A cutting tool used to cut the chamfered holes into which flush rivet heads fit. The cutter, which is driven by a shaft mounted in a hand-held drill motor, fits through a bushing in the body of the stop unit and extends out beyond a fiber collar on the end of the body.

The body is made in two parts threaded together to allow the length of the body to be adjusted. The body length determines the amount the cutter protrudes, and this, in turn, controls the depth of the hole made by the cutter. A fiber collar prevents the stop scratching the soft metal being countersunk.