# Re: Hobbing Prime Spurs using differential

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Posted by gearhead1 on April 03, 2013 at 13:25:22:

In Reply to: Re: Hobbing Prime Spurs using differential posted by Bill Simpson on March 29, 2013 at 12:23:46:

Example -

From that page's directions -

C = Pi/(NDP * Feed desired * sine of Helix Angle)
C = 3.1416 / (16.933333 * .019685" * sine of 15.625°)
C = 3.1416 / (16.933333 * .019685 * .26934)
C = 34.99262

K ( machine index costant) * C * hob threads
7.5 * 34.99262 * 1 = 262.44465

C * N - 1
34.99262 * 71 teeth - 1 = 2483.47602

From above - 262.44465 / 2483.47602 = .105698881

This should be the new index gears that will make a 15.625° RH Helix 71T gear of 1.5Mod ( 16.93333 DP )

Then, the new feed gears are calculated -

Feed desired / Machine's feed constant
.019685" / .8 = .02460625" ( happens to be .625mm )

So, I put those new index gears in, the new feed gears in, and the gears tha make a 15.625° LEFT hand helical gear in teh differential, and I get ... ... ...

NOT a spur gear.

I get something tat looks like a 1° LH helical...

WTF, over.

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