ROBERT GIBSON GRISWOLD

The tool chest of any amateur that has any pretensions at all in the line of nice machine work is not complete without a set of lathe arbors, known by some as mandrels. It is true that this set requires the use of a grinder, but one may be easily rigged up in the tool post of the lathe that will do admirable work, but the description of it will not be given until the next issue, as the lathe work of this set will require quite as much spare time as the amateur will care to put upon it in the interim.

All accurate turning upon pieces having central holes is done on these arbors, which are forced into the hole with some pressure, and the arbor is then swung between the lathe centers, being driven by a dog fastened to one end and driven by a face plate. The very truest work may be done on them, and their convenience more than pays for the expense of making.

Opinions as to the best material for this work varies, but the writer recommends the use of a good quality of tool steel, such as may be obtained in rolled steels. It is not well to trust to case hardening in such accurate tools, and a tool steel arbor may be hardened and ground with great nicety, and it is very hard to destroy the finished surface by an accidental contact with the tool.

The sizes that will be found most useful to the amateur, together with the best lengths over all are given in the following list:

The diameter of the smaller sizes may be out of proportion for their lengths when very heavy cuts are to be taken, but if such work is intended another set of shorter arbors should be made that will stand the strain better. It is seldom necessary, however, to use the smaller sizes in heavy work.

In Fig. 1 is shown an enlarged view of how an arbor should hang on the lathe centers. The holes should be drilled deep enough so that the point of the center will not touch the bottom, thus tending to wear it to a blunt point instead of a sharp point. The depth of the taper should be such that a considerable portion of the center is taken in, so that the load will not come near the point, which in very heavy work would crack it off. Then the wearing surfaces are much larger and the wear on both the arbor bearing and the lathe center is reduced. The countersunk hole in the end does not end flush with the surface, for the following reason. When the arbor is driven into the piece to the turned, any blow on the end would cause the metal to be bruised or spread into the hole, thus causing it to run out of true. To obviate this, the end is provided with another counterbore, as shown, leaving a ridge of metal to drive on around the hole, which will prevent the hammer striking the edge of the countersink. Some mechanics prefer the edge of the hole to be rounded off in a fillet, while others prefer it terminating in a sharp edge, as shown.

The pieces of stock having been cut for the various drill in the end of the first three sizes a large 1-16 in. hole and a 3-32 in. hole in the ends of the larger sizes. Then place on the centers and take a light cut on each so that they may be run in the steady rest. Ream out the centers with a 60° reamer held in a drill chuck in the tail spindle, guiding the end of the arbor in the steady rest. Make this hole as true and smooth as possible, and with a tool relieve the metal for a distance of 3-32 in. away from the edge of the hole in the larger sizes, and about 1-16 in. in the smaller sizes. Then mount the arbors on the centers and turn to within about .012 or .015 in. of the finished size, this being left for grinding. The ends are turned about 1-16 in.

less in diameter than the body part, and a flat is filed filed on the side of each to take the dog screw. This is shown in Fig. 3.

All arbors are made with a slight taper in the body, the exact size being maintained in the middle, and one end is made about .001 in. small, and the opposite end about .001 in. large. This small taper enables a very tight driving fit to be made in the piece so that the friction will hold it securely. In the sizes above 1/2 in perhaps the difference may be made .0015 in. on eithei end. This taper need not be made in the turning, but the tail spindle should be set over sufficiently to make this difference when grinding.

Having turned all the pieces approximately to size.

they are now ready to be hardened. Take time to heat them evenly throughout and then dip them in oil instead of water. In dipping, keep them perfectly vertical and do not move them to and fro, as that will have a tendency to make them bend out of shape. Move them up and down in a vertical line, while cooling. The temper should then be drawn evenly to a dark straw color, because if they are left too hard they are liable to crack when struck, or when the load is put on them.

The holes in the ends are now to be finished very carefully, because upon the accuracy of these depends the accuracy of the grinding to a great extent. Turn up a piece of copper to exactly 60° and hold in the drill chuck in the tail spindle. Then carry one end of the arbor in the steady rest and feed the copper into the hole, first covering it with very fine emery and oil. This forms a copper lay and will lap the hole out true and round if too much pressure is not used. Finish with a little crocus or rouge until a polished hole is the result. The opposite end is, of course, supported on the live center and driven by a dog. In order that it may not slip off, a strip of rawhide lacing may be tied tightly around the dog and then to the face plate.

The next operation is grinding the body, but the description of this process will be deferred until the next chapter. Prepare two ball centers, as shown in Fig. 4, making the balls about 3-16 in. in diameter and perfect spheres. Harden them and draw their temper to a purple, and polish the surface as smoothly as possible. These are to be used in grinding the arbors and correct the necessary errors introduced by setting over the tail stock. Balls can be obtained of dealers in machinists' supplies.