Chapter VII: Details in Lathe Construction (2)

Another plan is to let the back gear move endwise and bush its bearing holes with hardened steel bushes. This possesses the advantage that the gear is sure, if made right, to keep so, but it has some decided disadvantages: first, the pinion A, Fig. 563, must be enough larger than the smallest cone-step B to give room between B and C for the belt, and this necessitates that D also be larger than otherwise; secondly, the gear-spindle F projects through the bearing at _f_, and this often comes in the way of the bolt-heads used for chucking work to the face plate. The method of securing the spindle from end motion is as follows: On the back of the head is pivoted at _i_, a catch G, and on the gear shaft F are two grooves. As shown in the sketch, G is in one of these grooves while H is the other, but when the back gear is in, G would be in H.

Sometimes a simple eccentric bush and pin is used as in Fig. 564, in which _a_ is the spindle journal, _b_ a bush having bearing in the lathe head, and _d_ a taper pin to secure _b_ in its adjusted position.

In large heavy lathes having many changes of speed, there are various other constructions, as will be seen upon the lathes themselves in the various illustrations concerning the methods of throwing the back gear in and out. The eccentric motion shown in Fig. 573 of the Putnam lathe, is far preferable to any means in which the back-gear spindle moves endways, because, as before stated, the end of the back-gear spindle often comes in the way of the bolts used to fasten work to the large face plate. This occurs mainly in chucked work of the largest diameter within the capacity of the lathe.

In many American lathes the construction of the gearing that conveys motion from the live spindle is such that facility is afforded to throw the change gears out of action when the lathe is running fast, as for polishing purposes, so as to save the teeth from wear. Means are also provided to reverse the direction of lead screw or feed screw revolution. An example of a common construction of this kind is shown in Fig. 565, in which the driving wheel A is on the inner side of the back bearing as shown. It drives (when in gear) a pair of gears, one only of which is seen in the figure at B, which drives C, and through R, D, I, and S, the lead screw. A side view of the wheel A and the mechanism in connection therewith is shown in Fig. 566, in which S represents the live spindle and R is a spindle or shaft corresponding to R in Fig. 565. L is a lever pivoted upon R and carrying two pinions B and E; pinion B is of larger diameter than E, so that B gears with both C and E (C corresponding to wheel C in Fig. 565), while E gears with B only.

With the lever L in the position shown, neither B nor E engages with A, hence they are at rest; but if lever L be raised as in Fig. 567, B will gear with wheels A and C, and motion will be conveyed from A to C, wheel E running as an idle wheel, thus C will revolve in the same direction as the lathe spindle.

But if lever L be lowered as in fig. 568, then wheel E will gear with and receive motion from A, which it will convey to B, and C will revolve in the opposite direction to that in which the lathe spindle runs. To secure lever L in position, a pin F passes through it and into holes as I, J, provided in the lathe head. Lever L is sometimes placed inside the head, and sometimes outside as in Fig. 569, and it will be obvious that it may be used to cut left-hand threads without the use of an extra intermediate change gear, which is necessary in the construction shown in Fig. 570, in order to reverse the direction of lead screw revolution.

Sometimes the pin F is operated by a small spring lever attached to L, so that the hand grasps the end of L and the spring lever simultaneously, removing F from the hole in H, and therefore freeing L, so as to permit its operation. By relaxing the pressure on the small spring lever pin F finds its own way into the necessary hole in H, when opposite to it, without requiring any hand manipulation.

In larger lathes the lever L is generally attached to its stud outside the end bearing of the head H.

It is preferable, however, that the device for changing the direction of feed traverse be operative from the lathe carriage as in the Sellers lathe, so that the operator need not leave it when it is necessary to reverse the direction of traverse.

The swing frame, when the driving gear D is outside of the back bearing (as it is in Fig. 570), is swung from the axis of the lead screw as a centre of motion, and has two slots for receiving studs for change wheels. But when the driving gear is inside the back bearing as in Fig. 571, the swing frame may be suspended from the spindle (R, Fig. 565) that passes through the lathe head, which may also carry the cone for the independent feed as shown in Fig. 571, no matter on which side of the lathe the lead screw and feed rod are. This affords the convenience that when both lead screw and feed rod are in front of the lathe, the feed may be changed from the screw cutting to the rod feed, or _vice versâ_, by suitable mechanism in the apron, without requiring any change to be made in the driving gears.

