Modern Machine-Shop Practice Part 59

[Ill.u.s.tration: Fig. 795.]

When a mandrel is fitted to the sockets for the lathe centre, it should have a thread and nut, as shown in Fig. 795, so as to enable its extraction from the socket without striking it, as has been described with reference to lathe centres.

[Ill.u.s.tration: Fig. 796.]

[Ill.u.s.tration: Fig. 797.]

Mandrels may be employed to turn work, requiring its outside diameter to be eccentric to the bore, by the following means:--In Fig. 796, let the centre C represent the centre of the mandrel, and D a centre provided in each end of the mandrel, distant from C to one half the amount the work is required to be eccentric. The mandrel must be placed with the centres D receiving the lathe centres. In this operation great care must be taken that a radial line drawn on each end of the mandrel, and pa.s.sing through the centre of the centres D, shall exactly meet and coincide with the line L drawn parallel to the axis of the mandrel. If this be not the case the work will be less eccentric at one end than at the other. As it is a somewhat difficult matter to test this and ascertain if the mandrel has become out of true from use, it is an excellent plan to turn such a mandrel down at each end, as shown in Fig. 797, and draw on it the lines L, L, which correspond to the line L L in Fig. 796. If then a steel point be put in the lathe rest and fed in to the work, so that revolving the latter just causes the tool point to touch the lines L at each end, or if the tool point makes long lines as at _a_, _a_, the two lines L, L, should intersect the lines _a_, _a_ at the centre of their respective lengths. The lines L L should be marked as fine as possible, but deep enough to remain permanently, so that the truth of the eccentricity of the mandrel may be tested at any time. An equivalent device is employed in turning the journals of crank shafts, as is shown in Figs. 798 and 799, in which D, D are two pieces fitted on the ends of the crank shaft, being equal in thickness to the crank throw, as shown at A, B in the figure, so that when D, D lie in the same plane as the crank cheeks (as when all will lie level on a plate, as in the figure) the centres C will be in line with the journal in the crank throw.

Pieces D are broadened at one end to counterbalance the weight of the crank, which will produce more true work than counterbalancing by means of weights bolted to the face plate of the lathe, as is sometimes done, causing the crank throw to be turned oval instead of round. In the case of a double crank, however, the centre pieces cannot be widened to counterbalance, because what would counterbalance when the centres A in Fig. 799 were used, would throw the crank more out of balance when centres B were used for the throw B. In this case, therefore, the centre pieces are provided with seats for the bars E, E, which may be bolted on to carry the counterbalancing weights, the bars being changed on the centre pieces when the centres are changed. The bars, for example, are shown in their position when the centres A are being used to turn up the journal A, the necessary amount of weight for counterbalancing being bolted on them with a set-screw through the weight.

[Ill.u.s.tration: Fig. 798.]

[Ill.u.s.tration: Fig. 799.]

The centres are steel plugs screwed tightly into the pieces D, and are hardened after being properly centre-drilled and countersunk.

[Ill.u.s.tration: Fig. 800.]

To enable the pieces D to be easily put on and taken off, it is a good plan to make the bore a tight fit to the shaft and then cut it away as at E, as shown in Fig. 801, using set-screws to hold it.

[Ill.u.s.tration: Fig. 801.]

Great care is necessary in putting in the work centres, since they must, if the crank throws are to be at a right angle one to the other, as for steam engines, be true to the dotted lines in figure, these dotted lines pa.s.sing through the centre of the axle and being at a right angle one to the other. If the thickness of the centre pieces are greater than the crank throws they may be adjusted as in Fig. 800, in which B, B'

represent the centre pieces, and C the crank, while S is a straight-edge; the edge surfaces of B, B being made true planes parallel to each other on each arm, and parallel to the axial line of the bore fitting the end of the crank axle.

The straight-edge is pressed at one end, as at F, firmly to an edge face of B, the other end being aslant so as not to cover the edge of the piece B' at the opposite end of the crank (as shown at G, Fig. 801).

