What are Surface Gauges?A surface gauge is a very useful tool when you need to find the centre of a piece of round section material. It is used to scribe parallel lines. It has a heavy base which makes it stable when being used. Sometimes they come with magnetic bases which enables them to be locked onto metal surfaces making it easier to use.Used by machinists and toolmakers for accurately scribing lines, transferring measurements and probing during inspection work. The surface gauge is usually used with a V Block and angle plate as well.How they workTo find the centre place your round section steel object in a V block. The surface gauge is them moved across the surface of the steel, scribing a line. The steel is then rotated through 90 degrees and another line is scribed. Repeat this until a square is formed in the centre. Diagonal lines are then drawn from each corner of the square to highlight the exact centre of the circle.