the wheel in position through the medium of the feather as is shown in Fig. 470.
Through keys and keyways are employed to lock two pieces, and sometimes to enable the taking up of the wear of the parts. Fig. 471 represents an example in which the key is used to lock a taper shaft end into a socket by means of a key passing through both of them. When the keyway is completely filled by the key as in the figure it is termed a solid key and keyway, indicating that there is no draft to the keyway. Fig. 472 represents a key and keyway having draft. One edge, a c, of the key binds against the socket edges only, and the other edge e binds against the edge b of the enveloped piece or plug, so that by driving in the key with a hammer the two parts are forced together. The space or distance between the edge d and the key, and between edges e and f, is termed the draft. The amount of this draft is made equal to the taper of the key, hence, when the key is driven in so that its head comes level with the socket or work surface, the draft will be all taken up and the key will fill the keyway.
Draft is given to ensure all the strain of the key forcing the parts together, to enable the key to be driven in to take up any wear and to adjust movable parts, as straps, journal boxes or brasses, &c. When the bore of the socket and the end of the rod are parallel, the end of the rod f, Fig. 473, should key firmly against the end e of the socket, while the end d of the socket should be clear of the shoulder on the rod; otherwise instead of the key merely compressing the metal at f it will exert a force tending to burst the end f from g of the rod, furthermore, the area of contact at the shoulder d being small the metal would be apt to compress and the key would soon come loose.
In some cases two keys are employed passing through a sleeve, the arrangement being termed a coupling, or a butt coupling.
The usual proportions for this class of key, when the rod ends and socket boxes are parallel, is width of key equals diameter of socket bore, thickness of key equals one-fourth its width, with a taper edgeways of about 1⁄4 inch in 10 inches of length.
As the keys in through keyways often require to be driven in very tight, and as the parts keyed together often remain a long time without being taken apart and in some situations become rusted together, it is often a difficult matter to get them apart. First, it is difficult to drive it out because the blows swell the end of the key so that it cannot pass through the keyway, and secondly, driving the socket off the plug of the two parts keyed together often damages the socket and may bend the rod to which it is keyed. Furthermore, as the diameter of the socket is usually not more than half as much again as the diameter of the plug, misdirected blows are apt to fall upon the rod instead of upon the socket end and damage it. Hence, a piece of copper, of lead, or a block of wood should always be placed against the socket end to receive the hammer blows. To force a plug out of a socket, we may use reverse keys. These are pieces formed as shown in Fig. 474. a, a and b, b are edge and face views respectively of two pieces of metal, formed as shown, which are inserted in the keyway as shown in Fig. 475, in which a is the plug or taper end of a rod and b the socket, c is one and d the other of the reverse keys, while e is a taper key inserted between them, b driving e through the keyway, a and b are forced apart. The action of the reverse keys is simply to reverse the direction of the draft in the keyway so that the pressure due to driving e through the keyway is brought to bear upon the rod end in the part that was previously the draft side of the keyway, and in like manner upon the keyway in the socket on the side that previously served as draft.
Reverse keys are especially serviceable to take off cross heads, piston heads, keyed crank-pins, and parts that are keyed very firmly together.
Hubs are sometimes fastened to their shafts by pins passing through both the hub and the shaft. These pieces may be made parallel or taper, but the latter obviously secures the most firmly. If the pin is located as in Fig. 476, its resisting strength is that due to its cross sectional area at a and b. But if the pin be located as in Fig. 477 it secures the hub more firmly, because it draws the bore (on the side opposite to the pin) against the shaft, causing a certain amount of friction, and, furthermore, the area resisting the pressure of the hub is increased, and that pressure is to a certain degree in a crushing as well as a shearing direction.
If unturned pins are used and the holes are rough or drilled but not reamed, it is better that two sides of the pin should be eased off with a file or on the emery wheel, so that all the locking pressure of the pin shall fall where it is the most important that it should—that is, where it performs locking duty. This is shown in Fig. 478, the hole being round and the pin being very slightly oval (not, of course, so much as shown in the drawing), so that it will bind at a b, and just escape touching at c, d, so that all the pressure of contact is in the direction to bind the hub to the shaft.