While being so pressed the other end must be swung over the end arm of B' at the opposite end of the crank, when the edge of the straight-edge should just meet and have slight contact with the surface of the edge of B'. This test should be applied to all four edges of B, and in two positions on each, as at G, H--I, J, and for great exact.i.tude may be applied from each end of the crank. It is to be observed, however, that the tests made on the edges standing vertical, as at I, J, will be the most correct, because the straightness of the straight-edge is when applied in those positions not affected by deflection of the straight-edge from its own weight.

In shops where such a job as this is a constantly recurring one attachments are added to a press of some kind, so that the axle and the pieces B may be guided automatically and forced to their proper places, without requiring to be tested afterwards.

[Ill.u.s.tration: Fig. 802.]

When the work is sufficiently long or slender to cause it to sag and bend from its own weight, or bend from the pressure of the cut, it is supported by means of special guides or rests. Fig. 802 represents a steady rest of the ordinary pattern; its construction being as follows:--F is a base fitting to the [V]s of the lathe shears at F, and capable of being fastened thereto by the bolt C, nut N, and clamp A. F'

is the top half of the frame, being pivoted at P to F, the bolt P'

forming the pivot for both halves (F and F'), of the frame, which may be secured together by the nut of P'. On the other side of the frame the bolt is pivoted at _b_ to F. This bolt pa.s.ses through an open slot in F', so that its nut being loose, it may swing out of the way as denoted by the arrow _e_, and the top half frame _f'_ may be swung over in the direction of arrow _g_, the centre of motion or pivot being on the bolt P'. With F' out of the way the work may be placed within the frame, the nut of B and also that of P' may be tightened up so as to lock the two halves of the frame firmly together.

On this frame and forming a part of it are the three ways, G G' G", which contain cavities or slide ways to which are fitted and in which may slide the respective jaws J, and to operate these jaws are the respective square-headed screws S, which are threaded through the tops of the respective ways G, G', and G". The screws are operated until the ends of the jaws J have contact with the work W, and hold it axially true with the line of centres of the lathe, or otherwise, as the nature of the work may require. When adjusted the jaws are locked to the frame by means of the bolts D, which are squared to fit in the rectangular openings, shown at _h_ in the respective jaws, so as to prevent the bolts from rotating when their locking nuts _d_ are screwed home.

As an example of the use of this device as a steadying rest, suppose a long shaft to require turning from end to end and to be so slight as to require steadying, then a short piece of the shaft situated somewhat nearer the live centre than the middle of the length of the work is turned upon the work, so that this place shall be round and true to receive the jaws, or plates _p_, and revolve smoothly in them. The jaws are then adjusted to fit the turned part a close sliding fit, but not a tight fit, as that would cause the jaws to score the work. To prevent this even under a light pressure of contact, oil should be occasionally supplied. This steadies the work at its middle, preventing it from springing or trembling when under the pressure of the cut.

By placing the steady rest to one side of the middle of the work length, at least one half of that length may be turned before reversing the work in the lathe centres. After reversing the work end for end in the lathe centres, the jaws, or plates _p_, are adjusted to the turned part, and the turning may be completed.

In adjusting the plates _p_ to the work, great care is necessary or they will spring the work out of its normal line of straightness, and cause it to be out of parallel, or to run out of true in the middle of its length, as explained in the remarks referring to the cat head shown in Fig. 809.

The plates _p_ should be gripped to the frame by the nuts with sufficient force to permit them to be moved by the set-screw S under a slight pressure, which will help their proper adjustment. They should also be adjusted to just touch the work, without springing it, the two lower ones being set up to the work first, so that their contact shall serve to relieve the work of its spring or deflection, due to its own weight. This is especially necessary in long slender spindles, in which the deflection may occur to a sensible degree.

If the work does not require turning on its full length, the steady rest may be applied but a short distance from the length of the part to be turned, so as to hold the work more steadily against the pressure of the cuts.

