Undercutting tool

A rotary expansion tool for insertion deep into a pre-existing hole to undercut the walls of the hole at a location which is intermediate the ends of the tool and remote from the mouth of the hole, comprising:

• (a) an elongated shaft having an axial bore, extending all the way therethrough, the shaft having an outer surface fitting snugly in the hole and having two longitudinal guide slots extending from the bore through the shaft on diametrically opposite sides of the shaft axis at said intermediate location, the outer surface of the shaft providing bearing areas contacting the walls of the hole on both sides of said location;
• (b) cutter means including two identical elongated cutter members in the bore, the cutter members being pivotally supported intermediate their ends in back-to-back relationship by a pivot carried by the shaft, the cutter members each having a first longitudinal cutting edge on one side of the pivot pin and a second longitudinal stabilizing edge on the other side of the pivot pin and a portion of both of these edges being always contained within an associated one of the guide slots, the cutting edge of each cutter member being supported by the back of the other cutter member and occupying the same guide slot as the stabilizing edge of the other cutter member;
• and
• (c) elongated rod means reciprocably carried in the bore of the shaft and having means at its inner end operative to pivot the cutter members about the pivot pin when the rod is pushed toward the cutter members to extend the longitudinal cutting edge of each member through one of the guide slots beyond the outer surface of the shaft while extending a different portion of the longitudinal stabilizing edge of each member to occupy the diametrically opposite guide slot, and the rod having a tool drive coupling at its outer end.