Posted by Ron V on April 25, 2012 at 16:11:14:
In Reply to: Re: first helical gear posted by BillS on April 24, 2012 at 17:23:55:
Youíve done an excellent job explaining these difficult to understand subjects.
Iíd just like to extend on a few points made:
Form milling (I think thatís what you mean when you talk about shaper cut gears) cutters should never be referred to as involute cutters as they are not involute. The correct terminology to use for the form used is composite. They are called composite form milling cutters because the profile is a combination of cycloidal and involute. The involute curve is used near the pitch line and as the profile extends to the tooth tip and root, the profile is cycloidal.
The pitch diameter of a gear doesn't really exist. It's purely theoretical and is therefore only useful for calculations.
What is more important and more relevant is the operating pitch diameter. The operating pitch diameter changes with the profile shift. Itís the operating PCDs that are tangentially connected when two gears of equal base pitch are meshed together.
To make things even more confusing; when gears are generated, the toolís pressure angle doesnít necessarily translate into the operating or generated pressure angle. In other words, hobbing or gear shaping a blank with a 20 degree cutter will not mean that two gears in mesh operate with a 20 degrees pressure angle. This is why profile corrected gear teeth look different to teeth that havenít been corrected.
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