The lathe may be justly termed the most important of all metal-cutting machine tools. Not only on account of the rapidity of its execution which is due to its cutting continuously while many others cut intermittently, but also because of the great variety of the duty it will perform to advantage. In the general operations of the lathe, drilling, boring, reaming, and other processes corresponding to those performed by the drilling machine, are executed, while many operations usually performed by the planing machine, or planer as it is sometimes termed, may be so efficiently performed by the lathe that it sometimes becomes a matter of consideration whether the lathe or the planer is the best machine to use for the purpose.
The forms of cutting tools employed in the planer, drilling machine, shaping machine, and boring machine, are all to be found among lathe tools, while the work-holding devices employed on lathe work include, substantially, very nearly all those employed on all other machines and, in addition, a great many that are peculiar to itself. In former times, and in England even at the present day, an efficient turner (as a lathe operator is termed), or lathe hand, is deemed capable of skilfully operating a planer, boring machine, screw-cutting machine, drilling machine, or any of the ordinary machine tools, whereas those who have learned to operate any or all of those machine tools would prove altogether inefficient if put to operate a lathe.
In almost all the mechanic arts the lathe in some form or other is to be found, varying in weight from the jewellers’ lathe of a few pounds to the pulley or fly-wheel lathe of the engine builder, weighing many tons.
The lathe is the oldest of machine tools and exists in a greater variety of forms than any other machine tool. Fig. 479 represents a lathe of primitive construction actually in use at the present day, and concerning which the “Engineering” of London (England), says, “At the Vienna Exhibition there were exhibited wood, glasses, bottles, vases, &c., made by the Hucules, the remnant of an old Asiatic nation which had settled at the time of the general migration of nations in the remotest parts of Galicia, in the dense forests of the Carpathian Mountains. The lathe they are using has been employed by them from time immemorial. They make the cones b, b (of maple) serve as centres, one being fixed and the other movable (longitudinally). They rough out the work with a hatchet, making one end a cylindrical, to receive the rope for giving rotary motion. The cross-bar d is fastened to the trees so as to form a rest for the cutting tool, which consists of a chisel.” c, of course, is the treadle, the lathe or pole being a sapling.
In other forms of ancient lathes a wooden frame was made to receive the work-centres, and one of these centres was carried in a block capable of adjustment along the frame to suit different lengths of work. In place of a sapling a pole or lath was employed, and from this lath is probably derived the term lathe.
It is obvious, however, that with such a lathe no cutting operation can be performed while the work is rotating backwards, and further, that during the period of rest of the cutting tool it is liable to move and not meet the cut properly when the direction of work rotation is reversed and cutting recommences, hence the operation is crude in the extreme, being merely mentioned as a curiosity.
The various forms in which the lathe appears in ordinary machine shop manipulation may be classified as follows:—
The foot lathe, signifying that the lathe is driven by foot.
The hand lathe, denoting that the cutting tools must be held in the hands, there being no tool-carrying or feeding device on the lathe.
The single-geared lathe, signifying that it has no gear-wheels to reduce the speed of rotation of the live spindle from that of the cone.
The back-geared lathe, in which gear-wheels at the back of the headstock are employed to reduce the speed of the lathe.
The self-acting lathe, or engine lathe, implying that there is a slide rest actuated automatically to traverse the tool to its cut or feed.
The screw-cutting lathe, which is provided with a lead screw, by means of which other screws may be cut.
The screw-cutting lathe with independent feed, which denotes that the lathe has two feed motions, one for cutting threads and another for ordinary tool feeding; and
The chucking lathe, which implies that the lathe has a face plate of larger diameter than usual, and that the bed is somewhat short, so as to adapt it mainly to work held by being chucked, that is to say, held by other means than between the lathe centres.
There are other special applications of the lathe, as the boring lathe, the grinding lathe, the lathe for irregular forms, &c., &c.
This classification, however, merely indicates the nature of the lathe with reference to the individual feature indicated in the title; thus, although a foot lathe is one run by foot, yet it may be a single or double gear (back-geared) lathe, or a hand or self-acting lathe, with lead screw and independent feed motion.
Again, a hand lathe may have a hand slide rest, and in that case it may also be a back-geared lathe, and a back-geared lathe may have a hand slide rest or a self-acting feed motion or motions.
