July 14, 2022
There is an old saying that if the only tool you have is a hammer, everything looks like a nail. Imagine that you are tasked by your employer to design a new gearbox and your only cutting tools are for herringbone machines. You are stuck with a system of fixed transverse pitches because each machine has a specific pitch diameter on the cutter. If I recall correctly, the most popular machines used 4-inch and 6-inch pitch diameter cutters; a 6-TDP cutter would have 24 or 36 teeth. When you were stuck with those tools you had to get creative. If you did not get creative you were extremely limited in fitting “standard geometry” components together. A gear and a pinion have integer values for numbers of teeth; what do you do if the center distance you are working with works out to a fractional total number of teeth? You cannot simply adjust the helix angle to make the tooth count reach an integer value. The clever designers of the day took advantage of that wonderful involute curve to make things fit. Unlike some competing tooth curves, the involute is not sensitive to center distance changes, so long as the teeth remain in mesh. Early designers discovered that they could enlarge or contract the “standard” center distance to permit the tooth counts they needed to be made with off-the-shelf cutters. Centers could be contracted — or reduced — two or three percent or enlarged — expanded — up to five percent without compromising the smoothness of operation. They did not call it “rack offset,” but a well thought out calculation system was developed to permit a limited set of cutting tools to produce a wide range of parts. If you have old reference books in your technical library, there is much to be learned by actually reading them. The language may be a bit cumbersome, but the techniques are usually described very completely. You might be surprised that vestiges of the “old ways” are still a part of the “old family recipe” used in your product design.