Arbors for milling machines should be made from steel strong enough to resist without twisting or springing, the strain caused by tightening the nut. When a limited number of arbors are made, tool steel is generally used; but for many milling machines, necessitating a great many arbors, a lower priced steel having the necessary stiffness is selected.
Fig. 89. Mandrel Showing Two Seta of Centers.
After centering and squaring the ends, a chip is turned the entire length of the piece, to remove all the outer surface. The ends D and C, Fig. 90, are next turned to size, and the tenon milled to the desired dimensions. In milling for the tenon, the arbor should be held between centers, and the cutting done with an end mill of the form shown in Fig. 91, the circumference of the cutter leaving the proper shape at the end of the tenon. The centers should be hardened, and the temper drawn to a straw color. If the projection on the end of the arbor at C, Fig. 90, is to be run in a socket in the tail block of a milling machine, it must be hardened the entire length, in which case the thread for the nut should be cut before the end is hardened.
Fig. 90. Milling-Machine Arbor.
If a lathe having a taper attachment is used, there is no particular method of procedure other than roughing the arbor nearly to size before either the taper or the straight end is finished. It will save time, however, if the straight end A, Fig. 90, is roughed first, then the taper B roughed and finished, after which the shoulder E, and the straight part A, may be turned to size and finished. If the projection C is to run in a socket, it should be turned .010 or .015 inch above finish size, and ground to the proper dimensions. If it is necessary to use a lathe having no taper attachment, the necessary taper must be obtained by setting over the tailstock. In this case it is better to turn and fit the taper first, for otherwise the centers would become changed enough to throw the arbor out of true.
Fig. 01. Left-Hand End Mill Courtesy of Union Twist Drill Company, Athol, Massachusetts.
These instructions should be followed wherever a straight and taper surface are to be turned on the same piece of work, in a lathe having no means of turning tapers other than by setting over the tailstock. Where extreme accuracy is required, it is advisable to leave the straight and taper parts a few thousandths of an inch above size, and to grind to size all over after the spline cut is taken.
Milling-machine arbors should have a spline slot cut the entire length of the part that is to receive the cutters and this can best be done in a shaper. Before putting the arbor in the shaper vise, a hole should be drilled close to the shoulder into which the tool is to run. The drill used should be about 1/32 inch larger in diameter than the thickness of the splining tool, and the hole drilled a trifle deeper than the slot to be cut. When the arbor is placed in the vise, a piece of sheet brass or copper should be placed between the arbor and the vise jaws to prevent bruising the arbor.
The nut is usually made of machine steel, casehardened. A bar of steel 1/16 inch larger than the finish size of the nut is selected, and a piece 1/16 inch longer than finish length is cut; it is then put in a chuck on the lathe, the hole drilled, and the thread cut. If no tap of the desired size is at hand, the thread may be chased; if a tap can be obtained, the thread should be chased nearly to size and finished with the tap. Before being taken from the chuck, the end of the nut should be faced, and the hole recessed to the depth of the thread for a distance of two threads; after being removed from the chuck, it should be placed on a threaded mandrel the threaded portion of which fits the thread in the nut. The nut should be turned to size and length, and the two opposite sides milled to receive the wrench used in tightening. Fig. 92 gives two views of the nut. It should be made and casehardened before the thread is cut on the arbor, in order that the thread may be made to fit thenut. Milling-machine arbor nuts should fit the thread on the arbor in such a manner that they may be turned the entire length of the thread without the aid of a wrench, yet not be loose.