Maximum Number of Teeth in a Shaper Cutter—Ask the Expert
This question and answer also appeared in the June/July 2012 issue of Gear Technology (download PDF).
How can I determine the maximum number of teeth in a shaper cutter to avoid trimming or rub on up-stroke when cutting an internal gear?
Marv Snider, senior manufacturing engineer, Allison Transmission
The Easy Answer
The easy answer is to direct you to Table 12 in my book, An Introduction to Gear Design (available for free download at my website, www.beytagear.com). This chart (reprinted here) was developed using the method described in professor Earle Buckingham’s landmark book, Manual for Gear Design, Section 2—"Spur and Internal Gears" (The Industrial Press, copyright 1935; copyright renewed 1962; reprinted 1981). On pages 40–43, Buckingham describes those places where interference or trimming can occur and presents mathematical methods for determining whether or not they occur for a given cutter and gear combination. Unfortunately, many manufacturing engineers are not familiar with Buckingham’s book, or are unable to obtain a copy of it. Check online book sellers for a used copy.
Guidance may also be available from your cutter supplier or machine builder. I developed Table 12 (page 46) to avoid having to perform the calculations frequently and to aid in the design of parts that are manufacturing-friendly. It helps avoid tooth numbers that require special cutters.