# Re: barber coleman differential constant for type T

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Posted by BillS on September 29, 2007 at 20:23:46:

In Reply to: Re: barber coleman differential constant for type T posted by Drew on September 28, 2007 at 14:39:31:

Drew, I received your FAX with machine formulas (thanks!) and the differential constant for your 16-11 is:
7.5 / pi
which is:
2.387324146

If you use the formula notation used by CPC-HOB,
the formula converted from your FAX is:

NDP x Sine(helix angle)
Diff Ratio = 2.387324146 x ----------------------------
Hob Starts x Teeth

This formula is for cutting splines or helical gears without taper.

There are two angle settings on the machine: Hob swivel and Slide swivel.
Looks like Slide swivel positions the cutter to offset the inherent amount of hob cutter angle to provide cutter clearance.
Hob Swivel seems to remove the effects of axial cutter feed by adding or subtracting a small amount of differential action. If not compensated, axial cutter feed would impart a small helix angle of its own.
This is not entirely clear to me, so if you see how this works, please let me know.

Steve, Drew's FAX also shows calculations you might make to cut tapered splines by using axial infeed of a tapered hob. I always thought the purpose of axial infeed was to distribute wear and to take gradual roughing/finishing cuts on large in-fed worm gears.
But the FAXed pages from Drew indicate calculations for generating tapered splines, using a tapered cutter. This is something new to me!
The hob cutter would be specially ground with taper and decreases in numerical pitch as the hob OD increases. Also the cutter's infeed would be sync'ed with table feed. This is normally the case where table feed and hob infeed are "geared together" internally so that differential ratio can be offset by a known amount of axial infeed, or in this case the "A" Hob swivel angle compensates - otherwise the spline or helix angle would not be exactly as designed.
Cool!

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