Modern Machine-Shop Practice Part 40

[Ill.u.s.tration: Fig. 545.]

The face plate is 12 feet diameter, cast with internal gear at the back.

It is provided with [T]-slots and square holes for fixing work. It is bolted to a large f.l.a.n.g.e in one piece with the spindle, and fitted with four steel expanding gripping jaws worked with screws and toothed blocks. These are for doing chuck work, or for gripping work to be driven, as the collars of propeller or crank shafts, or work of a similar character. By the system of gearing adopted, when desired, the face plate can be revolved almost free, which facilitates balancing for turning crank shafts, as well as other operations. The thrust against the live spindle is taken by an adjustable steel tail piece.

[Ill.u.s.tration: Fig. 546.]

[Ill.u.s.tration: Fig. 547.]

[Ill.u.s.tration: Fig. 548.]

[Ill.u.s.tration: Fig. 549.]

The beds are double, 10 feet in width over all, the sections being joined together by ma.s.sive ground plates and bolts. They are made with square lips to resist the upward strain of cutting. The front bed is fitted with two saddles, each carrying a compound slide rest having the following movements: First, screw-cutting, by means of a leading screw, situated inside the bed, with a sliding disengaging nut and reversing motion for right or left-hand threads, or for instantaneously stopping the longitudinal movement of the saddle. This is accomplished by a set of clutch mitres placed inside the bed at headstock end, and actuated by a lever in front: Second, a self-acting surfacing motion to slide rest by means of a longitudinal shaft at the front of the bed, and clutch mitres for reversing the saddle screw.

[Ill.u.s.tration: Fig. 550.]

Third, power motion for moving the saddles quickly to position along the bed. This is done through the fast and loose pulleys at the headstock end of lathe.

Fourth, hand rack motion to saddle. The back bed is fitted with two saddles, each carrying a pillar rest, fitted for all movements in plain turning like the front rests, and also with swiveling motion for corner turning.

[Ill.u.s.tration: Fig. 551.]

The tailstock has a spindle 9 inches diameter. It is fitted in [V]s on the bed, and held down by three [T]-head bolts on each side. The top section is adjustable for turning tapers. It is moved along the ways by engaging a nut with the main screw. An end-cutting rest is fitted to the tailstock, which is adapted for operating on f.l.a.n.g.ed couplings and similar work.

There is a separate set of change wheels for each saddle, so arranged as to cut standard pitches up to 3-inch pitch, and for self-acting feeds down to 50 per inch. By this means, when both tools are in operation on a piece of work, one tool may be used with coa.r.s.e feed for roughing out, while the other may be taking a fine or finis.h.i.+ng cut either on the same or a different part of the piece; or one tool may be cutting towards and the other from the face plate, always maintaining the balance of a front and back cut.

[Ill.u.s.tration: Fig. 552.]

Complete counter driving motion, consisting of wall brackets, shaft, cone, and sets of fast and loose pulleys for quick reversing motion in screw cutting, also belt bar s.h.i.+pping motion, and full set of case-hardened wrenches are provided.

CHAPTER VII.--DETAILS IN LATHE CONSTRUCTION.

Although in each cla.s.s of lathe the requirements may be practically the same, yet there is a variety of different details of construction by means of which these requirements may be met or filled, and it may be profitable to enter somewhat into these requirements and the different constructions generally employed to meet them.

[Ill.u.s.tration: Fig. 553.]

The cone spindle or live spindle of a lathe should be a close working fit to its boxes or bearings, so that it will not lift under a heavy cut, or lift and fall under a cut of varying pressure. This lifting and falling may occur even though the work be true, and the cut therefore of even depth all around the work, because of hard seams or spots in the metal.

It is obvious that the bearings should form a guide, compelling the live spindle to revolve in a true circle and in a fixed plane, the axis of revolution being in line with the centre line of the tail spindle and that means should be provided to maintain this alignment while preserving the fit, or in other words taking up the wear. The spindle journals must, to produce truly cylindrical work, be cylindrically true, or otherwise the axis of its revolution will change as it revolves, and this change will be communicated through the live centre to the work, or through the chuck plate to the work, as the case may be.