In the lathe shown in Fig. 572, which is from the design of S. W. Putnam, of the Putnam Tool Company of Fitchburg, Massachusetts, the cone pinion for the back gear, and that for driving the feed motion, are of the same diameter and pitch, so that the gear-wheel L in Fig. 573 may (by means of a lever shown dotted in) be caused to engage with either of them. When the latter is used in single gear it would obviously make no difference which wheel drives L, but when the back gear is put in and L is engaged with the cone pinion, its speed corresponds to that of the cone, which being nine times faster than the live spindle, enables the cutting of threads nine times as coarse as if the back gear was not in use. This affords very great advantages for cutting worms and threads of coarse pitches.

An excellent method of changing the direction of feed motion, and of starting or stopping the same, is shown in Fig. 574, which represents the design of the Ames Manufacturing Company's lathe.

In the figure, A is the small step of the lathe cone, B the pinion to drive the back gear, C a pinion to drive the feed gear, giving motion to D, which drives E, the latter being fast to G and rotating freely upon the shaft F, G drives H, which in turn drives I. The clutch J has a featherway into which fits the feather _c_, on the shaft F, so that when the clutch rotates it rotates J through the medium of _c_; K is a circular fork in a groove in J, and operated by a lever operated by a rod running along the front of the lathe bed. This rod is splined so that a lever carried by the apron or feed-table, having a hub and enveloping the rod, may by means of a feather filling into the spline operate the rod by partly rotating it, and hence operate K. Suppose now that this lever stands horizontal, then the clutch J would stand in the position shown in the cut, and D, E, G, H, and I, would rotate, while F would remain stationary. By lifting the lever, however, J would be moved laterally on F (by means of K) and the lug _a_ on J would engage with lug _b_ on G, and G would drive J, which through _c_ would drive F, on which is placed a change gear at L, thus traversing the carriage forward. To traverse it backward the lever would be lowered or depressed below the horizontal level moving K, and therefore J, to the right, so that lug _a_ would engage with lug _b_ on I, hence F would be driven by I, whose motion is in an opposite direction to G, as is denoted by the respective arrows.

To throw all the feed motion out of gear, to run the lathe at its quickest for polishing, &c., the operation is as follows.

M is tubular and fast in N and affords journal bearing to wheel D. Through M passes stud O, having a knob handle at P. At the end of the hub of D is a cap fast in D, the latter being held endways between the shoulder shown on O and the washer and nut T. If then P be pulled outwards O will slide through M, and through the medium of T will cause D to slide over M, in the direction of the arrow, and pass out of gear from C, motion therefore ceasing at C.

Q is the swing frame for the studs to carry the change wheels, and R a bolt for securing Q in its adjusted position. S is a journal and bearing for H.

If it be considered sufficient the feed motion on small lathes (instead of feeding in both directions on the lateral and cross feeds as in the Putnam Lathe), may feed in the direction from the dead to the live centre, and in one direction only on the cross slide.

An example of a feed motion of this kind is shown in Figs. 575 and 576; _f_ _f_ is the feed spindle splined and through the medium of a feather driving the bevel pinion A having journal bearing in B. Pinion A drives the bevel gear C, which is in one piece with pinion D. The latter drives gear F, which drives pinion K, which is carried on a lever L, pivoted on the stud which carries F, so that by operating L, pinion K is brought into gear with pinion P, which is fast upon the cross-feed screw, and therefore rotates it to effect the automatic cross feed.

As shown in the cut, the lever L is in such a position as to throw K out of gear with P, and the cross feed screw is free to be operated by the handle by hand. At M is a slot in L in which operates a cam or eccentric, one end of which projects into L, while at the other end is the round handle R, Fig. 575, which is rotated to raise or lower that end of L so as to operate K. To operate the saddle or carriage the motion is continued as follows:--at the centre of F is a pinion gear G which operates a gear H, which is in one piece with the pinion I, and the latter is in gear with the rack running along the lathe bed.

If the motion from A to I was continuous, the carriage feed or traverse would be continuous, but means are provided whereby motion from F to I may be discontinued, as follows:--A hand traverse or feed is provided. J, Figs. 575 and 576, is carried by a stud having journal bearing in a hub on X and receiving the handle Q; hence by operating Q, J is rotated, operating the gear H, upon which is the pinion I, which is in gear with the rack running along the lathe bed.

To lock the carriage in a fixed position, as is necessary when operating the cross feed on large radial surfaces, the following device is provided:--N is a stud fixed in a hub on X, and having a head which overlaps the rim of H, as shown in figure. On the other side of that rim is a washer Z on the same stud having a radial face also overlapping the rim of H, but its back face is bevelled to a corresponding bevel on the radial face on the hub of lever O (the hub of O being pivoted on the same stud). When therefore O is depressed the two-bevel face of the hub of O forces the washer Z against the face of the wheel H, whose radial faces at the rim are therefore gripped between the face of the collar N and that of the washer Z, hence H is locked fast. By raising the end of lever O, Z is released and H is free to rotate.

