Split Ring Drill Stops that fits drill sizes:  1/8 in., 3/16 in., 1/4 in., 5/16 in., 3/8 in., 7/16 in., 1/2 in.  Slip the stop collar over the appropriaate drill bit and tighten the allen screw the desired depth. 

Promax 79600    $9.99

Promax 79038     $6.99