Modern Machine-Shop Practice Part 49

The tailblock of a lathe should be capable of easy motion for adjustment along the shears, or bed of the lathe, and readily fixable in its adjusted position. The design should be such as to hold the axial line of its spindle true with the axial line of the live spindle. If the lathe bed has raised [V]s there are usually provided two special [V]s for the tailblock to slide on, the slide rest carriage sliding on two separate ones. In this case the truth of the axial line of the tail spindle depends upon the truth of the [V]s.

If the lathe bed is provided with ways having a flat surface, as was shown in Fig. 622, the surfaces of the edges and of the projection are apt in time to wear, permitting an amount of play which gives room for the tailblock to move out of line. To obviate this, various methods are resorted to, an example being given in the Sellers lathe, Fig. 518.

In wood turners' lathes, where tools are often used in place of the dead centre, and in which a good deal of boring is done by such use of the tail spindle, it is not unusual to provide a device for the rapid motion of that spindle. Such a device is shown in Fig. 636; it consists of an arm A to receive the end C of the lever B, C being pivoted to A. The spindle is provided with an eye at E, the wheel W is removed and a pin pa.s.sed through D and E, so that by operating the handle the spindle can be traversed in and out without any rotary motion of the screw.

When the tailblock of a lathe fits between the edges of the shears, instead of upon raised [V]s, it is sometimes the practice to give them a slight taper fitting accurately a corresponding taper on the edges of the shears. This enables the obtenance of a very good fit between the surfaces, giving an increased area of contact, because the surfaces can be filed on their bearing marks to fit them together; but this taper is apt to cause the tailstock to fit so tightly between the shears as to render it difficult to move it along them, and in any event the friction is apt to cause the fit to be destroyed from the wear. An excellent method of obviating these difficulties is by the employment of rollers, such as shown at R in Figs. 637 and 638, which represent the tailstock of the Putnam Tool Company's lathe. In some cases such rollers are carried on eccentric shafts so that they may be operated to lift the tailstock from the bed when moving it.

[Ill.u.s.tration: Fig. 636.]

[Ill.u.s.tration: Fig. 637.]

[Ill.u.s.tration: Fig. 638.]

[Ill.u.s.tration: Fig. 639.]

A very ready method of securing or releasing a small tailstock to a lathe shears is shown applied to a wood turner's hand rest in Fig. 639, in which A A represents the lathe shears, B the hand rest, C the fastening bolt, D a piece hinged at each end and having through its centre a hole to receive the fastening bolt, and a counter-sink or recess to receive the nut and prevent it uns.c.r.e.w.i.n.g. E represents a hinged plate, and F a lever, having a cam at its pivoted end. A slot for the fastening bolt to pa.s.s through is provided in the plate E. In this arrangement a very moderate amount of force applied to bring up the cam lever will cause the plate D to be pressed down, carrying with it the nut, and binding the tailstock or the tool rest, as the case may be, with sufficient force for a small lathe.

When a piece of work is driven between the lathe centres, the weight of the work tends to deflect or bend down the tail spindle. The pressure of the cut has also to be resisted by the tail spindle, but this pressure is variable in direction, according to the shape of the tool and the direction of the feed; usually it is laterally towards the operator and upwards. In any event, however, the spindle requires locking in its adjusted position, so as to keep it steady. The pressure on the conical point of the dead centre is in a direction to cause the tail screw to unwind, unless it be a left-hand thread, as is sometimes the case.

If the spindle and the bore in which it operates have worn, the resulting looseness affords facility for the spindle to move in the bore as the pressure of the cut varies, especially when the spindle is far out from the tailstock.

Now, in locking the tail spindle to obviate these difficulties, it is desirable that the locking device shall hold that spindle axially true with the live spindle of the lathe, notwithstanding any wear that may have taken place. The spindle is released from the pressure of the locking device whenever it is adjusted to the work, whether the cut be proceeding or not. Hence, the wear takes place on the bottom of the spindle and of the hole, wear only ensuing on the top of the spindle and bore when the spindle is operated under a slight locking pressure, while the cut is proceeding in order to take up the looseness that may have arisen from wear in the work centres.

