As a correctly formed bevel gear tooth has the same sectional shape throughout its length but on a diminishing scale from the large to the small end, it is not possible to cut bevel gears properly by using a milling cutter. Finish milling of bevel gear teeth with a simple disc cutter is neither economical nor accurate. It may be used in the case of emergency, repairs etc.
A milling cutter simply reproduces its shape and if it was to have the exact curvature required at the large end of the tooth, then it would not be suitable for any other portion of the gear and the error would be considerable at the small end of the tooth.
Consequently, accurate bevel gears are cut by a generating process in order to form teeth having the proper curvature on a diminishing scale, the tooth tapering towards the apex of its pitch cone. However, bevel gears may be cut by means of formed milling cutters, in connection with repair or miscellaneous work. These milled bevel gears are sufficiently accurate if the speeds are relatively low, but these are unsatisfactory for high speeds and where accurate tooth curves are of importance.
In cutting a bevel gear on a milling machine, the gear blank is mounted on a mandrel inserted in the dividing head spindle, and the latter is set to the cutting angle (of pitch cone angle minus the dedendum angle). A formed cutter (which is usually made thinner than a spur gear cutter because it must pass through the narrow tooth spaces at the inner ends of the teeth) is used, and it is necessary to take two cuts through each tooth space, with the gear blank slightly off centre, first on one side and then on the other to obtain a tooth of approximately the correct form. The gear blank is also rotated proportionately to obtain the proper tooth thickness at the large and small ends.
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Most of the helical or spiral gears are cut in regular gear cutting machines of the hobbing type, although milling machines are sometimes used either in connection with jobbing work or in shops where such gear cutting operations are done on a small scale.
Fig. 31.2 shows a set up for cutting of spiral gear of angle greater than 45° on milling machine with milling cutter.
Worm gears may be cut on a milling machine by first milling gashes or grooves around the worm gear blank to form the teeth roughly and then using a hob for finishing. Gashing is done preferably the use of an involute spur gear cutter of a number and pitch corresponding to the number and pitch of the teeth in the worm-gear.
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The cutter used for gashing is held on an arbor in the usual way, and the gear blank is also mounted on an arbor which is supported between the centres of a dividing head. The cutter should be centered in both crosswise and lengthwise directions relative to the gear. It is also necessary to locate the table at an angle so that the inclination of the gashes will correspond to the lead or helix angle of the worn thread.
After the gashes have been cut, the machine table is set at right angles to the cutter spindle, and the gashing cutter is replaced by a hob. The dog is removed from the work arbor or the dividing head gearing is disengaged to permit free rotation of the spindle and the hob is placed in mesh with the gashes in the blank. As the hob is revolved, the worm-gear revolves with it. As this occurs, the gear blank is gradually elevated so that teeth are formed on it which mesh accurately with those of the hob.
Fig. 31.3 shows the operation of hobbing worm wheel in milling machine after gashing.
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Fig. 31.4 shows worm-wheel hob cutter used for single thread worms having small helix angles only. This type of cutter is usually set with its axis parallel to the axis of the work.
