Hollow mills are used in screw machines and turret lathes for roughing down and finishing. They are also used in drill-press work for finishing a projection which must be in some given position; in the latter case, they are generally guided by a bushing in a fixture, to bring the projection into the proper location.
For roughing out work on a screw machine or turret lathe, solid mills having strong stubbed teeth are preferred because of their rigidity. For finishing, they are made adjustable in order to get exact sizes. Fig. 162 shows a plain hollow mill having the cutting end hollowed out in the form of a V, in order that it may center itself when starting to cut. Fig. 163 shows a form of plain hollow mill intended for use in squaring up a shoulder at the end of a cut that has been made with a mill of the form shown in Fig. 162, or it may be used for roughing out a piece, but it will not center itself so readily as the former one. For small hollow mills, some tool-makers advise three cutting teeth, while others contend that better results are secured with four teeth on all sizes.
Fig. 162. Hollow Mill with V-End.
Fig. 163. Hollow Mill for Squaring Up Shoulder.
Fig. 164. Hollow Mill with Tapered Hole.
The rear end of the mill is bored somewhat larger than the cutting end, to allow it to clear on long cuts. The cutting end must be relieved, or it will bind and rough the work and probably twist it off in the mill. There are several methods of relieving mills; the most common one is to ream the hole tapering, making it larger at the back end, as shown in Fig. 164. Another method is to file back of the edges, as shown in Fig. 165.
For making several hollow mills having the same outside diameter, it is advisable to use a holder of the form shown in Fig. 166, which has a taper shank that fits the spindle of a lathe. The hole in the other end of the holder should be the size of the holder in the screw machine or turret lathe, which holds the mills when in use. The steel for the hollow mills should be cut to length, and turned to the proper diameter to fit the holder. After putting the blank in the holder, the ends may be squared, and the holes drilled and bored to the desired sizes. If the mill is to be one of the forms shown in Figs. 162, 163, and 164, the cutting end may be reamed with a taper reamer to give the necessary clearance. The reamer should be run in from the back end in order that this end may be larger. For the form shown in Fig. 164, the hole at the cutting end should be straight and of finish size. Cutting Teeth. The milt is now ready for cutting the teeth. If four cutting edges are to be given, a side milling cutter may be used, of a diameter about double the diameter of the hollow mill to be cut. The blank should be held in a chuck on the end of the spindle in the spiral head. For a strong tooth, the spiral head should be set at an angle that will produce the tooth shown in Fig. 167, by feeding the milling cutter through the blank. If a deeper tooth is desired, the spiral head must be set so that the blank will be in a vertical position, and the milling cutter fed in until the desired form and depth of tooth are obtained.
Fig. 165. Hollow Mill with Edges Filed.
Fig. 198. Holder for Hollow Milk.