A tool that makes the helical portion of a threaded hole by cutting material as it moves along a predrilled hole. Cutting taps usually have sharp "flutes", somewhat like the spirals of a drill, where the chips can accumulate and be ejected out of the hole NOTE: Cutting taps come in sets of 3; A starter tap, a midway tap, and the bottoming tap. Heli-Coil insertsare precision formed coils of extremely hard, diamond shaped 304 stainless steel. When installed into an STI tapped hole, they provide permanent conventional 60° internal screw threads. This assembled insert accommodates any standard bolt or screw as per MIL-S-7742 and AS8879 (UNJ controlled root radius). In order to install a helicoil you need to make a tapped hole large enough to accept the appropriately sized helicoil. The proper name for tapped hole of this size is: STI (Screw Thread Insert). An STI drill chart will inform you on the proper size drill to use in conjunction with an STI tap. Please reference the chart below from Balax as an.