# Re: Checking Accuracy of Pressure Angles

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Posted by BillS on June 08, 2015 at 13:03:16:

In Reply to: Re: Checking Accuracy of Pressure Angles posted by cnc mod on June 07, 2015 at 09:33:36:

The angle of profile is defined as the angle between a radial and a tangent to the tooth profile. Pressure angle is defined as angle of profile where tangent contacts tooth profile at pitch circle.
Pitch circle doesn't move unless circular pitch changes. Remember that in all discussions of tooth dimensions.

But to the casual observer pitch circle appears to drop down the tooth profile when OD is enlarged. Not true. Actually, the tooth root and profile shift upward to follow an enlarged OD when the same NDP cutter is tipped off at the enlarged OD and the depth of cut (measured from OD) is the same as before. In other words, if the only changes are gear OD and location of cutter, then PA and Pitch Circle are unchanged. It may not be obvious, but enlarging the pinion and reducing the mating gear will work correctly since pitch circles of both still intersect at contact points. This is not always the case, since one member can be cut to a different PA using a different NDP/PA cutter that is equivalent in base circle. And so far, these comments only apply to hobbing with rack form cutter. Shaping is a totally different topic entirely. I won't go there except to say there are other ways to solve gear cutting problems.

So a conversation about pressure angle can be confusing since PA is not directly measureable since pitch circle is not directly measureable. Thus PA and Pitch Circle diameter are two unknowns when looking at a gear.

By making over pin measurements with different pin sizes the pitch diameter and PA can be calculated. I won't go into detail other than to say it is a commonly accepted solution using two simultaneous equations. Also, there may be solutions to PA measurement using CMM inspection that maybe someone would like to describe.

I'm not sure how this might help to answer the original question(s), other than to point out that finding PA on a part is not easily determined once a part is not cut with normal hob and/or normal OD.

Just a question or two for 'cnc mod':
What is 'a balance of creep'?
How does the software accommodate pressure angle variation?
And, what is the source of the software?

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