Subject:- Metrology and Quality Engineering

Topic:- Screw Thread measurement

Difficulty:- High

Three Wire method:- The three-wire method is the accurate method. In this method three wires of equal and precise diameter are placed in the groves at opposite sides of the screw. In this one wire on one side and two on the other side are used. The wires either may held in hand or hung from a stand. This method ensures the alignment of micrometer anvil faces parallel to the thread axis.

This method is more accurate than two wire method as it ensures alignment of micrometer faces parallel to the thread axis. Here, three wires of exactly known diameters are used, one on one side & the two on the other side. The wires may be held in hand or hung from a stand. From the fig, M=diameter over the wires E= effective diameter (to be found) d= diameter of wires, h=height of wire center above the pitch line, r=radius of wire, H=depth of thread, D=major diameter of the thread.