How does this gage measure diameter?

 

Although we may call it a "chamfer" gage or a "countersink" gage, in all cases it is in fact a "depth" gage. Regardless of brand or trade name, every gage of this type simply measures the depth that a cone shaped member (called the gage "plunger") has fallen into a hole, where it comes to a stop, gently wedged in place.

There is always a direct linear relationship between the distance that the conical plunger travels into the part and the diameter on the part taper at which it comes to rest. The gage incorporates a base plate, through which the plunger passes, that is pressed against the face of the part above the taper and provides a plane from which the depth measurements are referenced.

The most common gages have a 90� included angle on the gage plunger resulting in a 2:1 ratio between the taper diameter observed and the dial indicator rack travel.

Another popular version has an included angle near 127� resulting in a 4:1 diameter to depth ratio.

Choosing which to use depends on the nature of your parts.