Modern Machine-Shop Practice Part 35

[Ill.u.s.tration: Fig. 480.]

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, Ma.s.sachusetts, 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 pa.s.sed through the spindle, which saves cutting the rod into short lengths. The front bearing of the headstock has two bra.s.ses 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 pa.s.ses 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.

[Ill.u.s.tration: Fig. 481.]

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_.

[Ill.u.s.tration: Fig. 482.]

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.

[Ill.u.s.tration: Fig. 483.]

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 f.l.a.n.g.e on it, the face of that collar or f.l.a.n.g.e 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.

[Ill.u.s.tration: Fig. 484.]

[Ill.u.s.tration: Fig. 485.]

[Ill.u.s.tration: Fig. 486.]

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.

[Ill.u.s.tration: Fig. 487.]

[Ill.u.s.tration: Fig. 488.]

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.

[Ill.u.s.tration: Fig. 489.]

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.

[Ill.u.s.tration: Fig. 490.]

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.

[Ill.u.s.tration: Fig. 491.]

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.

[Ill.u.s.tration: Fig. 492.]

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, Ma.s.sachusetts. 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 [V]s 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.

[Ill.u.s.tration: Fig. 493.]

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.

[Ill.u.s.tration: Fig. 494.]

Fig. 494 is a sectional side elevation of the headstock; A A' represents the headstock carrying the bearing boxes B and B', which are capable of bore closure so as to be made to accurately fit the spindle S by the construction of the front bearing B, being more clearly shown in Fig.

495; B is of composition bra.s.s, its external diameter being coned to fit the taper hole in the head; it is split through longitudinally, and is threaded at each end to receive the ring nuts C and C'. If C be loosened from contact with the radial face of A, then C' may be screwed up, drawing B through the coned hole in A, and, therefore, causing its bore to close upon S.

At the other end of S, Fig. 496, C" is a ring nut for drawing the journal box B' through _a'_ to adjust the bore of B' to fit the journal of S, s.p.a.ce to admit the pa.s.sage of B' being provided at _e_. D is a box nut serving to withdraw B' or to secure it firmly in its adjusted position, and also to carry the end adjusting step E. F is a check nut to lock E in its adjusted position.

The method of preventing end motion to S is more clearly shown in Fig.

496, in which _h_ is a steel washer enveloping S, having contact with the radial face of B' and secured in its adjusted position by the check nuts _g_, hence it prevents S from moving forward to the right. _f_ is a disk of raw hide let into E; the latter is threaded in D and is squared at the end within F to admit of the application of a wrench, hence E may be screwed in until it causes contact between the face of _f_ and the end of S, thus preventing its motion to the left. By this construction the whole adjustment laterally of S is made with the short length from _h_ to _f_, hence any difference of expansion (under varying temperature) between the spindle and the head A A', or between the boxes and the spindle S, has no effect towards impairing the end fit of S in its bearings.

The method of adjusting the bearings to the spindle is as follows:--C"

and C' are slackened back by means of a "spanner wrench" inserted in the holes provided for that purpose. C and D are then screwed up, withdrawing B and B' respectively, and leaving the journal fit too easy.

C' is then screwed up until B is closed upon the spindle sufficiently that the belt being loose on the cone pulley, the latter moved by the hand placed upon the smallest step of the cone can just detect that there is contact between the bore of B and the spindle, then, while still moving the cone, turn C' back very slowly and a very little, the object being to relieve the bore of B from pressure against S. C may then be screwed up, firmly locking B in its adjusted position. C" may then be operated to adjust B' in a similar manner, and D screwed up to lock it in its adjusted position. Before, however, s.c.r.e.w.i.n.g up D it is better to remove F and release E from pressure against _f_, adjusting the end pressure of E after D has been screwed home against A'.

To prevent B and B' from rotating in the head when the ring nuts are operated, each is provided with a pin, _q_, grooves _c_ and _c'_ permitting of the lateral movement of B and B' for adjustment. The boxes B, B' admit of being rotated in their sockets in A and A' so as to a.s.sume different positions, the pins _q_ and _q'_ being removable from one to another of a series of holes in the boxes B, B' when it is desired to partly rotate those boxes. The tops of the boxes are provided with oil holes, and the oil ways shown at _r_, _s_ being the oil groove through the head and _a_ simply a stopper to prevent the ingress of dust, &c.