Pipe Thread
What makes NPT different from a machine screw thread?
NPT is a 60° tapered pipe thread designed to seal pressure-containing joints, while a machine screw thread is generally parallel and designed for fastening. The taper creates interference on makeup, so the thread form and sealing mechanism are fundamentally different.
Does NPT seal on the threads alone?
No. The taper produces interference between male and female threads, but practical sealing usually relies on thread sealant or PTFE tape filling the spiral clearance. The interference plus sealant creates the pressure-tight joint.
How do you identify a pipe thread in the shop?
Measure the outside diameter with calipers, count the threads per inch with a pitch gauge, and compare the values to an NPT chart. Then confirm the 60° form and taper using the correct gauge or a mating fitting.
Pipe thread is a tapered 60° threaded connection used to join pipes, fittings, and pressure components, most commonly as NPT (National Pipe Thread Tapered). The external and internal threads interfere and deform on makeup, sealing by controlled interference rather than flank contact alone. Nominal pipe size plus threads per inch, such as 1/2–14 NPT, defines the callout.
On a CNC lathe or manual machining floor, pipe threads are a common job: cutting male NPT nipples, tapping manifolds, drilling and tapping valve blocks, and verifying fit with gauges or controlled hand makeup. The machinist must hold three things at once: the nominal major diameter and threads per inch from the callout, the 3/4 inch per foot taper, and the 60° form with flattened crests and roots. A straight-thread habit is the usual failure point: a CNC turning cycle programmed without taper motion produces the right pitch but a parallel thread that cannot seal. Tap selection and bore size must come from NPT chart dimensions, not the nominal pipe size name. Checking taper with a taper gauge, using go/no-go plug and ring gauges, and respecting the hand-tight plus wrench-tight makeup range are the practical checks.
Nominal size mix-up: “1/2 NPT” is not 0.500 in. OD; pipe sizes are nominal and actual diameter differs. The result is mismatched fittings, wrong tap selection, and rejected parts that look close but do not engage.
Wrong taper: Holding the right pitch but missing the 3/4 inch per foot taper happens when a CNC thread cycle is set up as straight. The joint leaks under pressure, and over-torque can crack valve bodies.
Over-tightening to seal: Pipe threads are designed for hand-tight plus wrench-tight engagement, not max torque. Excess force splits thin-wall castings, distorts aluminum manifolds, or gall steel fittings before a reliable seal is made.