279,166. Wheatley, W. H. July 21, 1926. Milling-cutters.-In a rotary helical cutter for cutting the teeth of straight or spiral bevel gears and the like in a single continuous operation such as that described in Specification 214,738, [Class 83 (iii), Metals, Cutting], the teeth 10, Figs. 1 and 2, are arranged in a helical path with cutting edges 9 on each side, the cross sections of the teeth gradually diminishing in size from the centre of the helix towards the two ends thereof, and the pitch of the helix being equal to or less than the pitch at the smaller end of the bevel wheel. The teeth may extend completely round the cutter. In a method of making a cutter of this form, a master bevel wheel 11 of the form which the cutter is to be designed to produce is first cut on any suitable machine and the teeth of this wheel cross cut by staggered serrations and relieved to form cutting edges after which the master wheel is hardened and preferably finished by grinding. A cutter blank is then formed with a rough helix 14 corresponding to the small-end pitch of the gear to be cut and the helix is then cross cut to provide the required number, preferably an odd number, of teeth. The cutter and master wheel are then placed in cutting relation with the master wheel, Fig. 6, and the cutter and wheel rotated together at the required speed ratio while the cutter is fed slowly across the wheel face, its axis moving in a plane which is parallel to a plane tangential to the cone containing the bottoms of the wheel teeth. In this way the cutter is formed to the correct shape for generating wheels similar to the master wheel and the teeth are then relieved by specially shaped relieving cutters so that the correct shape is retained when the teeth are reground. If desired the cross cutting of the cutter teeth may be performed after the shaping of the helix. If the cutter is intended for cutting spiral teeth it is offset in relation to the master wheel as shown in Fig. 8, the master wheel preferably being of spiral form although a straight-toothed master wheel may also be used. The finished helix on the cutter may be further shaped to form a cutter of the type described in Specification 214,738.
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