a surgical drilling guide for guiding a drilling tool , measuring its penetration depth , and protecting surrounding tissue . the guide has a sleeve and a plunger that telescopes within the sleeve . the plunger and the sleeve have axial bores adapted to slidably receive a portion of the tool that protrudes beyond a drill chuck . the extended length of the drilling guide is substantially as long as the length of the protruding portion of the tool , and a maximum extended length of the guide is preferably as long as the protruding portion . as the tool in drilled into a workpiece , the chuck collapses the drilling guide . gradations on the side of the guide indicate workpiece - penetration depth .