# Re: Checking Accuracy of Pressure Angles

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Posted by BillS on June 15, 2015 at 12:11:04:

In Reply to: Re: Checking Accuracy of Pressure Angles posted by Dimebag on June 13, 2015 at 14:55:18:

Hi Guys,
Here are some thoughts from memory and there may be errors - please let me know if you see something wrong.

From a job shop perspective, identifying a gear meant (to me) knowing enough about the gear, either sample or print, to duplicate it in the shop. The next comments usually work for spur and helical cylindrical gears.

With a sample, first I roll a gear gage or hob cutter (both are rack form)in the sample to determine the pitch and pressure angle. Rolling the right hob/gage will be very smooth - if PA doesn't match this will cause small "bumping" as you roll the two together.

Next I measure the OD. If the standard OD calculation

OD=(Teeth+2)/NDP

matches the measurement, allowing for a small negative tolerance on the sample, then the rest can be based on standard, or nominal design.

If the print or sample OD measurement is noticeably larger than OD calculation, then I assume an enlarged OD. This is common on a pinion with small number of teeth.

Way back when, it was common practice to enlarge an OD by simply adding one tooth in the calculation, i.e. to be equivalent to

OD = (Teeth+3)/NDP

This reduced undercut and actually made the design, shall we say, predictable to the job shop.
(notice that there is no change in actual number of teeth).

Modern pinion design uses Enlarged Addendum or Modified Profile, which increases OD by a calculated amount to reduce or eliminate undercut. Enlarged Addendum is descriptive since pitch diameter is unchanged, and addendum is larger than standard design.

Reducing undercut is always good practice since it strengthens pinion teeth and it has no ill effect that I know of. Cutting an oversize pinion uses the same NDP/PA cutter.

If the mating gear member is large in teeth, its profile change may be very small, but to mesh on the same CC Distance, the gear OD should be be reduced by the same amount as the pinion. If CC adjustment is possible then the original gear OD should work.

Modified Profile is commonly done on all gearing (spur, helical, bevel, spiral bevel).

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