In article ,
Larry Green wrote:


I need to drill a 1 3/4 inch hole in ABS plastic. I presently have great
luck doing this with smaller holes, up to an inch or so, using a step
drill bit. I've searched around the web, and the largest dia step drill
bit I can find is 1 3/8 inch - does anyone know of one that goes up to 1
3/4 inch?



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).


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.

--
Larry Green