In all cases the feed of the cut should be stopped while the centre is adjusted, so as to relieve the spindle and bore from undue wear; but most workmen pay little heed to this; hence the wear ensues, being, as already stated, mainly at the bottom. It is obvious, then, that, if the spindle is to be locked to the side of the bore on which it slides, it will be held most truly in line if it be locked to that side which has suffered least from wear, and this has been shown to be at the top.

[Ill.u.s.tration: Fig. 640.]

[Ill.u.s.tration: Fig. 641.]

The methods usually employed to effect this locking are as follows:--In Fig. 640, S is the tail spindle, B part of the tailblock in section, R a ring-bolt, and H a handled nut. s.c.r.e.w.i.n.g up the nut H causes R to clamp S to the upper part of the bore of B; while releasing H leaves S free to slide. There are three objections to this plan. The ring R tends to spring or bend S. The weight of R tends to produce wear upon the top of the spindle, and the spindle is not gripped so near to its dead centre end as it might be. If S is a close fit in B the pressure of R could not spring or bend S; but, so soon as wear has taken place, S becomes simply suspended at R, having the pressure of R, and the weight of the work tending to bend it. Another locking device is shown in Fig. 641. It consists of a shoe placed beneath S, and a wedge-bolt beneath it, operated by the handled nut C. Here the pressure is again in a direction to lift S, as denoted by the arrow; but when the wedge W is released the shoe falls away from S, hence the locking device produces no wear upon S. This device may be placed nearer to the end of B, since the wedge may pa.s.s through the front leg of the tailstock instead of to the right of it, as in Fig. 640. But S is still suspended from the point of contact of the shoe, and the weight of the work still bends it as much as its play in B will permit.

[Ill.u.s.tration: Fig. 642.]

Another clamping device is shown in Fig. 642. In this the cylindrical part B of the tailblock is split on one side, and is provided with two lugs. A handled screw pa.s.ses through the upper lug, and is threaded into the lower one, so that by operating the handle C, the bore may be closed, so as to grip S, or opened to relieve it. This possesses the advantages: First, that it will cause S to be gripped most firmly at the end of B, and give a longer length of bearing of B upon S; and, secondly, that it will grip S top and bottom, and, therefore, prevent its springing from the weight of the work. But, on the other hand, B will close mainly on the side of the split, as denoted by the dotted half-circle, and therefore tend to throw S somewhat in the direction of the arrow, which it will do to an amount answerable to the amount of looseness of S in B. In the Pratt and Whitney lathes this device is somewhat modified, as is shown in Fig. 643. A stud E screws into the lower lug D, having a collar at E let into the upper lug, with a square extending above the upper lug so that the stud may be screwed into D, exerting sufficient pressure to close the bore of B to a neat working fit to the spindle. The handled nut, when screwed up, causes B to grip the spindle firmly; but when released, leaves the spindle a neat working fit and not loose to the amount of the play; hence, the locking device may be released, and the centre adjusted to take up the wear in the work centres while the cut is proceeding, without any movement of the spindle in B, because there is no play between the spindle and B.

[Ill.u.s.tration: Fig. 643.]

[Ill.u.s.tration: Fig. 644.]

In the design shown in Fig. 644, the end B of the tailblock is threaded and is provided with a handled cap nut A A. In the end of the tailblock where the spindle emerges, is provided a cone, and into this cone fits a wedge-shaped ring, as shown. This ring is split quite through on one side, while there are two other slots nearly but not quite splitting the wedge-ring. When the handle C is pulled towards the operator it screws A up on the end B, and forces the wedge-ring up in the conical bore in B.

From the split the ring closes upon the spindle S, and grips it. Now, as the ring is weakened by slots in two places besides the split, it closes more nearly cylindrically true than if it had only a split, there being three points where the ring can spring when closing upon S; and from the cone being axially true with the live spindle of the lathe, S is held axially true, notwithstanding any wear of the spindle, because the locking device, being at the extreme end of B, is as near to the dead centre as it is possible to get it; and, furthermore, when C is operated for the release, the wedge-ring opens clear of S, so that S does not touch it when moved laterally. The wear of the bore of B has, therefore, no effect to throw S out of line, nor has the gripping device any tendency to bend or spring S, while the latter is held as close to the work as possible; hence the weight of the work has less influence in bending it. The pitch of the thread and the degree of cone are so proportioned that less than one-quarter rotation of A will suffice to grip or release S, the handle C being so placed on A as to be about vertical when the split ring binds S; hence C is always in a convenient position for the hand to grasp.

