Tapping Plastic: Drill Sizes, Inserts and Better Options

Quick answer: Most rigid thermoplastics tap fine with the standard 75% tap drill (for example, #7 for 1/4-20, 4.2 mm for M5, 2.5 mm for M3). Use a sharp two-flute tap, keep it clear of chips, and back up the exit side of brittle plastics like acrylic. For threads that will be tightened repeatedly or carry load, especially in 3D prints, a heat-set or press-in insert is the better choice. Thread-forming screws made for plastics are another strong option in molded or printed bosses.

Plastic threads fail differently from metal threads. Brittle plastics crack, soft plastics strip or creep, and printed parts have layers and infill that do not behave like solid stock. Here is how to get threads that hold.

Which plastics tap well

All the plastics below are thermoplastics, which matters for inserts later. Boedeker Plastics’ machining guidelines rate relative machinability on a 1 (easiest) to 10 scale.

Drill size choices

There is no separate drill chart for plastics. Common shop charts, such as the University of Florida design lab tap chart, group plastics with aluminum and brass at 75% thread. That gives these standard sizes:

Treat these as starting points and test in scrap. Plastics vary a lot in hardness, toughness and how much they spring back. If a tap feels very tight or the material cracks, try the next drill up; if the thread feels weak, use a longer engagement or an insert. For the math, see the tap drill formula guide.

Coarse threads are usually the better choice in plastic. Tapmatic notes coarse threads are less prone to strip in lower-strength metals and easier to tap in brittle materials, and the same reasoning carries over to most plastics. If you have a choice between 1/4-20 and 1/4-28, use 1/4-20. See UNC vs UNF.

How to drill and tap plastic

From Boedeker’s guidelines:

• Drill: use sharp drills, peck to clear chips on deeper holes, and use compressed air or water-soluble coolant to control heat. Back up the exit of through holes with scrap to prevent chipping. This is critical for acrylic.
• Drill geometry: many plastics drill best with a 90° to 118° point. Acrylic prefers a near-zero rake (Boedeker lists 0° to 4°). A standard twist drill can grab and crack acrylic, so feed gently.
• Tap: use the specified drill with a two-flute coated tap, keep the tap clean of chip build-up, and use coolant. Boedeker notes a coated tap creates radii at the thread root, giving a tougher thread that is less prone to cracking from over-tightening.

A few general habits help too:

• Chamfer or countersink the hole entrance slightly so the first thread does not chip or lift.
• Go slowly and avoid heat. Friction can melt or smear thermoplastics, especially PLA and ABS.
• Do not over-tighten. Plastic threads strip or crack long before metal ones. A brittle plastic like acrylic or polycarbonate under constant screw pressure can crack later, not just at assembly.

Threaded and heat-set inserts

When a plastic thread will be assembled often, carry real load, or sit in a 3D print, an insert is usually worth it. You get a brass or steel machine thread that will not wear out, and the load spreads over the insert’s larger outer surface.

Heat-set (heat/ultrasonic) inserts

These are knurled brass inserts pressed in while hot. The plastic melts around the knurls and grips them as it cools. They only work in thermoplastics, which covers every plastic in the table above.

SPIROL’s white paper on inserts for 3D printed assemblies gives practical guidance:

• Heat/ultrasonic inserts give the highest tensile and torque performance in thermoplastic prints (FDM and SLS). Press-in inserts give moderate performance and need no special equipment.
• Hole size is critical. Use the insert maker’s recommended hole. SPIROL notes desktop printers typically hold only ±0.3 to 0.5 mm, much looser than its recommended +0.08 mm hole tolerance, so either drill the hole after printing or iterate with test prints.
• Wall thickness: design a solid boss at least 1.5 times the insert’s knurl diameter; performance peaks around 2 times.
• Infill: insert performance rises with infill density. SPIROL recommends more than 50% infill around inserts.
• Installing with a soldering iron: SPIROL suggests a tip that engages the insert well, typically 650°F to 750°F, and a rule of thumb of the plastic’s heat deflection temperature plus 150°F. Push with steady pressure and keep it square.
• Resin (SLA) prints are thermosets and do not melt, so use press-in inserts there instead.

Threaded (self-tapping) inserts

These have a coarse external thread or cutting slots and screw into a plain drilled hole. They work in thermoplastics and in materials that cannot be heat-staked. They are easy to install with hand tools, but SPIROL notes self-tapping inserts in thermosets offer less back-out torque than press-in types. Follow the insert maker’s hole size.

Thread-forming screws: skip the tap entirely

Screws designed for plastics use a narrow thread angle (DELTA PT uses a 30° flank angle) so they form their own thread while keeping outward stress on the boss low. Examples include EJOT’s DELTA PT and the PLASTITE family. In molded or printed thermoplastic bosses, they are fast, cheap and strong.

• Pilot hole: Semblex, which makes the DELTA PT, recommends a hole of 0.8 times the screw’s nominal diameter, larger for heavily filled or stronger plastics (Semblex DELTA PT).
• Counterbore the hole entrance. Semblex calls this particularly important to prevent boss cracking.
• Use screws made for plastics. General-purpose sheet-metal and wood screws are not designed around plastic bosses, so stick to a plastics screw and its maker’s hole chart.

Where sources differ: Boedeker, a plastics machining supplier, advises against thread-cutting and thread-forming screws in machined plastic parts because of the stress they put around the hole. Fastener makers design screws like DELTA PT specifically to limit that stress in thermoplastic bosses. In practice: thread-forming screws suit molded or printed bosses with the right hole and wall thickness; for machined acrylic, polycarbonate or other notch-sensitive stock, a tapped thread or an insert is the safer bet. Test on scrap either way.

Which method should you use?

If you are also tapping metal parts in the same assembly, see our guides to tapping aluminum and cut vs form tapping.