Steady rests are often used to support the end of work without the aid of the dead centre, but it is not altogether suitable for this cla.s.s of work, because it has no provision to prevent the work from moving endways and becoming loose on the dead centre. A provision of this kind is sometimes made by tying the work driver to the face plate or to the pins driving the work driver or dog, or bolts and plates holding the work driver towards the lathe face plate; but these are all objectionable in that unless the pressure thus exerted be equal, it tends to spring or bend the work.

Another method of preventing this is to drive the work by means of a universal chuck; but this again is objectionable, because the jaws of these chucks do not keep dead true under the wear, and indeed if made to run concentrically true (in cases where the chuck has provision for that purpose) the gripping surfaces of the chuck jaws have more wear at the outer than at the inner ends, hence those surfaces become in time tapering. Again the jaws wear in time so easy a fit in their radial slots that they spring under pressure, and the wear not being equal, the amount of spring is not equal, so that it is impracticable to do dead true work chucked in this way.

The reasons that the chuck jaws do not wear equal in the radial slots may be various, as the more frequent presence of grit in one than in the other, less perfect lubrication, inequalities in the fit, less perfect cleaning, and so on, so that it is not often that the wear is precisely equal. In addition to these considerations there are others rendering the use of the steady rest in some cases objectionable; suppose, for example, a piece of cylindrical work, say 6 feet long, to have in one end a hole of 2 inches diameter, which requires to be very true (as, for example, the cone spindle for a lathe). Now let the face plate end be driven as it may, it will be a difficult matter to set the steady rest so as to hold the other end of the work in perfect line, so that its axial line shall be dead true with the line of lathe centres, because the work will run true though its axial line does not stand true in the lathe.

Here it may be added that it will not materially aid the holding of the work true at the live centre end, by placing it on the live centre and then tightening the universal chuck jaws on it, because the pressure of those jaws will spring it away to some extent from the live centres.

This will occur even though the work be placed between the two lathe centres, and held firmly by s.c.r.e.w.i.n.g up the dead centre tight upon the work, before tightening the chuck jaws upon the work, because so soon as the pressure of the dead centre is removed, the work will to some extent relieve its contact with the live one.

If the jaws of the chuck are not hardened, they may be trued up to suit a job of this kind as follows:--A ring (of such a size that when gripped in the outer steps of the chuck jaws, the inner steps will be open to an amount about equal to the diameter of the work at the live centre end) may be fastened in the chuck, and the inner ends of the jaws may be turned up with a turning tool, in which case the jaws will be made true while under pressure, and while in the locations upon the chuck in which they will stand when gripping the work, under which conditions they ought to hold the work fairly upon the live centre. But even in this case the weight of the work will aid to spring it, and relieve it from contact with the live centre.

[Ill.u.s.tration: Fig. 803.]

Now let us suppose that the piece of work is taper on its external diameter at each end, even truing of the chuck jaws will be of no avail, nor will the steady rest be of avail, if the taper be largest at the dead centre end. Another form of steady rest designed to overcome these objectionable features is shown in Fig. 803. In this case the stand that is bolted to the lathe bed is bored to receive a ring. This ring is made with its middle section of enlarged diameter, as denoted by the dotted circle C. Into the wide part of the stand fits a ring F, its external diameter fitting into C. The ring carries the jaws, hence the ring is pa.s.sed over the work, and is then inserted into the stand, while the work is placed between the lathe centres.

The ring revolves with the work and has journal bearing in the stand, the enlarged diameter C preventing end motion. There is nothing here to take up the lost motion that would in time ensue from the wear of the radial faces of the ring, hence it is better to use the cone-plate shown in Fig. 805.

[Ill.u.s.tration: Fig. 804.]

When, however, the work will admit of being sufficiently reduced in diameter, it may be turned down, leaving a face F in Fig. 804, that may bear against the radial faces of the jaws of the steady rest; or a collar may be set upon the work as in Fig. 804 at C. But these are merely makeshifts involving extra labor and not producing the best of results, because the radial face is difficult to keep properly lubricated, and the work is apt to become loose on the live centre.