[Ill.u.s.tration: Fig. 645.]

In this case, however, the spindle being locked at the extreme end of the hole, there is more liability of the other end moving from the pressure of the cut, or from the weight of the work; hence it would seem desirable that a tail spindle should be locked in _two_ places; one at the dead centre end of the hole, and the other as near the actuating wheel, or handle, as possible, and also that each device should either hold it central to the original bore, notwithstanding the wear, an end that is attained in the Sellers lathes already described.

Slide rests for self-acting or engine lathes are divided into seven kinds, termed respectively as follows: simple, or single, elevating, weighted, gibbed, compound, duplex, and duplex compound. A simple, or single, slide rest contains a carriage and one cross slide, as in Fig.

621. An elevating slide rest is one capable of elevation at one end to adjust the cutting tool height, as in Fig. 499. A weighted slide rest is one held to the shears by a weight, as in Fig. 577. A gibbed slide rest is held to the shears by gibs, as in Fig. 621. A compound slide rest has above the cross slide, a second slide carrying the tool holder, this second slide pivoting to stand at any required angle, as in Fig. 505. A duplex slide rest has two rests on the same cross slide, and in a compound duplex both these two rests are compound, as in Fig. 511. The rest shown on the Putnam lathe in Figs. 492 and 499, is thus an elevating gibbed single rest.

TESTING A LATHE.--To test a lathe to find if its live and dead spindles are axially in line one with the other and with the guides on the lathe bed, the following methods may be employed in addition to those referred to under the heading of Erecting.

To test if the live spindle is true with the bed or shear guides, a piece such as in Fig. 645 may be turned up between the lathe centres, the end A fitting into the live spindle in place of the live centre, and the collars B C being turned to an equal diameter, and the end face D squared off true. The end A must then be placed in the lathe in place of the live centre, the dead centre being removed from contact with the work; with the lathe at rest a tool point may be set to just touch collar C, and if when the carriage is moved to feed the tool past collar B, the tool draws a line along it of equal depth to that it drew along C, the live head is true; the dead centre may then be moved up to engage the work end D, and the lathe must be revolved so that (the tool not having been moved at all by the cross-feed screw) the tool may be traversed back to draw another line along C, and if all three lines are of equal depth the lathe is true. The tool should be fine pointed and set so as to mark as fine a line as possible.

[Ill.u.s.tration: Any View.

Fig. 646.]

[Ill.u.s.tration: Side View.

Fig. 647.]

[Ill.u.s.tration: Side View.

Fig. 648.]

[Ill.u.s.tration: Top View.

Fig. 649.]

Another method is to turn up two discs, such as in Fig. 646, their stems A and B fitting in place of the live and dead centres. One of these discs is put in the place of the live, and the other in that of the dead centre, and if then the lathe tailstock be set up so that the face of B meets that of A, their coincidence will denote the truth of the live and dead spindles. The faces of the discs may be recessed to save work and to meet at their edges only, but their diameters must be equal. If the discs come one higher than the other, as in Fig. 647, the centres are of unequal height. If the faces meet at the top and are open at the bottom, as in Fig. 648, it shows that the back bearing of the live spindle is too high, or that the tail spindle is too low at the dead centre end. If the discs, when viewed from above, come as in Fig. 649, it is proof that either the live spindle or the tail spindle does not stand true with the lathe shears. If the disc faces come so nearly fair that it is difficult to see if they are in contact all around, four pieces of thin paper may be placed equidistant between them, and the grip upon them tested by pulling.

If the tailstock has been set over to turn taper and it is required to set it back to turn parallel again, place a long rod (that has been accurately centred and centre-drilled) between the lathe centres, and turn up one end for a distance of an inch or two.