Both the carriage feed and cross feed can only be traversed in one direction so far as these gears and levers are concerned, but means are provided on the lathe headstock for reversing the direction of motion of the feed spindle _f_ so as to reverse the direction of the feeds. It will be observed that so long as _f_ rotates, A, C, D, and F rotate, the remaining motions only operating when S is screwed up.

In order to obtain a delicate tool motion from the handle Q it is necessary to reduce the motion between J and I as much as possible, a point in which a great many lathes as at present constructed are deficient, because Q, although used to simply traverse the carriage along the bed, in which case rapid motion of the latter is desirable, is also used to feed the tool into corners when the lathe has no compound rest to put on light cuts on radial faces, hence it should be capable of giving a delicate tool motion.

On account of the deficiency referred to it is often necessary to put on a fine radial cut by putting the feed traverse in gear, and, throwing the feed screw gear out of contact with the other change wheels, pull it around by hand to put on the cut. In compound slide rests these remarks do not apply, because the upper part of the rest may be used instead of the handle Q.

Many small lathes are provided with a tool rest known as the _elevating rest_, or weighted lathe.

An excellent example of an elevating rest for a weighted lathe is shown in Figs. 577 and 578, which represent the construction in the Pratt and Whitney lathe. A is the lathe shears upon which slides the carriage provided with [V] slideways R for the sliding piece B, and provided at the other end with the guides H. The cross slide S is pivoted upon B at D, and fits at the other end between the guides H. At E is the elevating screw which when operated raises or lowers that end of the elevating rest to adjust the tool height. This also affords an excellent means of making a minute adjustment for depth of tool cut. The tool rest F is bolted to S.

The weight W is suspended from S and, therefore, holds one end of S to B, the lathe to C, and C to A; at the other end the weight holds S to C (through the medium of the elevating screw E) and C to A. The cross feed nut N is fast to S, the cross feed screw being operated by hand wheel G. B is provided with the [V] slideways R, which slide upon corresponding [V] slides R´ upon C; P is a lug cast upon C, and K is a screw threaded in B. When the end of screw K abuts against P the motion of S, and, therefore, of the cutting tool T, towards the work is arrested, hence when the tool is adjusted to the proper depth of cut, K is operated to abut firmly against P, and successive pieces may be turned to the same diameter without requiring each piece to be measured for diameter. N is the handle for opening and closing the nut for the feed screw Q, and Z is the wheel for the hand feed traverse. The length of cross feed motion is determined by the length of the cross [V] slides R´.

This class of rest possesses the advantage that no lost motion in the slides occurs by reason of the wear, because the weight keeps the parts in constant contact notwithstanding such wear; on the other hand, however, the slide [V]s sustain the extra wear due to the weight W in addition to the weight of the carriage. Lathes of this class are intended for light work, and are less suited for boring than for plain turning; they are, however, very convenient, and are preferred by many to any other kind of lathe for short and light work.

The tool rest being removable may be supplanted by other special forms of rest. Thus Figs. 579 and 580 represent a special rest for carrying two tools to cut pieces of work to the exact same length. Bolts D and E are to secure the rest A to the elevating rest, and C C are the clamps for the two tools B.

Fig. 581 represents a cross sectional view of the Putnam Tool Company's gibbed elevating rest, there being a gib on the underneath side of the front shear. The elevating screw is pivoted by a ball joint. By employing a gib instead of a weight, the bed may be provided with cross girts or ribs joining the two sides of the shear, thus giving much greater stiffness to it.

Figs. 582, 583, and 584 represent a lathe feed motion by William Munzer, of New York. The object in this motion is to insure that no two feeds can be put into operation simultaneously, because putting the feed in motion in one direction throws it out of gear for either of the others. Another object is to have the transmitting motion as direct as possible so as to avoid the rotation of any wheels not actually necessary for the transmission of the motion; and a third object is to enable the throwing out of gear of all wheels (when no feed motion at all is required) without leaving the apron.

The means employed to effect these objects are as follows:--

In Fig. 582 _f_ represents the independent feed spindle and S the lead-screw: _f_ is splined to drive A, A´ and A´´, which is a sleeve in one piece, and consists of a circular rack at A, a bevel pinion at A´, and a second bevel pinion at A´´. This sleeve may be operated in either direction along _f_ by rotating the pinion B. As shown in the cut A´ and A´´ are both out of gear with the bevel-wheel C, but if B be rotated to the right then A´ will be in gear with C, or if it be operated to the left then A´´ will be in gear with C. Now the direction of rotation of C will be governed by which pinion, A´ or A´´, drives it, and these are the means by which the direction of the feed traverse and also of the cross feed is determined.