[Ill.u.s.tration: Fig. 805.]

For these reasons the cone plate shown in Fig. 805 is employed; A is a standard fitting the shears or bed of the lathe and carrying the circular plate C by means of the stud B, which is fitted so as to just clamp the plate C firmly to the frame A when the nut of B is screwed firmly home with a wrench.

The plate C contains a number of conical holes, 1, 2, 3, &c., (as shown in section at D) of various diameters to suit varying diameters of work.

The frame is fitted to the lathe bed so that the centre stud B stands sufficiently out of the line of lathe centres to bring the centres of the conical holes true with the line of lathe centres. The centres of the conical holes are all concentric to B. Around the outer diameter of the cone plate are arranged taper holes G, so situated with reference to the coned holes that when the pin, shown at G in the sectional view, will pa.s.s through the plate and into the frame A as shown, one of the coned holes will stand axially true with the line of lathe centres.

Hence it is simply necessary to place one end of the work in the live centre, with a work driver attached in the usual manner; to select a coned hole of suitable size; to move the frame A along the lathe bed until it supports the overhanging end of the work in a suitably sized coned hole without allowing the work any end motion, and to then fasten the frame A to the lathe bed, and the work will be ready to operate on.

The advantages of this device are that the pin shown at G in the sectional view holds the conical hole true, and thus saves all need of adjustment and liability to error, nor will the work be sprung out of true, furthermore the tool feed may traverse back and forth, without pulling the work off the live centre. With this device a coa.r.s.e pitch left-hand internal thread may be cut as easily as if it were an external thread and the work was held between the lathe centres, heavy cuts being taken which would scarcely be practicable in the ordinary form of steady rest.

The pins B and G and the coned holes should be of cast steel hardened, so as to avoid wear as much as possible. The plate may be made of cast iron with hardened steel bushes to fit the coned holes.

It is obvious that the radial face of the work at the cone plate end, as well as the circ.u.mference, must be trued up, so that the work end may have equal contact around the bore of the coned rings.

[Ill.u.s.tration: Fig. 806.]

[Ill.u.s.tration: Fig. 807.]

Figs. 806 and 807 represent a cla.s.s of work that it would be very difficult to chuck and operate on without the aid of a cone plate. The former requires to have a left-hand thread cut in its bore A, and the latter a similar thread in end A. A universal chuck cannot be used to drive the work, because in the former case it would damage its thin edge, and in the latter the jaws would force the work out of the chuck; a steady rest cannot be used on the former on account of its being taper, while if used on the latter there would be nothing to prevent the work from moving endwise, unless a collar be improvised on the stem, which on account of the reduced diameter of the stem would require to be made in two halves. It can, however, be driven on the live centre by a driver or dog, and supported at the other end by the cone plate without any trouble, and with an a.s.surance of true work.

[Ill.u.s.tration: Fig. 808.]

Fig. 808 represents a form of steady rest designed by Wm. MacFaul, of the Freeland Tool Works, for taper work. The frame affords journal bearing to a ring A, having four projections B, to which are a close but easy sliding fit, the steadying jaws C. These are held to the work or cue blank W by the spiral springs shown in the projections or sockets B, which act against the ends of C. It will be observed that the work being square could not move in any direction without moving sideways the two of the steadying jaws C which stand at a right angle to that direction.

But the jaws C fit the bore of the sockets, and cannot, therefore, move sideways; hence it is evident that the work is firmly supported, although the steadying jaws are capable of expanding or contracting to follow the taper of the blank cue or other piece of work. This enables the steady rest to lead the cutting tool instead of following it, so that the work is steadied on both sides of the tool. Obviously, the stand may be fastened to the leading side of the lathe carriage or fitted upon the cross-slide, as may be most convenient.

[Ill.u.s.tration: Fig. 809.]

To steady work that is unturned and of so great a length that it springs too much to permit of its being turned true, the sleeve or cat head shown in Fig. 809 is employed; it may contain three or four screws C, to true it upon the work. The body B is turned true.