Modern Machine-Shop Practice Part 58

[Ill.u.s.tration: Fig. 771.]

[Ill.u.s.tration: Fig. 772.]

When a piece of work to be turned between the lathe centres is of such a form that there is no place to receive centres, provision must be made to supply the deficiency.

[Ill.u.s.tration: Fig. 773.]

In Fig. 773, for example, a temporary centre B is fitted into the socket to receive the centre.

In small work that has been drilled or bored, a short mandrel is used instead of the piece B.

[Ill.u.s.tration: Fig. 774.]

If a half-round piece is to be turned it should be forged with a small projecting piece to receive the lathe centre, as in Fig. 774.

[Ill.u.s.tration: Fig. 775.]

When the end of the work is flat and not in line with the axial line of the main body of the work, a piece of metal to contain the centre may be held to the work by a driving clamp, as in Fig. 775, in which A represents the end of the work and B a temporary piece containing the centre C. In this case it is best to make the centre C after the piece B is clamped to the work.

[Ill.u.s.tration: Fig. 776.]

To provide a temporary centre for a piece having a taper hole, a taper plug is used, as shown in Fig. 776, W representing the work and P the plug, which must be an accurate fit to the taper of the hole, and must not reach to the bottom of the hole.

MANDRELS OR ARBORS.--Work (of about 6 inches and less in diameter) that is bored is driven by the aid of the mandrel or arbor, which is held between the lathe centres, as in Fig. 777, in which W represents a washer and M the mandrel, driven into the washer bore so as to drive it by friction. At A is a flat place to receive the set-screw of the driver or lathe dog, and at B a flat place upon which the diameter of the mandrel is marked. The mandrel diameter is made slightly larger at D than at C, so as to accommodate any slight variation in the diameter of holes bored by standard reamers, which gradually reduce in diameter by wear; thus if a reamer be made 1-1/1000 inch diameter, with a limit of wear of 1/1000 inch, then the mandrel may be made 1 inch at C and 1-1/1000 inch at D. It is well to taper the end of the mandrel from C to E about 1/2000 inch, so that it may enter the work easily before being driven in. Instead, however, of driving mandrels into work, it is better to force them in under a press. If driving be resorted to a lead hammer, or for very light mandrels a raw-hide mallet, may be used.

[Ill.u.s.tration: Fig. 777.]

[Ill.u.s.tration: Fig. 778.]

In the absence of a lead hammer, a driver, such as in Fig. 778, is a good subst.i.tute, consisting of a socket containing babbitt or some other soft metal at B (the mandrel being represented by M). If copper be used instead of babbitt a hole may be drilled through it, as denoted by the dotted lines.

[Ill.u.s.tration: Fig. 779.]

[Ill.u.s.tration: Fig. 780.]

The centres of mandrels should either have an extra countersink, as at A in Fig. 779, or else the cut should be recessed as at B, Fig. 780.

Mandrels are best made of steel hardened and ground up after hardening.

If the bore of the work is coned, and of too great a cone to permit the mandrel to be driven, and drive the work by friction, the cone mandrel shown in Fig. 781 may be used. M is the mandrel in one piece with the collar C. The work W is held between two cones A, A, which slide a close fit upon the mandrel, and grip the work by s.c.r.e.w.i.n.g up the nut N, there being a thread upon the mandrel, as at S, to receive the nut. It is obvious, however, that work having a parallel bore may also be held by the cone mandrel, as shown in Fig. 782.

[Ill.u.s.tration: Fig. 781.]

[Ill.u.s.tration: Fig. 782.]

To obviate the necessity of having the large number of mandrels that would be necessary so as to have on hand a mandrel of any size that might happen to be required, mandrels with provision for expanding or contracting the diameter of the parts used to hold the work are made.

[Ill.u.s.tration: Fig. 783.]

Thus in Fig. 783 is shown Le Count's expanding mandrel, in which G H is the body of the mandrel, turned parallel along a certain distance, to fit the bore of the sleeve A, which is a close-sliding fit on this parallel part of E.

From the end H of the mandrel there extends towards the end G four dovetail grooves, which receive four keys B. The heads of these four keys are enclosed and fit into an annular groove provided in the head C of the sleeve A, so that moving the sleeve A along the mandrel causes the four keys to slide simultaneously in their respective grooves.

Now these grooves, while concentric at any one point in their transverse section to the axis of the mandrel, are taper to that axis, so that sliding the sleeve A along the parallel part of the mandrel increases or decreases (according to the direction in which A is moved) the diameter of the keys.

If the sleeve be moved towards the end G, the keys while sliding in their taper grooves recede from the axis of the mandrel, while if moved towards H they approach the axis of the mandrel, or what is the same thing, if the sleeve be held stationary and the body of the mandrel be moved, the keys open or close in diameter in the same manner; hence all that is necessary is to insert the mandrel in the bore of the work, and drive the end G, when the keys will expand radially and grip the work bore.