If none of the feeds are required to operate, the sleeve occupies the position shown in the cut, and the circular rack at A simply rotates while B and all other parts remain at rest. On the same central pin as C is the pinion D driving a spur gear E´´. On the same centre pin as E is the gear F driving G, which is on the same central pin as C and D. The gear H is fixed to and rotates with G and drives I; all these gears serving to reduce the speed of motion when operating to feed the carriage traverse in either direction.

A gear J is carried on the end of a lever K, being pivoted at L. In the position shown J is out of gear with all gears, but it may be swung to the right so as to engage with wheel I and wheel M, and convey the motion of I to M. Upon the same spindle as M is the pinion N, engaging with the rack O, which is fast on the lathe bed. This completes the automatic feed traverse.

For a hand feed traverse, pinion P is employed to drive M, which is fast to N. The cross feed is self-acted by moving lever K to the left, causing it to engage with pinion Q as well as with T, Q being fast on the cross feed screw. To lock J in either of its three positions there is provided on lever K a spring locking pin R, shown clearly in Fig. 584, which represents an irregular section of the gearing viewed from the headstock of the lathe. The pin R is pressed inward by the spiral spring shown, and has a conical end fitting into holes provided in the apron to receive it. There are three of these holes, shown in dotted lines at _a_ _b_ _c_ in Fig. 582. When the pin is in _a_ the lever K, and therefore wheel J, Fig. 582, is locked out of gear; when it is in hole _b_ wheel J is locked in gear with I and M, and when it is in _c_ the wheel J is in gear with T and Q, and the cross feed is actuated.

A similar locking device is provided for the pinion B for actuating A; thus in Fig. 582 B is the lever, the spring pin being at R´´; or referring to Fig. 584, X is the lever fast at _x_ on the pin driving B, and R´´ is the spring pin.

The nut for the lead screw is secured either in or out of gear with the screw in the same manner, _x´_, Fig. 583, being the lever and R´ the spring pin.

In screw cutting the cutting tool requires to be withdrawn from the thread while the carriage traverses back, and it is somewhat difficult to know just how far to move the tool in again in order to put on a proper depth of cut. To facilitate this the device shown in Fig. 585 (which is taken from the "American Machinist,") is sometimes employed.

It consists of a ring C inserted between the cross slide D and the handle hub B having journal bearing on and rotating with the latter. When the first cut is put on, the mark on C is coincident with that on D, and the ring is then, while the first cut is traversing, moved (supposing the cross feed screw to have a right-hand thread) to the left, as shown in the figure, to the amount the handle will be required to move to the right to put on the next cut, and when the next cut is put on the handle will be moved the distance it was moved to withdraw the tool for the back traverse, and in addition enough to make the marks coincide, then while the second cut is being taken the ring is again moved to the left, as in the cut, to give the depth of cut for the next traverse, and so on.

If the cross feed screw has a left-hand thread, the mark on the ring would require to be moved to the right instead of to the left of the mark on D. It is obvious that this answers the same purpose, but is more exact than the chalk mark before referred to, and, indeed, that chalk mark could be used in the same way, leaving the chalk mark D and rubbing out that on C while the cut is proceeding and making a new one for the next cut.

Another device for use on lathes specially designed for screw-cutting is shown in Fig. 586, in which A represents the cross feed screw. It is fast to the notched wheel B, and is operated by it in the usual way. C is a short screw which provides journal bearing for the screw A by a plain hole. It is screwed on the outside, and the plate in which it fits acts as its nut. It is fast to the handle D, and is in fact operated by it. The handle or lever is provided with a catch E, pivoted in the enclosed box F, which also contains a means of detaining the catch in the notches of the wheel, or of holding it free from the same when it is placed clear. If, then, the lever D be moved back and forth the feed screw A, and hence the slide rest, will be operated; while, if the catch be placed in one of the notches on the wheel B, both the screws, A and C, will act to operate the rests. When, therefore, the tool is set to touch the diameter of the work, the catch E is lifted and the feed wheel B rotated, putting on the cut until the catch E will fall into the next notch in B, the lever D resting in the meantime on the stud G. When the cut is carried along the work to the required distance the tool is withdrawn by moving D over until it rests upon stud or stop H. While the slide rest is traversing back E is lifted and B rotated so that E will fall into the next notch, and when the tool starts forward again D is moved over from H to G, as shown in the figure, and the tool cut is put on.

