It Gets Complicated

Why did something as simple as a standard tooth form fail to get traction? While the old joke about the number of opinions at any meeting of engineers being N+1, where N is the number of engineers present certainly applies, there were actual technical and commercial reasons we could not all stick to a nice 20 degree normal pressure angle normal DP tooth.

The firm cited in my last posting adopted a 30 degree helix angle TRANSVERSE DP system as an outgrowth of its pioneering work on herringbone gears. If you have a building full of machines with a built in 30 degree helix angle it is very logical to have all your cutting tools made consistent with that infrastructure. As hobbing machines improved those tools got used to make other helix angles but the TDP system remained. The fundamental understanding of gear mathematics within that company diverged from the mainstream and only the advent of third party computer software has dragged them back. Kicking and screaming in some cases but back.

Math is hard. It was harder before calculators and computers. The trig functions needed to perform even the most basic gear calculations were explained well in a little orange paperback that most gear people kept close at hand. A few copies are probably still floating around your shop. To truly appreciate the degree of difficulty, I urge you to manually calculate an over pins measure for an odd number of teeth.

Another way to see the complexity of the “standard tooth form” problem is to look at an Ash Gear and Supply catalog. When we attempted to develop a computerized cutting tool inventory it was the Ash nomenclature system that kept us from going crazy. Besides module, NDP, and TDP systems you have to factor in a number of pressure angles, helix angles, and tooth depths.

To the naked eye many of these tools look alike. They do not cut alike, of course, and mixing them up in your shop can result in a disaster. A Sykes herringbone and a Sunderland herringbone both use a nominal 30 degree helix angle but they are not conjugate to each other. There are also TDP hobs with 20 degree pressure angles -only some are NORMAL pressure angle and others are TRANSVERSE pressure angle. Again, not conjugate. Some unfortunates learned this lesson in hardware.

Do yourself and your company a favor: know the tooth form you are using.

About Charles D. Schultz 463 Articles
Charles D. Schultz is President of Beyta Gear Service and one of Gear Technology's technical editors.

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