The keys, it will be observed, are stepped on their diametral or work-gripping surfaces, which is done to increase the capacity of the tool, since each step will expand to the amount equal to the whole movement of the keys in their grooves or slots.

Mandrels or arbors are sometimes made adjustable for diameter by forcing a split cone upon a coned plug, examples being given in the following figures, which are extracted from _Mechanics_. In Fig. 784, A is a cone having the driving head extending on both sides of the centre so as to balance it. Over its coned body fits the sh.e.l.l B, which is split, as shown in Fig. 785, the splits C, D being at a right angle to splits E, F.

It is obvious that the range of adjustment for such a sh.e.l.l is small, but several diameters of sh.e.l.l may be fitted to one cone, the thickness being increased to augment the diameter. The diameter of the sh.e.l.l should be made to enter the work without driving, the tightening being effected by s.c.r.e.w.i.n.g the nut up to force the sh.e.l.l up the cone.

[Ill.u.s.tration: Fig. 784.]

[Ill.u.s.tration: Fig. 785.]

[Ill.u.s.tration: Fig. 786.]

[Ill.u.s.tration: Fig. 787.]

[Ill.u.s.tration: Fig. 788.]

[Ill.u.s.tration: Fig. 789.]

[Ill.u.s.tration: Fig. 790.]

Figs. 786, 787, 788, and 789 represent an expanding mandrel designed by Mr. Hugh Thomas, of New York City. The body B of the mandrel is provided with a taper section _g_, and either three or four gripping pieces _a_, _a_, _a_, _a_, let through mortises or slots in a sleeve C, which fits the body of the mandrel at each end.

This sleeve when forced up the mandrel by the nut D, carries the gripping pieces along the cone at _g_, and causes them to expand outwards and grip the bore of the work, which is shown in the end view in Fig. 788 to be a ring or washer W.

The advantage of this form is that the cone at _g_ can be easily turned or ground to keep it true, and the gripping pieces _a_ may be fastened in their mortises by means of the screws shown at _h_ in the end view, and thus kept true. It is obvious that for long work there may be gripping pieces at each end of the mandrel, as in Fig. 789, and the work will be held true whether its bore be parallel, stepped, or taper, a valuable feature not usually found in expanding mandrels.

[Ill.u.s.tration: Fig. 791.]

When a mandrel is used upon work having its bore threaded the mandrel also must be threaded, and must abut against a radial face, as at _a_, in Fig. 790, because otherwise the pressure of the cut would hold the work still while the mandrel revolved, thus causing the work to traverse along the mandrel. If the thread of the mandrel be made so tight a fit that it will drive the work by friction it will require considerable force to remove the work from the mandrel, so much so, in fact, that finished pieces would be much damaged in the operation. It is better therefore to have the work such a fit that it can be just screwed home against the radial face of the mandrel under heavy hand pressure (if the work be not too heavy for this, in which case a clamp may be employed).

Small work, as nuts, &c., are turned on a mandrel of this kind, which has a stem, and fits into the cone or live spindle in the same manner as the live centre, which will drive work up to about 1 inch in diameter without fear of slipping. Threaded mandrels that are in frequent use soon become a loose fit to the work by reason of the thread wear, with the result that if the face of the work is not true with the thread, it meets the mandrel shoulder, as in Fig. 791, and as the nut cants over, one side as T in the figure, is turned too thick. When the nut is reversed on the mandrel, the turned face will screw up fair against the mandrel shoulder, and the faces of the nut, though true one with the other, are not square with the axis of the thread, and will not therefore bed fair when placed in position upon the work.

[Ill.u.s.tration: Fig. 792.]

To obviate this difficulty we have Boardman's device, which is shown in Fig. 792. It consists of a threaded mandrel provided with a ring, with two rounded projections A, A and B, B, on each radial face, those on one side being at a right angle to those on the other. This ring adapts itself to the irregular surface of the nut and by equally distributing the pressure on each side of the nut destroys the tendency to cant over, hence the nut may be turned true, notwithstanding any irregularity of its radial faces, and independently of its fitting the arbor or mandrel thread tightly.

[Ill.u.s.tration: Fig. 793.]

Another form of mandrel for the same purpose is shown in Fig. 793, the mandrel being turned spherical, instead of having a square shoulder, and the washer W being cupped to fit, so that the washer will cant over and conform to the nut surface.

[Ill.u.s.tration: Fig. 794.]

The mandrel thread may be caused to fill the nut thread better if it be provided with three or more splits A, B, C, Fig. 794, a hole D being drilled up the centre of the mandrel, the thread may then be turned somewhat large, the splits permitting the thread to close from the nut thread pressure.