"autolock" collet chucks (osbourn "pozi-lock" is a similar system) were designed to provide secure clamping of milling cutters with only hand tightening. they were developed in the 1940s by a now defunct uk company, clarkson (engineers) limited, and are commonly known as clarkson chucks. autolock collets require cutters with threaded shank ends to screw into the collet itself. any rotation of the cutter forces the collet against the collet cap taper which tightly clamps the cutter, the screw fitting also prevents any tendency of the cutter to pull out. collets are only available in fixed sizes, imperial or metric, and the cutter shank must be an exact match.