The construction of the bearings should be such, that end motion to the spindle is prevented in as short a length of the spindle as possible, the thrust in either direction being resisted by the mechanism contained in one bearing.

In Fig. 553 is a form of construction for the front bearing (as that nearest to the live centre is called), in which end motion to the spindle is prevented at the same time as the diametral fit is adjusted.

The spindle is provided with a cone at C and is threaded at T to receive two nuts N which draw the spindle cone within the bearing. In this case the journal at the back end may be made parallel, so that if the spindle either expands or contracts more under variations of temperature than the frame or head carrying the bearings or bearing boxes, it will not bind endwise, nor will the fit be impaired save inasmuch as there may be an inequality of expansion in the length of the front journal and its box. In this case, however, the end pressure caused by holding the work between the lathe centres acts to force the spindle into its bearing and increase the tightness of its fit, hence it is not unusual to provide at the back bearing additional means to resist the thrust of the dead centre.

[Ill.u.s.tration: Fig. 554.]

Fig. 554, which is taken from "Mechanics," represents Wohlemberg's patent lathe spindle, in which both journals are coned, fitting into bushes which can be replaced by new ones when worn; the end thrust is here taken by a steel screw, while the end fit is adjusted by means of a ring nut which binds the face of the large cone gear against the inside face of the front bearing and by the face of the gear that drives the change gears. It may be pointed out, however, that in this construction the spindle must be drawn within to adjust the fit of the front bearing, which can only be done by adjusting the pinion that drives the change gears, or by s.c.r.e.w.i.n.g up the nut that is inside the cone, and therefore cannot be got at. The back bearing can be adjusted by means of the ring nuts provided at each of its ends.

[Ill.u.s.tration: Fig. 555.]

Fig. 555 represents another design of cone bearing, in which the spindle is threaded to receive the nuts A which draw it within the front bearing and thus adjust the fit, and at the same time prevent end motion. The back bearing is provided with a bush parallel outside, and furnished with a nut at B to adjust the fit of the end bearing. To prevent the end pressure of the dead centre from forcing the spindle cones too tightly within their bearings a cross piece P is employed (being supported by two studs provided in the head), and through P pa.s.ses an adjusting screw D, having nuts N and C, one on each side of P. Between the end of D and of the lathe spindle a washer of leather or of raw hide is placed to prevent the end faces from abrading. A similar device for taking up the end thrust is often provided to lathes in which the journals are both parallel, fitting in ordinary boxes, a top view of the device being ill.u.s.trated in Fig. 556, in which B is the back bearing box, S S two studs supporting cross-piece P, and N and C are adjusting nuts. G is the gear for driving the change wheels for screw cutting or for ordinary feeding as the case may be. In this design the gear wheel G remains fixed and the combinations of gears necessary to cut various pitches of thread must be made on the lead screw and on the swing frame, which must be long enough to permit the change gear stud to pa.s.s up to permit the smallest change wheel to gear with wheel G, and which is provided with two grooves E and F, Fig. 557, for two studs to carry two compounded pairs of change wheels. This compounding in two places on the swing frame enables gear G to be comparatively large, and thus saves the teeth from rapid wear, while it facilitates the cutting of left-hand threads, because it affords more convenience for putting in a gear to change the direction of feed screw revolution.

[Ill.u.s.tration: Fig. 556.]

In many lathes of American design the journals are made parallel, and the end play is taken up at the back bearing, an example being given in Fig. 558, in which the back bearing boxes are made in two halves A and B, the latter having a set screw (with check nut) threaded through it and bearing against a washer that meets the end of the spindle.

[Ill.u.s.tration: Fig. 557.]

A simple method of preventing end motion is shown in Fig. 559, a bracket B affording a support for a threaded adjusting screw, which is sometimes made pointed and at others flat. When pointed it acts to support the spindle, but on the other hand it also acts to prevent the journal from bedding fairly in the boxes. In some cases of small lathes the back bearing is dispensed with, and a similar pointed adjusting screw takes its place, which answers very well for very small work.

