Tapping threads in plastics: pitch, pilot size and inserts

Tapping threads in plastic is not the same as tapping steel, and treating it that way is why so many threaded plastic parts strip on the assembly line. The material is softer, it springs back, and it has a much lower shear strength than metal. A thread that would be generous in aluminium can be marginal in POM and hopeless in a brittle glass-filled grade.

The good news is that the rules are simple and repeatable. Choose a coarser pitch, open the pilot hole slightly, match the tap style to the material, and be honest about when a metal insert is the right answer. Here is how each decision works, and how to prove a new thread design in a few minutes.

Why plastic threads need a coarser pitch

A thread carries load across its flanks, and the load has to spread over enough material to avoid shearing the turns. Coarse threads have deeper flanks and fewer turns per inch, so each turn presents more plastic in shear. Fine threads, which are ideal in steel where the material is strong, cut thin slivers of polymer that strip at low torque.

The practical rule is to stay with the coarse series and avoid fine threads unless the design demands them. Where the wall is thin, choose a smaller coarse thread rather than a fine pitch in the same nominal size, because the fine pitch will fail first. This single change fixes most stripping complaints we see.

Pilot hole sizes for the common materials

The pilot hole in plastic is generally larger than the equivalent hole in metal, because the material is elastic and the tap must not crowd the core. As a rule of thumb, add around 0.1 to 0.2 mm to the metal tapping-drill size, then test in the actual grade before committing, since filled materials behave differently from unfilled ones.

These figures are starting points, not a specification. Drill a test hole, tap it and torque it, because a supplier’s grade and your grade may sit at different ends of their tolerance bands. Get the pilot size right and tapping threads in plastic becomes predictable; get it wrong and no tap will save the joint.

Cut taps, form taps and roll forming

Cut taps remove material and produce a conventional thread with a sharp root. Form or roll taps displace material instead of cutting it, which leaves an unbroken grain flow and a slightly stronger thread in ductile polymers such as POM and the nylons. In brittle or heavily filled grades, form taps are risky because the displaced material can crack, so a sharp cut tap is the safer choice.

Whichever you use, keep the swarf clear. Plastic chips are light and tend to pack into flutes, and a tap that re-cuts its own debris produces a ragged thread and a hot, galled surface. A dull tap is worse in polymer than in metal, because it rubs rather than cuts and heats the very material it should be removing. Peck the tap, clear the flutes regularly, and use a light cutting fluid only where the material tolerates it. For reference on machining each family, see our page on POM and its well-known strength in cut and formed threads.

Threaded inserts: when metal wins

Any joint that will be assembled and disassembled repeatedly belongs on a metal insert, not on a tapped plastic hole. A plastic thread tolerates a handful of fastener cycles before the flanks wear and the strip-out torque falls; an insert moves the load to a knurled or bonded metal body that holds far better and protects the plastic from the fastener’s threads.

Choose press-fit or heat-set inserts for thermoplastics, and size the boss generously so the insert has wall thickness around it. Where the part is structural, our note on PEI and similarly stiff grades explains why high-modulus materials suit insert bosses better than soft ones, because they resist the hoop stress the insert imposes.

Assembly torque and the strip-out margin

Every plastic thread has a strip-out torque, the point at which the flanks shear, and a working torque should sit well below it. In practice, aim for a working torque around half the measured strip-out value, which leaves margin for a worn tap, a slightly oversized pilot or a powered driver that overshoots. State the torque on the drawing so the assembly line has a number to hit rather than a feel.

Where the design uses a stiff or brittle grade such as carbon-filled PEEK, the margin matters more, because failure is sudden rather than a gradual tightening. In that case tapping threads in plastic leaves far less room for error, so a controlled driver and a written figure turn a fragile joint into a reliable one.

Questions buyers ask about tapping plastics

Should I tap the hole or mould a thread where possible?

For machined stock shapes the thread is cut after machining, so the tap and pilot decisions above apply directly. Where a part is moulded, a moulded thread can be stronger, but it is outside what we do from sheet and rod.

How do I fix a stripped plastic thread?

Move to the next size up, install a threaded insert, or rebuild the boss. Simply running a tap of the same size rarely restores strength, because the sheared flanks are gone. If the joint will be opened again, the insert is the durable fix.

Do I need a different pilot hole for filled grades?

Usually a slightly smaller one, because the filler makes the material stiffer and less elastic, so it springs back less. Test in the real grade, since glass-filled materials vary more between suppliers than unfilled ones.

How can I test a new thread quickly?

Drill three pilot sizes, tap each, and torque to failure with a small torque wrench. Twenty minutes on a scrap piece tells you the working torque and whether a different rod diameter would give more wall thickness. If the numbers are marginal, talk it through with us before tooling up.