How to Tap Threads in Acrylic

Short answer: You can tap machine-screw threads in acrylic 1/4 in. or thicker: drill the standard tap drill size with a plastic-ground bit, then cut coarse threads (UNC or metric coarse) with a sharp tap, backing out often and using wax or mineral oil on larger holes. Sheet makers advise against sharp V threads, threadlocker, and tapping parts that see heavy torque, shock, or frequent reassembly. For those, use threaded metal inserts or through bolts.

Key facts

Thread form: coarse threads (UNC, metric coarse). Plaskolite says avoid sharp V threads because the crests fracture; Trinseo prefers Acme over V threads above 1 in. diameter.
Tap speed: about 25% slower than you'd run for brass (Plaskolite). Back the tap out often to clear chips.
Lubricant: none needed when hand-tapping holes 0.200 in. or smaller if you're careful; mineral oil for larger holes; a wax stick gives the clearest threads.
Minimum sheet: 1/4 in. for face threads; 3/8 in. or more for threads into an edge.
Don't use: liquid threadlocker or countersunk screws, which Plaskolite says can crack and craze acrylic.

When does tapping acrylic work?

Tapped threads in acrylic are fine for light, occasional loads: holding a lid, mounting a circuit board, attaching a bracket that won't be pulled hard. They work best when:

The sheet is 1/4 in. or thicker. Thinner sheet doesn't leave enough threads to hold. Tapping into an edge needs 3/8 in. or thicker.
The threads are coarse. UNC and metric coarse pitches have bigger, stronger thread forms. A 1/4-20 holds far better in acrylic than a 1/4-28.
There's enough engagement. Aim for thread depth of at least 1.5 times the screw diameter; twice is better. A 6-32 in 1/4 in. sheet is a reasonable pairing; a 1/4-20 in 1/4 in. sheet is marginal.
The hole has room around it. Keep at least twice the hole diameter, and twice the sheet thickness, between the hole and any edge.
The part doesn't see big temperature swings. Plaskolite notes that acrylic expands much more than a steel screw, and that with extreme temperature changes threading isn't recommended at all.

What size drill for tapping acrylic?

Start with the standard tap drill for a cut tap (about 75% thread). Some fabricators go one number size larger in brittle plastic to cut tap torque and stress, giving up a little thread strength.

Thread	Tap drill	Decimal
4-40	#43	0.089 in.
6-32	#36	0.1065 in.
8-32	#29	0.136 in.
10-24	#25	0.1495 in.
10-32	#21	0.159 in.
1/4-20	#7	0.201 in.
M3 × 0.5	2.5 mm	0.098 in.
M4 × 0.7	3.3 mm	0.130 in.
M5 × 0.8	4.2 mm	0.165 in.
M6 × 1.0	5.0 mm	0.197 in.

Metric tap drills follow the usual rule: major diameter minus pitch. These sizes are for cutting taps. Thread-forming (roll) taps need larger holes and push material outward, which is the wrong thing to do to acrylic.

How to tap a hole in acrylic

Drill the tap drill size with a bit ground to a 0° rake, over a backing board (how to drill acrylic). For blind holes, drill deeper than the thread so the tap never bottoms out.
Chamfer the top of the hole slightly so the first thread doesn't chip.
Use a sharp tap. A spiral-point tap pushes chips ahead in through holes; a spiral-flute tap pulls them out of blind holes. Dull taps tear threads.
Lubricate the right way. Small holes (0.200 in. and under) can be hand-tapped dry with care. On larger holes, Plaskolite recommends mineral oil, and Trinseo suggests pushing a wax stick into the hole first: it lubricates, supports the chips, and leaves cleaner, more transparent threads. Skip general-purpose cutting oils and emulsions, which can contain ingredients that craze acrylic.
Turn slowly and back out often, about every turn or two, to clear chips. Heat from speed gums the chips and jams the tap.
Blow out the chips and test with the actual screw.

Why are inserts usually better?

A tapped acrylic thread has two weak points: it wears if the screw goes in and out repeatedly, and it strips or cracks with a little too much torque. Plaskolite recommends threaded metal inserts wherever threads see shock, heavy torque, or frequent removal. Options:

Heat-set or ultrasonic brass inserts. SPIROL, an insert maker, lists acrylic (PMMA) among the amorphous plastics these suit. Drill the insert maker's recommended hole exactly: SPIROL warns that undersized holes create stress and cracks. Its general guide is a surrounding wall of two to three times the insert diameter, and a hole depth of the insert length plus two thread pitches.
Inserts bonded with epoxy in a slightly oversized hole. Low stress and strong.
Avoid cold press-in and tapered inserts in thin walls. SPIROL notes cold-pressed inserts need thicker walls, and tapered inserts in thin walls crack the plastic.
Nylon screws for light duty, where you'd rather the screw strip than the acrylic crack.

Through bolts: the strongest option

When a joint carries real load, skip threads in the acrylic. Drill a clearance hole larger than the bolt and use a washer on each side and a nut. Plaskolite recommends a compressible gasket or rubber washer where metal meets acrylic, and shoulder bolts to limit clamping pressure. The acrylic is only clamped, never threaded, and the oversized hole allows for expansion. More options in can you screw into acrylic.

Don't overtighten

Most cracked tapped holes come from torque, not the threads. Acrylic gives no warning before it strips or cracks.

Tighten until snug, then stop. A small screwdriver, not a power driver.
Skip liquid threadlocker: Plaskolite lists thread-locker products as incompatible with acrylic, causing cracking and crazing (why acrylic cracks later). Use a nylon-insert lock nut instead.
Avoid tapered pipe threads, which wedge the hole open.

We can tap holes for you

We drill and tap holes in 1/4 in. and thicker acrylic. Mark tapped holes on your drawing with the thread size (for example, "6-32 tap" or "M4") and add them to your quote or upload your file. For enclosures and mounts, see acrylic electronics enclosures.

Questions

Can I tap metric threads in acrylic?

Yes. Metric coarse pitches (M3 × 0.5, M4 × 0.7, M5 × 0.8, M6 × 1.0) work, with tap drills of 2.5, 3.3, 4.2, and 5.0 mm. The same thickness and engagement guidelines apply.

Can you tap 1/8 in. acrylic?

Not usefully. There aren't enough threads to hold, and they strip easily. Use a through bolt and nut, or bond a thicker block behind the sheet and tap that.

Can I tap into the edge of acrylic sheet?

Only in thick sheet, and carefully. The hole needs plenty of material on each side, so a small screw in 3/8 in. or thicker is the practical minimum. Edge threads split the sheet more easily than face threads.

Do heat-set inserts work in acrylic?

Yes. Acrylic is an amorphous plastic that softens gradually, which suits heat and ultrasonic inserts. Use the insert maker's hole size, go slowly so the surrounding plastic isn't overheated, and keep plenty of wall around the insert.

What lubricant should I use to tap acrylic?

Small holes can be tapped dry by hand. For larger holes, sheet makers suggest mineral oil or a wax stick pressed into the hole. Clean off oil afterward, and avoid cutting fluids that aren't known to be acrylic-safe.

How to tap threads in acrylic