Machining
A thread cut straight into 5 mm acrylic will hold a light load and fail a heavier one. Knowing which is which decides whether the part survives its first service visit.
Acrylic machines beautifully, which tempts designers to treat it like aluminium — drill a hole, run a tap through it, done. It works often enough to be misleading. A thread in acrylic is a thin spiral of material with low shear strength, and its capacity depends almost entirely on how much thread depth there is.
As a rule of thumb, an M6 thread in acrylic needs about 1.5× the diameter of engagement to reach a useful strength — roughly 9 mm of full thread. That means at least a 12 mm thick panel, or a boss, or an insert. In 5 mm sheet an M6 thread has about five turns of partial engagement and will strip under hand torque.
| Thread | Minimum sheet thickness for full engagement | Realistic use | Risk in thinner sheet |
|---|---|---|---|
| M3 | 5 mm | Light covers, small brackets | Strips at low torque |
| M4 | 7 mm | Panel fixings, light equipment | Strips if over-tightened |
| M5 | 9 mm | Equipment panels, covers | Marginal in 6 mm |
| M6 | 12 mm | Machine guards, load-bearing covers | Not viable below 10 mm |
| M8 | 16 mm | Rarely appropriate in acrylic | Use an insert or a through-bolt |
These are working rules from fabrication practice, not a design code. Where a fixing is safety-related or carries a structural load, the joint should be through-bolted or use a metal insert, and the calculation belongs with the machine designer rather than the fabricator.
Self-tapping screws for plastics — the type with a coarse thread and a trilobular shank — form a thread rather than cutting one, and they are stronger than a cut thread in the same hole because the material is compressed rather than removed. They are the right choice for a light cover, a small panel or a one-time assembly.
They are the wrong choice for anything that will be opened repeatedly. Each reinsertion cuts a little more material away, and after four or five cycles the screw will not hold. If a panel is serviced regularly, the joint should be a machine screw into an insert or a through-bolt with a nut.
A brass or stainless insert, either moulded in, thermally set or press-fit with a knurled body, gives a metal thread in an acrylic panel. It is the standard answer where a joint must be opened repeatedly, or where the sheet is too thin for a cut thread.
For most acrylic parts the best fixing is no thread in the acrylic at all:
This detail costs almost nothing, allows the panel to move, spreads the load over a washer, and removes the stress concentration that causes most cracked acrylic in service. It is what we put on a drawing when a customer has not specified a fixing.
A countersunk screw in acrylic looks neat and weakens the part. The countersink removes material exactly where the load is highest, and the conical head crushes the seat over time until the screw sits proud. If a flush face is essential, use a metal cup washer or a screw with an integral nylon washer and accept a small visible ring.
Ø6.5 THRU for M6, SLOTTED ±3 mm ON X — clearance and movement in one line.NO THREAD IN ACRYLIC if that is the intent; it prevents a well-meaning machinist from tapping it.INSERT: brass M6 heat-set, 8 mm deep where a metal thread is genuinely required.WASHER: nylon, both faces — the cheapest fatigue life improvement available.It will cut, but the rake angle is wrong for plastics: it grabs, generates heat and can crack the part. A tap ground for plastics, with a shallower rake and more clearance, gives a cleaner thread and far fewer failures.
Roughly 2 to 4 N·m in 12 mm cast sheet with full engagement, and considerably less in thinner material. Finger-tight plus a quarter turn is the practical instruction for an assembly line; a torque wrench set for metal will strip the thread.
Because the hole was too close to an edge, or the panel was under stress, or the screw was driven in with a lubricant that attacked the material. A crack rather than a stripped thread usually means the wall around the hole was too thin — at least one diameter of material should surround a thread.
Not usually. A Helicoil needs a solid wall to grip and spreads load outwards, which is the wrong direction in a brittle material. A bonded or heat-set insert with a knurled or flanged body is the better choice.
Yes — a machined boss bonded to the face with a solvent bond gives a deep thread and is common on covers. The bond area must be large enough: a boss needs a base diameter of at least twice the thread diameter to carry the load into the panel.
Tell us how often the joint is opened and what load it carries, and we will specify a thread, an insert or a through-bolt with the hole sizes to match.