Page:Practical Treatise on Milling and Milling Machines.djvu/161

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Twice four is 8, and 8 x 8 is 64, from which it can be seen that the cutter must be of Shape No. 2, as 64 is between 55 and 134, the range covered by a No. 2 cutter.

The number of teeth for which the cutter should be selected can also be found by the following formula:

$\tan \propto ={\frac {Na}{Nb}}$

No. of teeth to select cutter for gear $={\frac {Na}{\cos \propto }}$

No. of teeth to select cutter for pinion $={\frac {Nb}{\sin \propto }}$

If the gears are mitres or are alike, only one cutter is needed; if one gear is larger than the other, two may be needed.

Setting Cutter out of Centre. As the cutter cannot be any thicker than the width of space at small end of teeth, it is necessary to set it out of centre and rotate the blank to make the spaces of the right width at the large end of the teeth.

The amount to set cutter out of centre can be calculated with the table on page 326 and the following formula:

Set-over = Tc/2 - factor from table/P

P = diametral pitch of gear to be cut.

Tc = thickness of cutter used, measured at pitch line.

Given as a rule, this would read: Find the factor in the table corresponding to the number of the cutter used and to the ratio of apex distance to width of face; divide this factor by the diametral pitch and subtract the quotient from half of the thickness of the cutter at the pitch line.

As an illustration of the use of this table in obtaining the set-over, take the following example: A bevel gear of 24 teeth, 6 pitch, 30 degrees pitch cone angle and 1 1/4" face. These dimensions call for a No. 4 cutter and an apex distance of 4 inches.

In order to get the factor from the table, the ratio of apex distance with length of face must be known. This ratio is 4/1.25 = 3.2/1, or about 3 1/4/1. The factor in the table for this ratio with a No. 4 cutter is 0.280. Next, measure the cutter at the pitch line. To do this, refer to the regular "Table of Tooth Parts" on pages 323 and 324, and get the depth of space below pitch line s + f. This depth of space below pitch line can also be found by dividing 1.157 by the diametral