KC Stern, a design engineer for Boeing Commercial Airplane Group, is doing some design work and has a question regarding the hole size that is drilled before tapping internal threads. The book states, "A hole diameter slightly larger than the minor diameter of the thread must already exist, made by drilling/reaming, boring or die casting."
Conventional wisdom would lead one to believe that a hole slightly smaller should exist before tapping threads. The Machinery's Handbook seems to agree with this. Most tap drill sizes listed are smaller than the minimum minor diameter listed for a particular thread size in MIL-S-008879B. KC has found at least one exception to this but the difference was only a couple of thousandths of an inch.
KC consulted local sources on this matter and no one will commit to the correctness or incorrectness of the book. A number of people are scratching their heads on this one. Your assistance in this matter is greatly appreciated. Should the hole be slightly smaller or slightly larger than the minor diameter of the thread and why?