Fig. 480 represents a simple form of foot lathe. The office of the shears or bed is to support the headstock and tailstock or tailblock, and to hold them so that the axes of their respective spindles shall be in line in whatever position the tailstock may be placed along the bed. The duty of the headstock is to carry the live spindle, which is driven by the cone, the latter being connected by the belt to the wheel upon the crank shaft driven by the crank hook and the treadle, which are pivoted by eyes w to the rod x, the operation of the treadle motion being obvious. The work is shown to be carried between the live centre, which is fitted to the live spindle, and the dead centre fitting into the tail spindle, and as it has an arm at the end, it is shown to be driven by a pin fixed in the face plate, this being the simplest method of holding and driving work. The lathe is shown provided with a hand tool rest, and in this case the cutting tools are supported upon the top of the tool rest n, whose height may be adjusted to bring the tool edge to the required height on the work by operating the set screw s, which secures the stem of n in the bore of the rest.
To maintain the axes of the live and dead spindles in line, they are fitted to a slide or guideway on the shears, the headstock being fixed in position, while the tailstock is adjustable along the shears to suit the length of the work.
To lock the tailstock in its adjusted position along the shears, it has a bolt projecting down through the plate c, which bolt receives the hand nut d. To secure the hand rest in position at any point along the shears, it sets upon a plate a and receives a bolt whose head fits into a T-shaped groove, and which, after passing through the plate p receives the nut n, by which the rest is secured to the shears.
To adjust the end fit of the live spindle a bracket k receives an adjusting screw l, whose coned end has a seat in the end j of the live spindle, m being a check nut to secure l in its adjusted position.
The sizes of lathes are designated in three ways, as follows:— First by the swing of the lathe and the total length of the bed, the term swing meaning the largest diameter of work that the lathe is capable of revolving or swinging. The second is by the height of the centres (from the nearest corner of the bed) and the length of the shears. The height of the centres is obviously equal to half the swing of the lathe, hence, for example, a lathe of 28-inch swing is the same size as one of 14-inch centres. The third method is by the swing or height of centres and by the greatest length of work that can be held between the lathe centres, which is equal to the length of the bed less the lengths of the head and tailstock together.
The effective size of a lathe, however, may be measured in yet another way, because since the hand rest or slide rest, as the case may be, rests upon the shears or bed, therefore the full diameter of work that the lathe will swing on the face plate cannot be held between the centres on account of the height of the body of the hand rest or slide rest above the shears.
Fig. 481 shows a hand lathe by F. E. Reed, of Worcester, Massachusetts, the mechanism of the head and tail stock being shown by dotted lines. The live spindle is hollow, so that if the work is to be made from a piece of rod and held in any of the forms of chucks to be hereafter described, it may be passed through the spindle, which saves cutting the rod into short lengths. The front bearing of the headstock has two brasses or boxes, a and b, set together by a cap c.
The rear bearing has also a bearing box, the lower half d being threaded to receive an adjustment screw f and check nut g to adjust the end fit of the spindle in its bearings. In place of grooved steps for the belt the cone has flat ones to receive a flat belt.
The tail spindle is shown, in Fig. 482, to be operated by a screw h, having journal bearing at i, and threaded into a nut fast in the tail spindle at j. To hold the tail spindle firmly the end of the tail stock is split, and the hand screw k may be screwed up to close the split and cause the bore at l to clasp the tail spindle at that end.
To lock the tail stock to the shears the bolt m receives the lever n at one end and at the other passes through the plate or clamp o, and receives the nut p, so that the tail stock is gripped to or released from the shears by operating n in the necessary direction. The hand rest, Fig. 483, has a wheel w in place of a nut, which dispenses with the use of a wrench.
What are termed bench lathes are those having very short legs, so that they may for convenience be mounted on a bench or fastened to a second frame, as shown in Fig. 484.
It is obvious that when work is turned by hand tools, the parallelism of the work depends upon the amount of metal cut off at every part of its length, which to obtain work of straight outline, whether parallel or taper, involves a great deal of testing and considerable skill, and to obviate these disadvantages various methods of carrying and accurately guiding tools are employed. The simplest of these methods is by means of a slide rest, such as shown in Fig. 485.
The tool t is carried in the tool post p, being secured therein by the set screw shown, which at the same time locks the tool post to the upper slider. This upper slider fits closely to the cross slide, and has a nut projecting down into the slot shown in the same, and enveloping the cross feed screw, whose handle is shown at c, so that operating c traverses the upper slider on the cross slide and regulates the depth to which the tool enters the work, or in other words, the depth of cut.
