Subject:- Metrology and Quality Engineering

Topic:- Screw Thread measurement

Difficulty:- High

Two wire method:-

The effective diameter can not be measured directly but can be calculated from the measurements made. Wires of exactly known diameters are chosen such that they contact the flanks at their straight portions. If the size of the wire is such it contacts the flanks at the pitch line, it is called the ‘best size’ of wire which can be determined by geometry of screw thread. The screw thread is mounted between the centers & wires are placed in the grooves and reading M is taken. Then the effective diameter E =T+P where T =M-2d, & P is a value which depends on diameter of wire, pitch & angle of the screw thread.