Spring loaded drill stops for sheet metal work. Slide one onto the bit, set the depth with the hex socket set screw, and the bit cannot go deeper than you set it. The rounded nose keeps the chuck off the skin and the spring cushions the breakthrough, which is what saves small bits from snapping.

Sizes cover the standard aircraft rivet and cleco drills, #40, #30, #21 and #10, plus #20 for 5/32" blind rivets and 1/4" and 5/16" for bolt holes. Springs are colour coded to the matching cleco size: zinc for 3/32", copper for 1/8" and 1/4", black for 5/32" and brass for 3/16".