1 3/4 inch step drill bit ?

Can you use a high-end hole saw type bit instead of a step drill? If so, try Hougen.

If not, there are folks who will make you (read $$$) special tooling.

Hougen has a center point which is spring loaded and can be reversed to guarantee plug ejection on flat or large tube holes. We have had tremendous success with their product on metals and hard plastics. Haven't yet tried them on wood...

Regards, Joe Agro, Jr. (800) 871-5022

I have had good luck so far cutting plastics with 'wood' spade bits. I recently needed a few 1.5" holes in some 3/4" thick nylon/polyethylene blocks and only have a tiny Unimat DB200 lathe/mill. I had to go up in steps as the Uni just didn't have the 'balls' to cut the 1.5" hole right off. I was concerned about the bits 'wandering' as the point was not cutting but with a rigid set up (using the Uni as a lathe) the holes came out true and with an almost glass like finish. The size was bang on to within a thou too!

I am fairly sure spade bits are available at 1.75" and they might be worth a try (at least they don't cost much to experiment with).

A forstner or sawtooth style bit would give you a better finish and size, I find they work very well in polyethylene and nylon.

Lee valley have them in 1-3/4 for $15.40cnd

Try a holesaw. They are effective and relatively cheap.

I've tried one, and had terrible results, with a very rough edge and melting of the plastic. Perhaps it was the particular one I used, or the speed of the drill?

Thanks for the suggestion! What speed did you find worked best?

I found that the slowest speed on my Unimat was too fast and had to put a router speed controller 'inline' to reduce the speed. I have no idea what speed it was actually running at but I adjusted the speed by 'feel' until the bit was cutting nicely without chatter/squeal. You may have to experiment a bit with some scrap to 'dial in' the best speed for your application.

Bradrad make step drills up to 1-1/2" but that's no consolation. You could try one of these:

Tom

Melting means it got too hot, try cutting with ice water.

I use a spray bottle of isopropyl rubbing alcohol to cool plastic cuts. You just take little bites, enough feed to make a chip, but back off frequently keeping everything near room temp. Be very patient; plastic is an insulator of electricity and heat, so it takes a lot of time for the heat to dissipate, thus the evaporative coolant, rather than a flow-on coolant.

If you can get the cut started with a holesaw, you can finish it. Withdraw, stop, and feel with your hand until you have a feel for the frictional heat buildup. Patience! A 1:10 duty cycle is not out of the question.

Open a window, of course.