# Re: Helical Gears

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Posted by BillS on February 16, 2008 at 21:37:08:

In Reply to: Re: Helical Gears posted by shawn cooper on February 15, 2008 at 16:09:32:

I think a constant value is missing in your formula for differential ratio, N/TL. For most B-C machines, the formulas for differential look like this:

0.15915494 x Gear Teeth
------------------------------
Gear Lead x Hob Starts

This is what I call a type "A" machine, and the actual constant value (Diff Constant) can differ by machine model.

There are other B-C's that use the following formula:

Diff Constant
------------------------------
Gear Lead x Hob Starts

and I call these machines type "B".

Assuming that N/TL is correct as the basic formula (plus a constant, of course) for the 10-20, that would make it a type "A" machine.

Are you sure of the N/TL formula, or did you take that from the 14-15? A 14-15 is a type "A" by the way...

look at the gear FAQ to see the discussion of differential formulas and values for Diff Constant for B-C machines. Types and formulas are described at the end of the table.

Are you using the correct index constant for the 10-20? Index may not be the same for both machines.

Does it cut all teeth of in a few rounds, or does it seem to be cutting the wrong helix angle?

If it cuts teeth off in a few rounds, look for a problem with the index.

If it cuts the wrong helix, I suspect the index is correct, but the diff. gear set is wrong. Try playing with the formulas above and let us know what happened.

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