Machinery's Handbook, 31st Edition
Spur Gearing
2229
former standard 14 1 ∕ 2 -degree full-depth teeth (20 diametral pitch and coarser) are given in Table 8 on page 2225 . Example: A 14-tooth, 20-degree pressure angle pinion of 6 diametral pitch is to be enlarged. What will be the outside diameters of the pinion and a 60-tooth mating gear? If the mating gear is to have the minimum number of teeth to avoid undercut, what will be its outside diameter? . . . . D P N a D P N a 2 6 14 2 6 1234 2745 2 6 60 2 6 0766 10255 pinion inches gear inches o P o G = + = + = = + = + = ^ ^ a a h h k k For a mating gear with minimum number of teeth to avoid undercut: . . D P N a 2 6 21 2 6 0766 3755 gear inches o G = + = + = ^ a h k Series of Involute, Finishing Gear Milling Cutters for Each Pitch Number of Cutter Will cut Gears from Number of Cutter Will cut Gears from 1 135 teeth to a rack 5 21 to 25 teeth 2 55 to 134 teeth 6 17 to 20 teeth 3 35 to 54 teeth 7 14 to 16 teeth 4 26 to 34 teeth 8 12 to 13 teeth The regular cutters listed above are used ordinarily. The cutters listed below (an intermediate series having half numbers) may be used when greater accuracy of tooth shape is essential in cases where the number of teeth is between the numbers for which the regular cutters are intended. Number of Cutter Will cut Gears from Number of Cutter Will cut Gears from 1 1 ∕ 2 80 to 134 teeth 5 1 ∕ 2 19 to 20 teeth 2 1 ∕ 2 42 to 54 teeth 6 1 ∕ 2 15 to 16 teeth 3 1 ∕ 2 30 to 34 teeth 7 1 ∕ 2 13 teeth 4 1 ∕ 2 23 to 25 teeth … … Roughing cutters are made with No. 1 form only. Dimensions of roughing and finishing cutters are given on page 891 . Dimensions of cutters for bevel gears are given on page 892 . Enlarged Fine-Pitch Pinions: American Standard ANSI B6.7-1977, Information Sheet A provides a different system for 20-degree pressure angle pinion enlargement than is used for coarse-pitch gears. Pinions with 11 through 23 teeth (9 through 14 teeth for 25-degree pressure angle) are enlarged so that a standard tooth thickness rack with addendum 1.05/ P will start contact 5 ° of roll above the base circle radius. The use of 1.05/ P for the addendum allows for center distance variation and eccentricity of the mating gear outside diameter; the 5 ° roll angle avoids the fabrication of the involute in the troublesome area near the base circle. Pinions with less than 11 teeth (9 teeth for 25-degree pressure angle) are enlarged to the extent that the highest point of undercut coincides with the start of contact with the standard rack described previously. The height of undercut considered is that produced by a sharp-cornered 120 pitch hob. Pinions with less than 13 teeth (11 teeth for 25-degree pressure angle) are truncated to provide a top land of 0.275/ P . Data for enlarged pinions may be found in Table 9a, Table 9b, Table 9c, and Table 9d.
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