Sturdy surface gauge with adjusting knob and hardened scriber. Round base of close grain cast iron. A surface gauge is used to find the centre of a piece of round section material.First place the section in a vee block. The scriber on the surface gauge is moved across an end surface of the steel section and a line is etched. Rotating the work piece through 90 degrees another line is scribed. Repeat this process twice more and a square is visible in the centre of the surface. The point at which two diagonal lines from the corners of the square cross is the centre.