deep hole drilling in Plexi ?

Greetings Jon, I just looked at my supply of miniature drills and found that I have some .026 diameter drills. They have a flute length of about 9mm so I imagine the drills you want to use will be similar. So this will require lots of pecking as the drill gets deeper. I have machined lots of plexiglass (acrylic) plastic. The first lesson I learned was to avoid the use of oils in any form because it will almost certainly cause tiny fractures to radiate from the hole. Even milling with water soluble oil based coolant can lead to cracks if the plastic is stressed. I have seen this happen to vacuum chamber lids that were milled around the OD. The parts were washed off after machining and looked great but fine cracks appeared when the lids were stressed by the air pressure when a high vacuum was pulled. So use water only unless the plastic maker or supplier can recommend a particular water soluble coolant that won't cause the cracking or crazing. If I had to do these holes I would make a nozzle that could force water under pressure into the hole between pecks to clear the chips and make the job go faster. Maybe something that would break the surface tension of the water like "Water Wetter" that is sold in auto parts stores would help the water stick to the drill. Or maybe some of that stuff that is added to water jet cutting fluid that helps keep the water from stream from spreading too fast. It would of course need to be compatible with the plexi and I don't know how you would check that. Are you going to use a sensitive drill chuck? The kind that has a ring mounted on a bearing that you use to provide the drilling force? The good ones work very well because you can feel the drill cutting and can tell very quickly when the chips start to pack up in the flutes. Good luck. Cheers, Eric

You might look at drills made for plastic. They have a high helix angle so the swarf is cleared out the hole. Maybe it is a low helix angle. Not sure how one measures the helix angle.

Dan

Even more important is the rake angle - 0 or even negative to prevent digging in. But you already knew that.

I am not sure that is right. I think you want a sharp edge so it cuts a co ntinuous chip that goes up the drill and out the hole. You do not want chi ps that are short and jam in the hole , get hot from friction and tend to g lue themselves to the sides of the hole.

But you need to have a drill press that does not have any slop. You do n ot want the drill to dig in and then break.

Dan

" snipped-for-privacy@krl.org" fired this volley in news:9a1b5105- snipped-for-privacy@googlegroups.com:

Yeah... that IS right, Don.

Lloyd

I stand corrected. I was thinking of less brittle plastics as polyethylene. Dan

This will be done on a CNC mill, using a peck cycle.

Jon