The cross slide is formed on the top of the lower slider, which has beneath a nut for the feed screw, whose handle is shown at a, hence rotating a will cause the lower slider to traverse along the lower slide and carry the tool along the work to its cut. To maintain the fit of the sliders to the slides a slip of metal is inserted, as at e and at c, and these are set up by screws as at f, f and b, b.
The lower or feed traverse slide is pivoted to its base b, so that it may be swung horizontally upon the same, and is provided with means to secure it in its adjusted position, which is necessary to enable it to turn taper as well as parallel work. To set this lower slide to a given degree of angle it may be marked with a line and the edge of base b may be divided into degrees as shown at d.
When a piece of work is rotated between the lathe centres its axis of rotation may be represented by an imaginary straight line and the lower slides must, to obtain parallel work, be set parallel to this straight line, while for taper work the slide rest must be set at an angle to it. Now, in the form of slide rest shown in figure the cross slide is carried by the lower or feed traverse slide, hence setting the lower slide out of parallel with the work axis sets the cross slide out of a right angle to the work axis, with the result that when a taper piece of work is turned that has a collar or flange on it, the face of that collar or flange will be turned not at a right angle to the work axis as it should be, but at a right angle to the surface of the cone. Thus in Fig. 486 a represents the axis of a piece of work, and the slide nut having been set parallel to the work axis, the face c will be at a right angle to the surface b or axis a, but with the slide nut set at an angle to turn the cone d, the cross slide will be at an angle to a, hence the face e will be undercut as shown, and at a right angle to the surface d instead of to a a. This may be obviated by letting the cross slide be the lower one as in the English form of slide rest shown in Fig. 487, in which the upper slide is pivoted at its centre to the cross slide and may be swung at an angle thereto and secured in its adjusted position by the bolt at f. The projection at the bottom of the lower slider fits between the shears of the lathe and holds the lower slider parallel with the line of lathe centres, which causes the slide rest to cut all faces at a sight angle to the work axis whether the feed traverse slide be set to turn parallel or taper. In either case, however, there is nothing to serve as a guide to set the feed traverse slide parallel to the work axis, and this must, therefore, be done as near as may be by the eye and by taking a cut and testing its parallelism.
The rest may be set approximately true by bringing the operator’s eye into such a position that the edge a a, Fig. 488, of the slide rest come into line with the edge b b of the lathe shears, because that edge is parallel to the line of lathe centres, and therefore to the work axis.
Slide rests which have a slide for traversing the tool along the work to its cut are but little used in the United States, being confined to very small lathes, and then (except in the case of watchmakers’ lathes whose forms of slide rest will be shown hereafter), mainly as an expedient to save expense in the cost of the lathe, it being preferred to feed the tool for the feed traverse (as the motion of the cutting tool along the work is termed) by mechanism operated from the live spindle and to be hereafter described. In England, however, slide rests are much used, a specimen construction being shown in Fig. 489. The end face a of the rest comes flush so that the tool shall be carried firmly when taking facing cuts in which solidity in the rest is of most importance. The tool is held by two clamps instead of by single tool posts, because the slide rest is employed to take heavy cuts, and when this is the case with boring tools whose cutting edges stand far out from the slide rest, a single tool post will not hold the tool sufficiently firm.
The gib e, Fig. 485, is sometimes placed on the front side of the slider, as in the figure, and at others on the back; when it is placed in the front the strain of the cut causes it to be compressed against the slide, and there is a strain placed upon the screws f which lifts them up, whereas if placed on the other side the screws are relieved of strain, save such as is caused by the setting of the gib up.
On the other hand, the screws are easier to get at for adjustment if placed in front. When the screws b of the upper gib c, Fig. 485, are on the right-hand side, as in that figure, there is considerable strain on the screws when a boring tool is used to stand far out, as for boring deep holes. On the other hand, however, the screws can be readily got at in this position, and may therefore be screwed up tightly to lock the upper slider firmly to the cross slide, which will be a great advantage in boring and also in facing operations. But the screws must not in this case have simple saw slot heads, such as shown on a larger scale in Fig. 490, but should have square heads to receive a wrench, and if these four screws are used, the two end ones may be set to adjust the slicing fit of the slider, while the two middle ones may be used to set the slider form on its slide when either facing or boring. The corners of the gibs as well as those of the slider and slide may with advantage be rounded so that they may not become bruised or burred, and, furthermore, the slider is strengthened, and hence less liable to spring under the pressure of a heavy cut.
