Machinery's Handbook, 31st Edition
Spur Gearing
2227
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H S
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Pitch Circle
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Pitch Circle
Method of setting a gear tooth caliper Selection of Involute Gear Milling Cutter for a Given Diametral Pitch and Number of Teeth.— When gear teeth are cut by using formed milling cutters, the cutter must be selected to suit both the pitch and the number of teeth, because the shapes of the tooth spaces vary according to the number of teeth. For instance, the tooth spaces of a small pinion are not of the same shape as the spaces of a large gear of equal pitch. Theoretically, there should be a different formed cut- ter for every tooth number, but such refinement is unnecessary in practice. The involute formed cutters commonly used are made in series of eight cutters for each diametral pitch (see Series of Involute, Finishing Gear Milling Cutters for Each Pitch ). The shape of each cutter in this series is correct for a certain number of teeth only, but it can be used for other numbers within the limits given. For instance, a No. 6 cutter may be used for gears having from 17 to 20 teeth, but the tooth outline is correct only for 17 teeth or the lowest number in the range, which is also true of the other cutters listed. When this cutter is used for a gear having, say, 19 teeth, too much material is removed from the upper surfaces of the teeth, although the gear meets ordinary requirements. When greater accuracy of tooth shape is desired to ensure smoother or quieter operation, an intermediate series of cutters having half-numbers may be used provided the number of gear teeth is between the number listed for the regular cutters (see Series of Involute, Finishing Gear Milling Cutters for Each Pitch ). Involute gear milling cutters are designed to cut a composite tooth form, the center por tion being a true involute while the top and bottom portions are cycloidal. This composite form is necessary to prevent tooth interference when milled mating gears are meshed with each other. Because of their composite form, milled gears will not mate satisfactorily enough for high grade work with those of generated, full-involute form. Composite form hobs are available, however, which will produce generated gears that mesh with those cut by gear milling cutters. Metric Module Gear Cutters: The accompanying table for selecting the cutter number to be used to cut a given number of teeth may be used also to select metric module gear cutters except that the numbers are designated in reverse order. For example, cutter No. 1, in the metric module system, is used for 12–13 teeth, cutter No. 2 for 14–16 teeth, etc. Increasing Pinion Diameter to Avoid Undercut or Interference.— On coarse-pitch pin- ions with small numbers of teeth (10 to 17 for 20-degree and 10 and 11 for 25-degree pressure angle involute tooth forms) undercutting of the tooth profile or fillet interfer- ence with the tip of the mating gear can be avoided by making certain changes from the standard tooth proportions that are specified in Table 3 on page 2213 . These changes consist essentially in increasing the addendum and hence the outside diameter of the pin- ion and decreasing the addendum and hence the outside diameter of the mating gear.
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