The Star pinion cutter was manufactured by the A. Gentil & Company of La Brevine, Switzerland. A fundamental tool in the horological workshop, a pinion cutter is used to shape to perfection the small, toothed components that drive a watch or clock's gear train. Its purpose is straightforward: to produce pinions with teeth, often referred to as 'leaves', cut to an form enabling them to mesh cleanly with the corresponding wheels (wheels is the term used by watch and clock makers when referring to gears). Accuracy in these teeth is critical; even minor irregularities will introduce friction in the drive and cause wear and, consequently timing errors.
In operation, the cutter used is first ground to the precise profile required for the finished tooth. It rotates at high speed and is brought to bear against  a pinion blank is held securely in an indexing head. This indexing mechanism, of course, divides the blank into the correct number of positions, ensuring each leaf is cut at the proper angle and spacing. After the cutter machines one tooth, the blank is rotated to the next division, and the process repeats until the full set is formed. Some pinion cutters can be set to work automatically on a batch of blanks, but this simpler example shown here is designed to handle only one at once.
Many makers of high-class machine tools intended for watch, clock and similar trades have offered pinion cutters including the American Watch Tool Co., Bechler, Cataract , Petermann, Sloan & ChaceStark, Wade and Waltham..