Machining
Drilling acrylic: the bit, the speed, the backing and the order of operations
A hole in acrylic is a stress riser. Get the bit, the feed and the edge distance right and it will last; get any of them wrong and it cracks weeks later.
Machining
A hole in acrylic is a stress riser. Get the bit, the feed and the edge distance right and it will last; get any of them wrong and it cracks weeks later.
Acrylic drills easily, which is why so many holes in acrylic are drilled badly. A wood bit at high feed will make a hole; it will also leave stress around the hole, melt the swarf onto the flutes, and produce a crack that appears days later when the part is in service.
| Feature | What you want | Why |
|---|---|---|
| Point angle | 60–90 degrees, sharper than the 118 degrees used for steel | Stops the bit skating and reduces thrust |
| Rake angle | Zero or slightly negative | Prevents the bit grabbing and screwing in |
| Flutes | Polished and wide, for chip clearance | Acrylic swarf welds to a rough flute and then the bit grabs |
| Lip relief | Generous | Reduces friction and heat |
| Material | HSS or carbide, sharp | A blunt bit generates heat instead of cutting |
Bits sold as "for plastics" or "for acrylic" have this geometry. A general-purpose wood or masonry bit does not, and a brad-point wood bit works in thin sheet but grabs badly in thick material.
The general rule is high speed, low feed: the bit should cut with the cutting edge, not push the material ahead of it. In practice, for a 5 mm hole in 5 mm acrylic, roughly 1,500–2,500 rpm with a slow, steady feed and frequent pecking to clear the swarf.
Acrylic breaks out on the exit side. As the bit breaks through, the unsupported material around the hole is pushed away and a chip or a star crack appears. The fix is to clamp a sacrificial backing, a piece of scrap acrylic, MDF or plywood, hard against the exit face.
For a visible face, drill from the face that matters towards the backing. The entry side is clean; the exit side is against the backing and never seen.
| Feature | Minimum | Notes |
|---|---|---|
| Hole to edge | 1.5 times the hole diameter | Use 2 times if the part sees load, vibration or thermal cycling |
| Hole to hole | 2 times the hole diameter centre to centre | Measured edge to edge, this leaves one diameter of material |
| Hole to a bend | 3 times the thickness from the bend tangent | A hole in a bend zone will ovalise or crack |
| Hole diameter in sheet | Sheet thickness or less | A hole larger than the sheet is thin around its edge |
| Threaded hole | One diameter of material around the thread | See the threading guide for engagement depth |
These are working minimums for acrylic parts in normal service. Where a fixing is safety-related, the design is the machine builder's responsibility and should be checked against the appropriate standard, not against a rule of thumb from a fabricator.
Acrylic expands about 0.07 mm per metre per degree Celsius. A 1,000 mm panel in a frame that swings 20 degrees moves 1.4 mm. If the hole is the same size as the fastener, that movement is absorbed as stress at the hole edge.
The standard detail is a hole 1.5–2 mm larger than the fastener, a nylon washer under the head, and a slot on one axis for anything over 800 mm. It costs nothing and it prevents the majority of cracked parts we are asked to replace.
For a hole that takes a dowel, a pin or a bearing, drill undersize and ream. A reamed hole in acrylic can be held to ±0.02 mm with a sharp reamer and a slow feed. Do not try to achieve that tolerance with a drill alone; the drill will cut slightly oversize and out of round.
Both remove material where the load is highest. A countersink for a flat-head screw in acrylic is a weak detail: the head crushes the seat and the panel cracks outward. A counterbore with a metal washer is better; a plain clearance hole with a shoulder washer is best.
If the part will be solvent bonded, cleaned with alcohol daily, or see temperature cycling, anneal it after drilling. Annealing relaxes the stress introduced by the hole and substantially reduces later crazing at holes. It is a separate process step and should be quoted as such.
No. The geometry is completely wrong: it crushes and hammers rather than cutting, and it will crack the part or melt the edge. Use a bit ground for plastics.
Friction heat from too high a feed, too low a speed, or a blunt bit, usually combined with swarf not being cleared. Reduce the feed, increase the speed, clear the chips and add a little water.
At least 1.5 times the hole diameter from the edge, and 2 times where the part is loaded or sees temperature cycling. Closer than that, the edge will crack under load even if it looks fine at first.
About 8 mm, which is 1.5 to 2 mm larger than the fastener, with a nylon washer. Do not drill 6 mm; the movement of the panel will crack the hole edge.
Yes, with a hole saw designed for plastics, a slow speed, light pressure and both faces backed. A bimetal hole saw for wood or metal will grab and crack the sheet. Better still, cut a large hole on the router.
Hole sizes, edge distances and the fixing detail are what decide whether a part lasts. We check them before cutting.