How to drill plastic without the bit "grabbing" it?

Question

I am attempting to use a drill press to drill a 13mm hole in the ends of a small electronics enclosures (a Hammond case). The wall of the case is about 2mm thick and made of ABS plastic.

Started off by drilling a 5mm pilot hole and followed it up carefully and slowly with the 13mm bit. Immediately the box was grabbed by the bit and the workpiece was ruined (split down the side). After massaging my bruised thumb I realised that I need a better way to do this.

Reckon I'd have no better luck with a hand-cranked drill or a cordless drill.

My available tools are severely limited and there's no hope of getting any budget to buy new tools (not for a small project like this). I've got a decent selection of drill bits and a small, fixed-speed drill press and not much else.

I did consider just drilling a 5mm hole and simply reaming it up to the 15mm hole we actually need but this is a real pain in the neck.

What can I do to prevent this grabbing problem?

Answer 1

Sandwich the workpiece between two pieces of wood, clamp it all together, clamp the whole assembly so that it can't go spinning off into oblivion, then drill through the whole thing. You'll also want to use really light pressure when drilling through the plastic, but don't go so slow that you melt the stuff.

If you have some, you could use a step drill instead of a twist drill.

If you have a bunch of in between size bits, you can simply start small and slowly work your way up to the final size. However, you're still going to want to properly brace the material. Adding support behind the piece is also a good idea.

If it's within your budget, a drill bit designed to drill plastics might work. However, you're still going to have to secure the workpiece while drilling. I'd also recommend using a wood backer, to support the piece as you drill.

Answer 2

Considering how many times this type of question has been asked, I'm not sure why it wasn't closed as a duplication.

Reference at least one other posting: Yet another plastic drilling question

Use a drill bit made to drill plastic.

Sandwiching the material will not prevent grabbing when using a conventional bit.

The step bit may also grab and crack is used incorrectly, that is inappropriate speed for the material, but it has a much smaller chance of doing so than using a conventional steel cutting bit.

Answer 3

I was going to propose using an endmill. However this requires precise positioning of the work over an existing hole (endmills can't cut in their center since it has zero velocity), which brings us back to the crux of the problem: positioning and clamping.

When I have several same workpieces, I have often built a tray by nailing scrap to a piece of plywood, that the piece drops into snug. And then clamp the tray to the drill press. If I had a CNC router handy I would just mill the tray to the piece's exact dimensions.

Feeds and speeds will be everything, otherwise it will gall or melt.

I occasionally use endmills to square the bottoms of holes in wood, as it is impossible to cut past where the drill bit tip stopped, without feeds and speeds feeling really wrong.

Answer 4

Put it in a bowl of boiling water for a minute take it out and dry it and immediately drill it - it will not crack

Answer 5

When I work with plastic boxes I find it best to start with a small pilot hole just like you did but I then use a hand tool that looks like the following. It works very well at shaving out the hole to the diameter needed. It is amazing how fast and easy this tool works in the thin plastic walls of a project box.

Some hand tools like this will come to more of a point allowing them to start in a smaller hole. The one pictured above may not be optimally pointed but does demonstrate another desirable feature. A tool with more flutes will cut a smoother round hole.