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curve can be drawn by either the single-curve method or, as is more usual, by stopping the curve from two or more points on this same circle.

The difference between the 141- and 20-degree pressure angles can be seen by comparing Figs. 6 and 7. Not only is the tooth shorter, but the base is broader. The base circle for the tooth arcs is found in the same way as before.

This form of tooth is largely used in automobile transmission and similar work. William Sellers & Co. use a 20 degree pressure angle with a tooth of standard length.

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ANY tooth shorter than the regular standard length is called a "stub" tooth, but like the bastard thread there have been many kinds. In 1899 the Fellows Gear Shaper Company introduced a short tooth with a 20-degree pressure angle instead of the usual 141degree. This gives a broader flank to the tooth and makes a stronger gear, especially for small pinions where strength is needed. While the Fellows tooth is shorter than the standard tooth there is no fixed relation between them, as, on account of the tooth depth graduations of the gear shaper, it was thought best to give the new tooth depth in the same scale which is shown in the following table. This means that if the pitch is 4 it has the depth of a 5-pitch standard tooth divided as shown. The clearance is one-quarter the addendum or dedendum.

TABLE OF TOOTH DIMENSIONS OF THE FELLOWS STUB-TOOTH GEAR

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The Nuttall Company also use a 20-degree stub tooth, but have a fixed length or depth in the following proportions.

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GEAR TOOTH CUTTERS

Brown & Sharpe and other makes of standard cutters are made in sets of eight cutters for each pitch and are to be used as follows: No. I will cut wheels from 135 teeth to a rack. No. 2 will cut wheels from 55 teeth to 134 teeth. No. 3 will cut wheels from 35 teeth to 54 teeth. No. 4 will cut wheels from 26 teeth to 34 teeth. No. 5 will cut wheels from 21 teeth to 25 teeth. No. 6 will cut wheels from 17 teeth to 20 teeth. No. will cut wheels from 14 teeth to 16 teeth. No. 8 will cut wheels from 12 teeth to 13 teeth.

TABLE SHOWING DEPTH OF SPACE AND THICKNESS OF TOOTH IN SPUR WHEELS, WHEN CUT WITH THESE CUTTERS.

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BLOCK INDEXING IN CUTTING GEAR TEETH

Block or intermittent indexing is a method to increase the output gear cutters by allowing the feed and cutting speed to be increased without unduly heating the work. This is done by jumping from the tooth just cut to a tooth far enough away to escape the local heating and on the following rounds to cut the intermediate teeth. While the indexing takes a trifle more time, the heat is distributed so that faster cutting can be done without heating and dulling the

cutter.

The following table gives the indexing of gears from 25 to 200 teeth and is worked out for the Brown & Sharpe gear cutter but can be modified to suit other machines.

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