Checking the Functional Diameter of a Thread with a “No Go” Gage.

For external threads the “Not Go” thread ring gage ensures parts are not functionally undersize. The main differences in thread forms between “Go” and “Not Go” thread gages is that the “Go” gage has a full form crest while the “Not Go” gage has a truncated crest, and while both gage’s roots are “cleared”, the “Not Go” ring is cleared to a greater extent. The purpose of this is so that the “No Go” gage checks functional diameter with as little effect as possible from any other thread elements. A functional diameter of an external threaded part is considered to be undersized if it can enter a “No Go” thread gage, in most cases by more than 2 - 3 turns depending on the applicable standard. In most cases it is better to hold the ring gage and screw the work piece into it. This method typically results in a more sensitive measurement with less wear on the gage. If the part is large and heavy then the opposite is typically true where the ring gage should be turned onto the part. While it is better to introduce the part to the ring gage, in most cases the reverse is true for thread plug gages.