Then turn it end for end in the lathe and let it run a few moments so that the work centre, running on the dead centre of the lathe, may wear to a proper bed or fit to the lathe centre, and then turn up a similar length at the dead centre end, taking two cuts, the last a fine finishing cut taken with a sharp tool, and feeding the finishing cut from left to right, so that it will be clear of the work end when the cut is finished. Without moving the cross-feed screw of the lathe after the finishing cut is set, take the bar out of the lathe and wind the slide rest carriage, so that the turning tool will stand close to the live centre. Place the bar of iron again in the lathe, with the turned end next to the live centre, and move the lathe carriage, so that the tool is on the turned end of the bar.

Rotate the bar by hand, and if the tool just touches the work without taking a cut the line of centres is parallel with the ways. If there is s.p.a.ce between the tool point and the turned end of the bar, the tailstock requires setting over towards the back of the lathe, while if the tool takes a cut the tailstock requires to be set over towards the operator. If a bar is at hand that is known to be true, a pointed tool may be adjusted to just make a mark on the end of the bar when the slide rest is traversed. On the bar being reversed, the tool should leave, when traversed along the bar, a similar mark on the bar.

To test the workmanship of the back head or tailstock, place the forefinger on the spindle close to the hub whence it emerges, and observe how much the hand wheel can be moved without moving the spindle; this will show how much, if any, lost motion there is between the screw and the nut in the spindle. Next wind the back spindle about three quarters of its length out of the tailstock, take hold of the dead centre and pull it back and forth laterally, when an imperfect fit between the spindle and the hole in which it slides will be shown by the lateral motion of the dead centre. Wind the dead centre in again, and tighten and loosen the spindle clamp, and see if doing so moves the spindle in the socket.

To examine the slide rest, move the screw handles back and forth to find how much they may be moved without giving motion to the slides; this will determine the amount of lost motion between the collars of the screws and between the screws themselves and the nuts in which they operate. To try the fit of the slide rest slides, in the stationary sliding ways or [V]s, remove the feed screws and move the slide so that only about one-half inch is in contact with the [V]s, then move the slide back and forth laterally to see if there is any play. Move the slide to the other end of the [V]s, and make a similar test, adjusting the slide to take up any play at either end. Then clean the bearing surfaces and move the slide back and forth on the [V]s, and the marks will show the fit, while the power required to move the slide will show the parallelism of the [V]s.

If the lathe carriage have a rack feed, operate it slowly by hand, to ascertain if it can be fed slowly and regularly by hand, which is of great importance. Then put the automatic feed in gear, and operate the feed gear back and forth, to determine how much it can be moved without moving the slide rest. To test the fit of the feed screw to the feed nut, put the latter in gear and operate the rack motion back and forth.

To determine whether the cross slide is at a right angle with the ways or shears, take a fine cut over a radial face, such, for example, as the largest face plate, and test the finished plate with a straight edge. If the face plate runs true and shows true with a straight edge, so that it is unnecessary to take a cut over it, grind a piece of steel a little rounding on its end, and fasten it in the tool post or clamp, with the rounded end next to the face plate. Let the rounded end be about 1/4 in.

away from the face plate, and then put the feed motion into gear, and, with the steel near the periphery of the face plate, let the carriage feed up until the rounded steel end will just grip a piece of thin paper against the face plate tight enough to cause a slight strain in pulling the paper out, then wind the tool in towards the lathe centre and try the friction of the paper there; if equal, the cross slide is true.

To find the amount of lost motion in the screw feed gear, adjust it ready to feed the saddle, and pull the lathe belt so as to revolve the cone spindle backward, until the slide rest saddle begins to move, then mark a fine line on the lathe bed making the line coincident with the end of the lathe saddle or carriage. Then revolve the cone spindle forward, and note how much the cone spindle rotates before the saddle begins to traverse.

If the lathe has an independent feed motion it may be tested in the same manner as above.

In large lathes this is of great consideration, because the work revolves very slowly, and if there is much lost motion in the feed gear, it may take considerable time after the feed is put in gear before the carriage begins to travel. Suppose, for example, a 14-foot pulley is being turned, and that the tool cuts at 15 feet per minute, it will take nearly three minutes for the work to make a revolution.

CHAPTER VIII.--SPECIAL FORMS OF THE LATHE.