Abstract: An abrasive drill of small diameter or "micro" drill, on the order of tenths of a millimeter to about a millimeter in diameter, is provided with a concentric cored portion occupied by relatively non-abrasive material. The filled portion is small enough that during drill rotation all of the surface of a workpiece within the outline of the end of the drill is abraded sufficiently by some part of the rotating end of the drill to crumble away rather than leaving any solid core or large fragments of the workpiece. The drill composition comprises abrasive particles in a less abrasive metallic matrix. The drill is formed by deposition of matrix material entraining abrasive particles onto a taut filamentary mandrel. The mandrel may be metallic, such as copper wire or may be non-metallic, such as nylon or other synthetic polymeric textile fiber composition.

Abstract: An abrasive drill of small diameter is provided with a cored portion that is eccentric, i.e., lacking a substantial concentric portion, relative to the longitudinal axis of the drill. The core, regardless of shape, is located so that during drill rotation all of the surface of a workpiece within the outline of the end of the drill is abraded by some part of the rotating end of the drill. The drill composition comprises abrasive particles in a less abrasive matrix, and the core is not hollow but comprises relatively non-abrasive material. The drill is formed by deposition of matrix material entraining abrasive particles onto a taut filamentary mandrel. The mandrel preferably comprises one or more round filaments, either straight or twisted together, alongside the longitudinal axis of the drill, and may be metallic, such as copper wire or may be non-metallic, such as nylon or other synthetic polymeric textile fiber composition. The drill diameter may be on the order of a millimeter or so or smaller.