This is a set of chucking reamers designed to give you precise round holes. A standard drill chuck creates a hole which is OK for most bolts, but upon close examination you will find that it is not truly round. Toolmakers require precise holes for jigs and fixtures. With this set of reamers you can drill your holes 1 size under, then ream them for precision. The set comes in sizes 1/8" to 1/2" each in 0.001" under size for press fit and 0.001" oversize for slip fit. Held in a handy plastic pouch for protection of the cutting edges.