Since the strain of the cut carried by the cutting tool falls mainly upon the live centre end of the cone spindle, it is obvious that the bearing at that end has a greater tendency to wear.

[Ill.u.s.tration: Fig. 558.]

In addition to this the weight of the cone itself is greatest at that end, and furthermore the weight of the face plate or chuck, and of the work, is carried mainly at that end. If, however, one journal and bearing wears more than the other, the spindle is thrown out of line with the lathe shears, and with the tail block spindle. The usual method of obviating this as far as possible is to give that end a larger journal-bearing area.

[Ill.u.s.tration: Fig. 559.]

The direction in which this wear will take place depends in a great measure upon the kind of work done in the lathe; thus in a lathe running slowly and doing heavy work carried by chucks, or on the face plate, the wear would be downwards and towards the operator, the weight of the chuck, &c., causing the downward, and the resistance or work-lifting tendency of the cut causing the lateral wear. As a general rule the wear will be least in a lateral direction towards the back of the lathe, but the direction of wear is so variable that provision for its special prevention or adjustment is not usually made. In the S. W. Putnam lathe, provision is made that the bearing boxes may be rotated in the head, so that when the lathe is used on a cla.s.s of work that caused the live spindle to wear the bearing boxes on one side more than on another, the boxes may be periodically partly rotated in the head so that further wear will correct the evil.

The coned hole to receive the live centre should run quite true, so that the live centre will run true without requiring, when inserted, to be placed in exactly the same position it occupied when being turned up at its conical point. But when this hole does not run true a centre punch dot is made on the end of the spindle, and another on the centre, so that by placing the two dots to coincide at all times, the centre will run true.

The taper given to lathe centres varies from 9/16 per foot to 1 inch per foot. In the practice of Pratt and Whitney a taper of 9/16 per foot is given to all lathes, the lengths of the tapers for different sizes of lathes being as follows:

Length of Taper Socket Swing of Lathe. for Live Centre.

13 inches 5 inches.

16 " 3-3/4 "

18 and 19 inches 7-11/16 "

" " with hollow spindle 5 inches long and 1-1/16 diameter at the small end.

The less the amount of taper the more firmly the centre is held, but the more difficult it becomes to remove the centre when necessary.

[Ill.u.s.tration: Fig. 560.]

The princ.i.p.al methods of removing live centres are shown in Fig. 560, in which is shown at B a square part to receive a wrench, it being found that if not less than about 1/2-inch taper per foot of length be given to the live spindle socket, then revolving the centre with a wrench will cause it to release itself, enabling it to be removed by hand. Another method employed on small lathes is to drill a hole through the live spindle to receive a taper pin P, the live centre end being shown at C.

Another and excellent plan for large lathes, is to thread the centre and provide it with a nut M, which on being screwed against the end face of the live spindle will release the centre. The objection to the use of the pin P is that it is apt to become mislaid, and it is not advisable to use a hammer about the parts of the lathe, especially in such an awkward place as between the journal bearing and the cone, which is where the pin hole requires to be located. The square section is, therefore, the best method for small lathes, and the nut for large ones.

In cases where the live spindle is made hollow a bar may be pa.s.sed through from the rear end to remove the centre; this also enables rods of iron to be pa.s.sed through the spindle, leaving the end projecting through the chuck for any length necessary for the work to be turned out of its exposed end.

The dead centre may be extracted from the tail spindle by a pin and hole as in Fig. 560, or, what is better, by contact with the end of the tail screw as described when referring to the tail stock of the S. W. Putnam lathe.

The cone pulley should be perfectly balanced, otherwise at high speeds the lathe will shake or tremble from the unbalanced centrifugal motion, and the tremors will be produced to some extent on the work. The steps of the cone should be amply wide, so that it may have sufficient power, without overstraining the belt, to drive the heaviest cut the lathe is supposed to take without the aid of the back gear.