When backing off the teeth for clearance by any of the means described, it is first necessary to form the blank, then to gash it or to cut the notches as described; then to back off the teeth. After backing off, it is necessary to mill the face of the tooth back 1/32 inch or so, to cut away the "jump", as it is termed, caused by the forming tool drawing in a trifle when it first strikes the edge of the tooth.
Cutters of this description are sharpened by grinding on the face of teeth, as shown in Fig. 229.
It is often necessary to make milling cutters with threaded holes. This happens in the case of small angular cutters, and in many styles of cutters for use on profiling (edge milling) machines.
The general instructions given for making the other forms of cutters apply to those with threaded holes, except that instead of reaming the hole to a given size, the thread is cut with a tap of the proper size and pitch, or it is chased in the lathe. After threading, the cutter should be screwed on to a threaded arbor. Fig. 230 shows an arbor of this description. The end A is threaded slightly tapering, for short cutters about .002 inch in one inch of length. On the taper end of the arbor, a thread should be cut of a size that will not allow the cutter to screw on the arbor quite the entire length; that is, the cutter should overhang the threaded portion of the arbor a trifle, say one thread. This allows the outer end to be squared up without mutilating the threads on the arbor. The reason for using the taper end of the arbor when squaring the first end of the cutter is that the shoulder is true with the thread in the cutter. After squaring this shoulder, the cutter blank may be removed and placed on the opposite end of the arbor with the side that has been squared against the shoulder of the arbor.
Fig. 229. Method of Grinding Formed Cutters.
Fig. 230. Typical Threaded Arbor.
This method of machining pieces of work having a threaded hole, where it is desirable that the outer surfaces be true with the hole, is applicable to all classes of work. The cutter may be machined to length and shape on the straight end of the arbor.
The simplest form of milling machine cutter is known as a fly cutter. It has only one cutting edge, but is particularly valuable when making but one or two pieces of a kind for experimental work, and when making and duplicating screw-machine and similar tools of irregular shape. As these cutters have but one cutting edge, they produce work very accurate as to shape, but they cut very slowly and do not last so long as those having more teeth. However, they are used on special work, on account of the small cost of making. It is necessary to hold the cutters in a fly-cutter arbor, Fig. 231.
The cutter to be used in a fly cutter arbor may be filed to a templet, giving the necessary amount of clearance in order that the back edge, or "heel", may not drag. If it is desirable to make the impression in the fly cutter with a milling cutter of the regular form, the piece of square steel from which the cutter is to be made may be held in the milling machine vise, and the shape cut with the milling cutter. The desired amount of clearance may be given by holding the piece in the vise at an angle of a few degrees.
To make a fly cutter from the forming tool, the piece of steel may be held in the fly cutter arbor in such a position that the face is somewhat back of a radial line, as shown in Fig. 232. After hardening, the cutter should be set so that the cutting edge will be radial, and the clearance will be as shown in Fig. 233.
Fig. 231. Fly-Cutter Arbor.
Another method of getting the clearance for the cutter is to place the top of the cutter blank as near the arbor as possible, and then to cut the desired shape. If the cutter is set in the arbor so that it projects from the surface, it will have the necessary clearance, as shown in Fig. 234. A represents the position of the blank while being cut, and B the cutter in position for cutting; as the dotted line shows the circle through which the cutting edge travels, the amount of clearance is apparent.
Fig. 232. First Step in Making Fly Cutter.
Fig. 233. Fly Cutter Set Radial with Clearance Provided.
Fig. 234. Simple Method of Getting Clearance on Fly Cutter.