When the device is not required to be used E is thrown out, D rests on E, and the feed is operated in the ordinary manner.

A simple attachment for regulating on a slide rest the depth of tool cut in screw cutting or for adjusting the cut to a requisite diameter when a number of pieces are to be turned to diameter by a finishing cut, is shown in Fig. 587, in which B represents the slide rest carriage, and E the cross slide on which the slide rest A is traversed by means of the cross feed screw _f_. A screw is screwed into the rest, as shown, carrying the two circular milled edge nuts R P; the screw passes an easy fit through the piece C, which is capable of being fixed in any position along the slide E by means of the set screw S; the nut R is set in such a position on the screw that it will abut against C when the tool is clear of the work surface (for the back traverse) while P may be used in two ways:--First it may be set so that when it comes against C the thread is cut to the required depth, and thus act as stop to give the thread depth without trying the gauge: or it may be used to answer the same purpose and in the same way as the ring C in Fig. 585.

The use of this device as a stop to gauge the thread depth is confined to such lengths of work as enable the tool to cut several pieces without requiring regrinding, because when the tool is removed to grind it, it is impracticable to set it exactly the same distance out from the tool post, hence the adjustment of P becomes destroyed. It is better, therefore, in most cases where a number of threads of equal pitch and diameter are to be cut, to rough them all out, cutting the threads a little above the gauge diameter so as to leave a finishing cut to be taken. In roughing out, however, the nut P may still be used to regulate the depth.

For the finishing cut the tool may be ground and P adjusted to give the requisite depth of cut, taking a single traverse over each thread to finish it. This, of course, preserves the tool and enables it to finish a larger number of threads without regrinding, and the consequent readjustment of P.

It is obvious that the nut P may be employed in the same manner to turn a number of plain pieces to an equal diameter.

It is preferable in a device of this kind, however, to employ the two adjusting nuts P and Q in Fig. 588, Q being a clamp nut that can be closed by a screw so as to firmly grip the threaded stud. Q is adjusted so that when P abuts against it the tool will cut to the correct diameter when it is moved in as far as nuts P Q will permit. The use of the second nut P is as follows:--Suppose a first cut has been taken and P may be screwed up to just meet the face of clamp C. Then while the carriage is traversing, P may be screwed back towards Q sufficiently to put on the next cut, and so on, so that P is used to adjust the depths of the roughing, and Q that of the finishing cut.

Sometimes a feed motion to a slide rest is improvised by what is known as the _star feed_, the principle of action of which is as follows: Upon the outer end of the feed screw of the boring bar or slide rest, as the case may be, is fastened a piece of iron plate, which, from having the form in which stars are usually represented, is called the star. If the feed is for a slide rest a pin is fastened to the lathe face plate or other revolving part, in such a position that during the portion of the revolution in which it passes the star it will strike one of the star wings, and move it around sufficiently to bring the next wing into position to be struck by the pin during its succeeding revolution. When the feed is applied to a revolving boring bar the construction is the same, but in this case the pin is stationary and the star revolves with the feed screw of the bar.

In Fig. 589 is shown a star feed applied to a slide rest. A is the slide rest, upon the end of the feed screw of which the star, B, is fitted. C is a pin attached to the face plate of the lathe, which, as it revolves, strikes one of the star wings, causing it to partly rotate, and thus move the feed screw. The amount of rotation of the feed screw will depend upon the size of the star and how far the circle described by the pin C intersects the circle described by the extreme points of the star wings. Thus the circles denoted by D E show the path of the pin C; the circle F H the path of the star points, and the distance from F to G the amount which one intersects the other. It follows that at each revolution of C an arm or wing of the star will be carried from the point G to point F, which, in this case, is a sixth of a revolution. If more feed is required, we may move the pin C, so that it may describe a smaller circle than D E, and cause it to intersect F H to a greater extent, in which case it will move the star through a greater portion of its revolution, striking every other wing and doubling the amount of feed.

It will be observed that the points F and G are both below the horizontal level of the slide rest's feed screw, and therefore that the sliding motion of the pin C upon the face of the star wings will be from the centre towards the points. This is better, because the motion is easier and involves less friction than would be the case if the pin contact first approached and then receded from the centre, a remark which applies equally to all forms of gearing, for a star feed is only a form of gearing in which the star represents a tooth wheel, and the pin a tooth in a wheel or a rack, according to whether its line of motion is a circle or a straight line.

