R Thread

An R thread is a BSPT/ISO tapered external pipe thread with a 1:16 taper, used on pipes and fittings to create a leak-tight metal-to-metal seal by wedging interference. It is not a straight machine screw thread; internal counterparts are designated Rc in some standards. In CNC context, R may also appear as a cycle parameter, but in thread documentation it denotes this tapered pipe thread.

On a CNC lathe, cutting an R thread commonly occurs on tube ends, hydraulic fittings, pneumatic manifolds, and sealing ports where leak-tight assembly is required. The taper must be programmed as a conical helical path, not a constant-diameter helix, so the machinist needs a threading cycle that handles taper or must calculate the changing diameter along Z. Setup demands accurate pilot diameter, Z start/end control, and verification with the proper ring gauge or mating fitting, because a straight thread gauge can accept the wrong geometry. Sealing comes from controlled interference, so flank quality, taper angle, and surface finish matter; tool wear or chatter can create leak paths. This is especially critical under fluid pressure, vibration, thermal cycling, or repeated assembly, where the taper thread's deformation under torque is what makes the seal.