BSPP Thread Milling Cutter

BSPP thread mills are solid carbide thread mills used for machining 55° BSPP (G) parallel pipe threads, the British Standard Pipe Parallel form defined by ISO 228-1. BSPP threads — also written as G threads — retain a constant diameter along their length and are sealed with a gasket or bonded washer rather than relying on thread interference on a taper. This makes them the standard connection form for hydraulic and pneumatic fittings, valve bodies, manifolds, pump housings, and other fluid-handling components built to European and international specifications.

Thread milling machines these threads by helical interpolation: the cutter enters the prepared bore and follows a programmed helical path, keeping cutting forces moderate and allowing the finished thread diameter to be directly controlled through cutter compensation. Thread size can be corrected by adjusting the compensation value without changing the tool, and because the cutter is not loaded the same way as a tap, the risk of breaking a tool inside the workpiece is greatly reduced — a decisive advantage in valve bodies and fittings where a broken tap usually scraps the part. The same helical motion also produces external threads, and thread milling remains predictable in blind holes, interrupted cuts, and difficult-to-machine alloys where tapping is least reliable.

Each BSPP (G) thread is specified by its designation: the letter G, the nominal pipe size in inches, and the threads-per-inch count from ISO 228-1. A 1/2-inch BSPP thread with 14 TPI, for example, is written G1/2-14. The nominal size is a pipe designation, not the measured diameter of the finished thread or of the cutter, so the prepared bore, required thread depth, and cutter diameter should be checked against the component drawing before programming. The table below lists the designation and pitch for common BSPP sizes.