A slide rest for turning spherical work is shown in Fig. 491. a is the lower slide way on which is traversed the slide b, upon which is fitted the piece c, pivoted by the bolt d; there is provided upon c a half-circle rack, shown at e, and into this rack gears a worm-wheel having journal bearing on b, and operated by the handle f. As f is rotated c would rotate on d as a centre of motion, hence the tool point would move in an arc of a circle whose radius would depend upon the distance of the tool point from d as denoted by j, which should be coincident with the line of centres of the lathe.
The slide g is constructed in the ordinary manner, but the way on which it slides should be short, so as not to come into contact with the work. If the base slide way a be capable of being traversed along the lathe shears s s by a separate motion, then the upper slide way and slide may be omitted, g and c being in one piece. It is to be noted in a rest of this kind, however, that the tool must be for the roughing cut set too far from d to an amount equal to about the depth of cut allowed to finish with, and for the finishing cut to the radius of the finished sphere in order to obtain a true sphere, because if b be operated so that d does not stand directly coincident with the line of lathe centres, the centre of motion, or of the circle described by the tool point, will not be coincident with the centre on which the work rotates, hence the work though running true would not be a true sphere but an oval. This oval would be longest in the direction parallel with the line of centres whenever the pivot d was past the line of centres, and an oval of largest diameter at the middle or largest diameter turned by the tool whenever the pivot d was on the handle h side of the line of centres. To steady c it may be provided with a circular dovetail, as shown at the end i, provision being made (by set screw or otherwise) for locking c in a fixed position when using the rest for other than spherical work.
To construct such a rest for turning curves or hollows whose outline required to be an arc of a circle, the pivot d would require to be directly beneath the tool post, which must in this case occupy a fixed position. The radius of the arc would here again be determined by the distance of the tool point from the centre of rotation of the pivot, or, what would be the same thing, from that of the tool post.
Next to the hand slide rest lathe comes the self-acting or engine lathe. These are usually provided with a feed motion for traversing the slide rest in the direction of the length of the bed, and sometimes with a self-acting cross feed, that is to say, a feed motion that will traverse the tool to or from the line of centres and at a right angle to the same.
In an engine lathe the parallelism or truth of the work depends upon the parallelism of the line of centres with the shears of the lathe, and therefore upon the truth of the shears or bed, and its alignment with the cone spindle and tail spindle, while the truth of the radial faces on the turned work depends upon the tool rest moving on the cross slide at a true right angle to the line of centres.
Fig. 492 represents an 18-inch engine (or self-acting) lathe designed by and containing the patented improvements of S. W. Putnam, of the Putnam Tool Company, of Fitchburg, Massachusetts. The lathe has an elevating slide rest self-acting feed traverse and self-acting cross feed, both feeds being operative in either direction. It has also a feed rod for the ordinary tool feeding and a lead screw for screw-cutting purposes.
Fig. 493 represents a cross-sectional view of the shears beneath the headstock; a a are the shears or bed having the raised Vs marked v′ and v on which the headstock and tailstock rest, and v′′ and v′′′ on which the carriage slides. a and a′ are the shears connected at intervals by cross girts or webs b to stiffen them. c c are the bolts to secure the headstock to the shears. d is a bracket bolted to a′ and affording at e journal bearing for the spindle that operates the independent feed spindle. e is split at f and a piece of soft wood or similar compressible material is inserted in the split. The bolt f is operated to close the split, and, therefore, to adjust the bore e to properly fit the journal of the feed spindle, and as similar means are provided in various parts of the lathe to adjust the fits of journals and bearings the advantages of the system may here be pointed out. First, then, the fit of the bearing may be adjusted by simply operating the screw, and, therefore, without either disconnecting the parts or performing any fitting operation, as by filing. Secondly, the presence of the wood prevents the ingress of dust, &c., which would cause the bearings and journals to abrade; and, thirdly, the compression of the wood causes a resistance and pressure on the adjusting screw thread, which pressure serves to lock it and prevent it from loosening back of itself, as such screws are otherwise apt to do.
As the pressure of the tool cut falls mainly on the front side of the carriage, and as the weight of the carriage itself is greatest on that side, the wear is greatest; this is counteracted by forming the front V, marked v′′′ in figure, at a less acute angle, which gives it more wearing area and causes the rest to lower less under a given amount of wear.
The rib a′′ which is introduced to strengthen the shears against torsional strains, extends the full length of the shears.