How to drill acrylic without cracking it

Key facts

• Rake angle: 0° (flat, scraping cutting edges). Acrylite allows 0° to 4°.
• Point angle: 60° to 90°. Plexiglas recommends 60° for holes that go all the way through; Acrylite uses 90° for most work. A standard bit is 118° to 135°.
• Lip clearance: about 8° to 12° (Plexiglas) or 12° to 15° (Acrylite).
• Speed: about 1,800 to 2,000 rpm for 1/4 in., 800 to 1,000 rpm for 1/2 in., about 500 rpm for 1 in. (Plexiglas chart).
• Edge distance: hole center at least 1.5 times the hole diameter from the edge (Acrylite); Plaskolite asks for twice the hole diameter and twice the sheet thickness.
• Coolant: needed once the hole is deeper than the bit is wide, and for holes 1/2 in. and larger (Acrylite).

Why does acrylic crack when you drill it?

A standard twist drill for metal or wood has a positive rake: its cutting edges lean forward to bite. In acrylic that bite turns into a grab. The bit screws itself into the plastic, worst of all as it breaks through the back, and the sudden load cracks the sheet or blows a chip crater out around the exit. Heat makes it worse: a dull or over-fast bit melts the plastic, which then grips the bit.

What drill bit is best for acrylic?

• Plastic drill bits come ground with a 0° rake and a 60° to 90° point. They scrape a clean hole and don't grab. Plaskolite's Optix guide calls a drill press with plastic-cutting bits the best setup.
• Modify a standard high-speed steel bit. "Dub off" the cutting edges: hold the side of a grinding wheel or a stone against the front of each cutting lip and grind a small flat so the edge is vertical instead of hooked. That is the 0° rake. If you can, also regrind the point to about 60° to 90°. Many shops keep a set of old bits ground this way just for plastic.
• Step bits (Unibit style) are fine in thin sheet. Acrylite rates them for sheet up to 0.118 in. (3 mm) with the sheet fully supported.
• Hole saws and circle cutters work for big holes. Acrylite lists hole saws from 3/4 to 6 in. with cooling, and circle cutters at 400 to 600 rpm for holes over 1 in. Use a pilot hole and a backing board.
• Skip spade bits and brad-point wood bits. They chip and crack acrylic.

How do you drill acrylic step by step?

1. Leave the protective film on and mark the hole on it. Don't center-punch acrylic, which can start a crack; the Perspex design guide says to drill a small pilot hole instead.
2. Clamp the sheet flat over scrap wood, MDF, or scrap acrylic. All the sheet makers call for backing: it supports the plastic when the bit exits, which is when most chips happen.
3. Use a drill press if you can. It keeps the bit square and lets you control feed. A bit that wobbles leaves a rough hole.
4. Set the speed from the table below and feed steadily with light pressure. Plaskolite's sign of a good setup is two continuous spiral chips coming up the flutes. Melted strings mean too much heat; powder means too little feed or a dull bit.
5. Slow down as you break through. Ease the feed for the last bit of the hole so the point exits gently.

What speed should you drill acrylic?

Slower for bigger bits. These are the general speeds in Trinseo's Plexiglas fabrication guide; Plaskolite gives a wider 700 to 3,500 rpm window, lower for thicker sheet.

Drilling thick acrylic

Once a hole is deeper than the bit is wide (say, a 1/4 in. bit through 3/8 in. sheet), heat builds and chips pack up. Peck drill: go in a little, back the bit out to clear the chips, and repeat. Acrylite calls for coolant at that point and for any hole 1/2 in. or larger. Water is the simplest; the makers also list compressed air, a 10% compatible oil-in-water mix, kerosene, or mineral oil. Avoid random cutting fluids and emulsions, which Acrylite lists among the materials that can craze acrylic. For deep blind holes, Plexiglas suggests a 90° point when the hole is less than 3 times as deep as it is wide, and 118° for deeper holes, since the blunter point clears chips better.

How close to the edge can you drill acrylic?

Acrylite's rule is that the hole center should sit at least 1.5 times the hole diameter in from the edge. Plaskolite is stricter for fastened parts: leave at least twice the hole diameter, and twice the sheet thickness, between hole and edge. We use the stricter one. A 1/4 in. hole in 1/4 in. sheet should sit at least 1/2 in. from the edge, and the same spacing applies between holes. Holes too close to an edge leave a thin wall that cracks when the part is bolted, bumped, or grows with heat.

Make mounting holes oversized

Acrylic moves with temperature several times more than metal (Plaskolite puts it at about four times). A 48 in. panel grows about 0.2 in. over a 100 °F swing. Screwed down through snug holes, that movement has nowhere to go, and the sheet cracks at the holes.

• For small indoor parts, about 1/16 in. over the screw diameter is usually enough.
• For large or outdoor panels, Acrylite recommends a hole at least twice the bolt diameter, or slot the holes along the long direction.
• Use washers under screw heads and tighten only until snug.

Our thermal expansion calculator works out the clearance for your panel size. More background in acrylic thermal expansion.

What about screws and threads?

Driving wood screws straight into acrylic usually cracks it. See can you screw into acrylic for what works, and how to tap threads in acrylic for threaded holes in thick sheet.

Or let us drill it

Holes cut on our lasers come out clean and round with no chipped exit. We also drill and countersink holes, and tap them in 1/4 in. and thicker sheet. Add holes when you order acrylic cut to size, or draw them on your file and upload it.

Questions

Sources

• Trinseo: PLEXIGLAS acrylic sheet fabrication guide (drilling, drill speeds)
• Acrylite: Drilling ACRYLITE acrylic sheet
• Plaskolite: OPTIX acrylic sheet fabrication guide (drilling, mechanical fastening)
• Plaskolite: OPTIX-L continuous cast acrylic sheet fabrication guide
• Perspex: Design guide (drilling)