It is obvious that in designing a star feed, the pitch of the feed screw is of primary importance. Suppose, for example, that the pitch of a slide rest feed screw is 4 to an inch, and we require to feed the tool an inch to every 24 lathe revolutions; then the star must have 6 wings, because each revolution of the screw will move the rest 1/6 in., while each revolution of the pin C will move the star 1/6 of a revolution, and 4 × 6 = 24. To obtain a very coarse feed the star attachment would require to have two multiplying cogs placed between it and the feed screw, the smaller of the cogs being placed upon the feed screw.

In many lathes of European design, the feeds or some of them, are actuated by ratchet handles, operated by an overhead shaft, having arms which rock back and forth. Thus in Fig. 590 is a lathe on which there is provided at A crank disc, carrying in a dovetail slot a pin P, for rocking the overhead shaft from whose arms a chain is attached which may be connected to the ratchet handle shown on the cross-feed screw, the weight being for the purpose of carrying that handle down while the chain pulls it up. To regulate the amount of feed the pin P is adjusted in the slot in A, or the chain may be attached in different positions along the length of the ratchet arm, the weight being provided with a set screw so that it may be set in any required position along the ratchet arm.

TOOL-HOLDING DEVICES.--Perhaps no part of a lathe is found in American practice with so many different forms of construction as the device for holding the cutting tool. The requirements for a lathe to be used on light work and where frequent changes in the position of the tool are necessary, are quite different from those for a lathe intended to take as heavy a cut as the lathe will properly drive, and wherein tools having the cutting edge at times standing a long way out from the tool post (as sometimes occurs in the use of boring tools). In the former case a single holding screw will suffice, possessing the advantage that the tool may be quickly inserted, adjusted for height and set to one side or the other, with a range of motion which often permits of a tool that has taken a parallel cut being moved in position to capacitate it to take a facing one, which would not be the case were its capacity for side adjustment limited.

In the case of the common American lathe having a self-acting feed and no compound rest, the tool post is usually employed, the rest being provided with a [T] slot such as shown in Fig. 577. This enables the tool post to be moved from side to side of the tool rest, and swing around in any required position. In connection with such tool posts various contrivances are employed to enable the height of the cutting edge of the tool to be readily adjusted. Thus in the Fig. 591, the tool post is surrounded by a cupped washer W, and through the slot in the tool post passes a gib G, which may be moved endways in the slot and thus elevates or depresses the tool point.

The objection to this is that the tool is not lifted parallel, or in other words is caused to stand out of its proper horizontal position which alters the clearance of the tool, and by presenting the angles forming the tool edge in an improper position, with relation to the work, impair its cutting qualification, as will be shown hereafter when treating of lathe cutting tools.

An improvement on this form has been pointed out by Professor John E. Sweet, whose device is shown in Fig. 592. Here the washer or ring is rounded and the bottom surface of the gib is hollowed, so that chips or dirt will to a great extent fall off, and every time the tool post is swung the gib acts to push off whatever dirt may lodge on the washer.

In the design shown in Fig. 593, the tool rests upon two washers W that are tapered, and its height is adjusted by revolving one of these washers, it being obvious that the limit of action to depress the tool point is obtained when the two thin sides of the washers are placed together, and on the same side of the tool post as the cutting edge of the tool, while the limit of action to raise the tool point is obtained when the washers have their thick sides together and nearest to the tool point.

Here again the tool is thrown out of level, and to obviate this difficulty the stepped washer shown in Fig. 594 may be used, the steps on opposite sides of the washer being of an equal height. This enables the tool to be raised or lowered without being set out of the horizontal position; but it has the defect that the adjustment cannot be made any finer than the height of the steps, and if the height is made to vary but slightly, in order to refine, as it were, the adjustment, the range of tool elevation or depression is correspondingly limited. Another form of stepped washer is shown in Fig. 595, in which no two steps are of the same height. This affords a wider range of adjustment, because the same two steps will alter the height of the tool by simply turning the washer one-half revolution. It has two defects, however; first, the least amount of adjustment is that due to the difference in height of the steps; and, second, when the tool is elevated it grips the washer at A, so that the tool is not supported across the full width of face of the washer, as it should be.

A defect common to all devices in which the tool is thrown out of level, is that the binding screw does not bed fair upon the tool, and as a result it is apt, if screwed home very firmly, as is necessary to hold boring tools that stand far out from the tool post, to spread the screw end as in Fig. 596, or to bend it.

A very convenient tool-adjusting device is shown in Fig. 597. It consists of a threaded ring N receiving the threaded bush M, the tool height being adjusted by screwing or unscrewing one within the other.

