Drilling Acetal Nightmare...(or other plastics for that matter)

[StuartJPP]

I have been making a small clamp and had to drill some 6mm holes into a small sheet of 10mm black Acetal...I first marked them all off, drilled pilot holes (2mm)...unfortunately the part would not fit into the vice of the pillar drill so I clamped it down with a G-clamp...well it didn't work very well and the drill bit "bit" into the part and lifted/moved the clamp with the result being a fairly oblong shaped hole...

Luckily this wasn't a critical hole...the others were which is why I had pilot holes in the first place...

What I found worked better (on some sacrificial Acetal) was obviously clamping it down more securely, much more than you'd think, and to "nibble" at the hole until it was deep enough to take the whole cutting edge of the drill bit, once past that point it was okay... It also didn't seem to make much difference on drill speed though I was on something quite low...also having pilot holes or not didn't make much difference either (except for the initial accuracy)...but I suppose if I went up in 1mm steps it would have been okay...

This may be obvious to others, and should have been to myself since it has caught me out in the past, but I forgot just how quickly and violent the process can happen...buy a bigger drill press vice is my advice...

[Gina]

Yes, plastics are very tricky to drill - very carefully, slowly and steadily is my advice.

[Zakalwe]

A wood bit (spade drill) works best.

[Astrobits]

You need a specialised drill bit for plastics. It is easy to make one by just touching the cutting edge of a drill on a grindstone. This produces a scraping rather than a cutting edge.  No more "digging in" but not much use for metals

Nigel

[Bizibilder]

And a drop of "spit" works wonders keeping the drill and plastic lubricated whilst cutting.

[StuartJPP]

I have always enjoyed making bits and bobs, even though I am not very good at it...so it is good to pick up some tips...thanks everyone.

[vulkan]

I machine Delrin quite often and it can be a joy compared to some materials.

As has been mentioned you need to have drills with the cutting edge dulled slightly so they don't have a tendency to grab and lift the material.

Take the drill and lay it parallel to a sharpening stone, with the cutting lip on the front of the drill resting on the stone and give it a few swipes, you don't need to remove much at all, you now have drill bits that will do a much better job in grabby materials such as plastics and brass (I have a separate set that i built up every time I did this to a specific drill size).

For what you are doing, I'd say skip the pilot hole (its not really necessary on such small drill bits) and instead use a spot drill to create a starter hole slightly wider than your drill bit, the much more rigid spot drill will cut easily on centre and will not pull the material due to how it cuts, it will give you a nice start for your drill and make it run nice and true.

[Stub Mandrel]

You can actually get 'slow spiral' drills that are better for plastics and also good for sheet metals and other metals that tend to grab. The stoning off of sharp edges works as well but can spoil the drill for other materials until it is resharpened. flat bits as @Zakalwe suggests work too.

Excellent clamping is essential as you found - once a drill grabs in plastic it bites right in so the pull is huge.

[redtail]

Cone drill works for me.