14 - Screw Threads Specification, Tolerancing, Gauging and Measurement

Abstract

The chapter discusses the principles of screw threads specification, tolerancing, gauging, and measurement. The main methods for screw threads are taps and dies, screw cutting and thread cashing, thread milling, thread grinding, and thread rolling. A tap is a fluted screw of hardened steel with its leading threads tapered off and ground back to provide cutting clearance. It cuts threads in a plain hole generally drilled somewhat larger than the minor thread diameter. Dies, based up a circular nut, cut external threads using similar principles. The principles are of historical significance; the screw-cutting lathe was an important development of the early machine tool. By replacing the single-point tool method of screw cutting and thread chasing by a form milling cutter (hob) set to cut the full depth of a thread, short chips will be cut and the thread will be completed in one pass. The process originated attempts to finish grind pre-cut hardened blanks for the use as screw gauges. Modern taps are ground from prefluted hardened HSS blanks in a single pass that illustrates subsequent progress.