The objection to this is, that it occupies so much vertical height that there is not always room to admit it, which occurs, for example, in compound slide rests on small lathes.

On these rests, therefore, a single washer is more frequently used, which answers very well when the tool post is in a slot, so that it can be moved from side to side of the rest as occasion may require. When, however, the position of the tool post is fixed it has the disadvantage that the point P, Fig. 598, where the tool takes its leverage, is too far removed, and the tool is therefore liable to bend or spring from the pressure of the cut.

In Fig. 599 is an elevating device sometimes used on the compound rests of large lathes. The top of the rest is provided with a hub H, threaded externally to receive a ring nut R, around whose edge there are numerous holes to receive a pin for operating the nut. The tool-post is situated central in the hub. When the tool is loose the ring nut can be operated by hand or the tool may be gripped lightly and the ring nut operated by a pin. The level of the tool is here maintained; it is supported to about the edge of the rest on account of the large diameter of the ring nut, and a very delicate adjustment for height can be made, but such a device is only suitable for large lathes on account of the depth of the ring nut and hub.

On small slide-rests the device shown in Fig. 600 is often found. It consists of a holder H, in which is cut a seat for the tool, this seat being inclined to give the piece of steel used as a tool a certain constant degree of angle, and at the same time to permit of the tool being moved endwise in the holder to set it for height; but, as the tool requires to be pushed farther and farther through the holder to raise it, it is not so well supported as is desirable when slight tools are used, unless the holder is made long, so as to pass through the tool post with the tool. Again, it does not support the tool sideways unless the tool steel is dressed up and closely fits the groove in the holder.

In Fig. 601 W W are two inverted wedges which afford an accurate adjustment, but the range is limited, because if the wedges have much taper they are apt to move endways when the tool is fastened.

A convenient device for the compound rests of small lathes is shown in Fig. 602. It consists of a holder pivoted upon a central post and carrying two tool-binding screws, hence it can be revolved to set the tool in any required position. A similar device is shown in Fig. 603, in which the central post is slotted at A to receive the tool, and also carries a plate C, held by the nut N, and provided with tool-holding screws B and B´, which abut against the top of the rest, a top view of the device being shown in Fig. 604. Plate C may thus be swung around to set the tool in any required position on either side of the rest.

In Maudslay's slide rest, the tool clamp shown in Fig. 605 is employed. Screws A are employed to grip the tool moderately firm, and a turn of screws B (whose ends abut against the top of the slide rest) very firmly secures the tool, since it moves the clamp C as a lever, whose fulcrum is the screw A.

Figs. 606 and 607 represent the Whitworth tool clamp, the clamping plates of which change about upon the four studs, and are supported at their inner ends by a block equal in height to the height of the tool steel.

Figs. 608, 609, 610, and 611 represent the "Lipe" tool post, so called from the name of its inventor. The top of the cross slide is cylindrical, and is bored to receive the tool post which has a cylindrical stem. The cylindrical part of the tool post is split vertically, and has two lips, the bolt D passing through one lip and threading into the other, so that by operating bolt D the tool post may be gripped very firmly or released, so that it may be revolved to bring the tool into any required position after it is fastened in the tool post, which is a great advantage because the tool is brought to a solid seating in the post before its height is adjusted, and will not therefore be altered in height by setting up the set screws as often occurs in ordinary tool posts. From the shape of the tool post, the tool may be gripped by one set screw only, when required for light duty, or by two set screws for heavy duty or for boring, while in either case it is supported clear to the edge of the rest.

Fig. 591.

Fig. 592.

Fig. 593.

Fig. 594.

Fig. 595.

Fig. 596.

Fig. 597.

Fig. 598.

Fig. 599.

Fig. 600.

Fig. 601.

Fig. 602.

Fig. 603.

Fig. 604.

Fig. 605.

Fig. 606.]

Fig. 608 shows the tool in position, held by a single screw, for work requiring the tool to be close up to the work driver. In Fig. 609 a tool is shown held as is required by work between centres, but both set-screws are used. Fig. 610 shows a tool in position for boring, two set-screws being used. Fig. 611 shows a tool being held for the same purpose, but by a single screw, and it will be observed that the advantage of the second set-screw is obtained without in any way sacrificing the handiness of the post, when used with a single screw. Whether one or two set-screws are used, the boring tool may be forged from a single bar of octagon steel, which can be seated in a piece like that shown at E in Fig. 610, which is grooved so as to receive and hold the tool. As is well known, boring tools are the most troublesome both to forge and to adjust in the lathe, and, as the result, a light tool is often used because no other is at hand and it is costly to make a new one. When, however, the tool can be forged from a plain piece of steel, these objections are overcome, and a sufficient number of tools may be had so that one can always be found suitable for any ordinary sized hole, the object being to use as rigid a tool as can be got into the hole bored. The feature of maintaining the tool level is of great importance in boring work, because when the tool requires to be set out of level to adjust its height, it will generally strike against the mouth of the hole if the latter is of much depth. This annoyance is also frequently met with in boring tools which are forged out of rectangular steel, because the rounded stem is generally left taper. The largest end of the taper is generally nearest the tool post. Hence the capacity to use octagon steel and keep it level while adjusting its height, added to the fact that the tool is supported clear to the edge of the tool rest, and the tool post is so blocked as to virtually become a part of the rest, constitute a very important advantage.

A common device on large lathes is shown in Fig. 612, the two clamps being shown in position for outside turning, and being changed (so as to stand at a right angle to the position they occupy in the figure) for holding boring tools. The bolts are enveloped by spiral springs which support the clamps.

Figs. 613 and 614 represent the tool holders employed in the Brown and Sharpe small screw machines. In the front rest, Fig. 613, the piece R receives two adjusting and tool-gripping screws S, upon which sits the gib G, and upon this the tool is placed. The surface E at the top of the tool post slot is curved so that it will bear upon the top of the tool at a point only. The tool is here supported along the full length of the gib, and there is no set-screw at the top of the tool post, which enables a much more unobstructed view of the tool.

Fig. 614 is the tool post used at the back of the rest, the piece B passing through the tool post slot. The tool rests upon the top of screw E and upon the top of B at F, and is secured by set-screw S; its height is therefore adjusted by means of screw E, which is threaded in B. The set-screw S is not in this case objectionable, because it is at the back of the rest, and therefore does not obstruct the view of the work, while it is at the same time convenient to get at.

When the screw for traversing a lathe carriage is used for plain feeding, it is termed the feed screw, but when it is used to cut threads it is termed the lead screw.

A lead screw should be used for screw cutting only, so that it may be preserved as much as possible from wear. As the greater portion of threads cut in a lathe of a given size are short in comparison with the length of the lathe, it follows that the part of the lead screw that is in operation when the carriage or saddle is traversing over short work is most worn, while the other end is least worn, hence it is not unusual to so construct the screw and its bearings that it may be changed end for end in the lathe, to equalize the wear. By turning a lead screw end for end, therefore, to equalize the wear, the middle of the length of the screw will become the least worn, and, therefore, the most true. Hence it is better to use one end of the lead screw for general work, and to reverse it and use the other end only for screws requiring to be of very correct pitch.

To obviate the wear as much as possible the feed nut should embrace as great a length of the screw as convenient, and should be of a material that will suffer more from wear than the lead screw, or in other words shall relieve the feed screw from wear as much as possible. The wear on the nut being equal from end to end, the wearing away of one side of its thread does not vary its pitch; hence the only consideration as to its wearing qualification are the expense of its renewal and the length of time that may occur between its being engaged with the lead screw and giving motion to the lathe carriage, this time increasing in proportion as the nut thread is worn. Under quick speeds or when the lathe is in single gear, the rotation of the feed screw is so quick that not much time is lost before the carriage feeds, but when the back gear is in operation at the slowest speeds, the loss of time due to a nut much worn is an item of importance.

In some lathes the feed screw is employed for screw cutting and for operating an independent feed also. This is accomplished by cutting a feather way or spline along it, so that a worm having journal bearing in the apron of the rest carriage may envelop the lead screw and be driven by it, through the medium of a feather fast into the worm gear. The motion obtained from the worm gear is transferred through suitable gearing to the rack pinion.

The spline is cut deeper than the thread, so as to prevent the latter as far as possible from wear, by reason of the friction of the spline.

The lead screw if long should be supported, to prevent its sagging of its own weight. In some cases the lead screw is supported in a trough along its whole length, as is done in the Sellers lathe. In other cases, bearings hanging from the [V]-slides, and movable along the bed, are employed.

It is desirable that the feed screw and nut be as near the middle of the carriage as possible, so that it shall pull the carriage at as short a leverage as possible, thus avoiding the liability to tilt or twist the carriage; but it is not practicable to place it midway between the lathe shears, because in that case the cuttings, &c., from the work would fall upon it, and cause excessive and rapid wear of the screw and nut.

In general the lead screw is located either in front, or at the back of the lathe, and in considering the more desirable of the two locations, we have as follows: