PDA

View Full Version : Drilling Acrylic



elf
01-05-2013, 04:47 PM
I need to drill a handful of holes approximately .25 inches in diameter in .170 inch thick acrylic. Several online how-to sites say to dull the drill bit first, but unfortunately don't specify how dull or what the correct method of dulling a bit is :) Does anyone have recommendation on how to regrind a bit to work well in acrylic?

beanbag
01-05-2013, 05:02 PM
I need to drill a handful of holes approximately .25 inches in diameter in .170 inch thick acrylic. Several online how-to sites say to dull the drill bit first, but unfortunately don't specify how dull or what the correct method of dulling a bit is :) Does anyone have recommendation on how to regrind a bit to work well in acrylic?

plunge with a 1/4 endmill instead

Toolguy
01-05-2013, 05:23 PM
A dull drill bit sounds like a bad idea. One way is to grind the front face of the cutting edges so they are vertical (parallel to the axis of the drill) instead of inclined. The edge is still sharp and cuts good. The difference is it won't catch when it breaks through. This also works good on brass and sheet metal. Drills ground like this will still cut steel too, though not as well as original.
It takes about 30 seconds on the bench grinder to do.

John Stevenson
01-05-2013, 07:15 PM
Used to look after some machines that only drill acyrlic sheet.
We used normal jobber drills ground up as what we knew as wing and spur.

http://www.bizrice.com/upload/20120402/Brad_Point_Drill_Bit_Lip_spur_bit.jpg

The tip has to be just slightly longer than the spurs, these will cut very cleanly and not spelch the plastic.

Bob Fisher
01-05-2013, 07:41 PM
I always back up acrylic with a scrap piece clamped tightly and use a normal drill. I have also used brad point drills with good results. For good measure , I back those as well. Bob.

Mcgyver
01-05-2013, 07:49 PM
I need to drill a handful of holes approximately .25 inches in diameter in .170 inch thick acrylic. Several online how-to sites say to dull the drill bit first, but unfortunately don't specify how dull or what the correct method of dulling a bit is Does anyone have recommendation on how to regrind a bit to work well in acrylic?

if you read 'dull the drill' on a web page, they don't know what they're are talking about and have misled you. What they mean is remove the positve rake and give it some neutral or negative rake - same as you do brass the drills don't grab. If you have a lot, grind the point a much smaller angle than the standard 118 degrees. I'd also go slowly and add some coolant - both to stop the plastic from melting. here's some pics of drills with zero rake....middle one is a mock up to show how to put on zero rake for smaller drills, that point is in no condition to drill anything :)

http://i20.photobucket.com/albums/b201/michael0100/drills%20brass/grindingzerorake.jpg

http://i20.photobucket.com/albums/b201/michael0100/drills%20brass/stoningzerorake.jpg

http://i20.photobucket.com/albums/b201/michael0100/drills%20brass/DSCN0159.jpg

bborr01
01-05-2013, 09:53 PM
Not really dull the drill but grind the cutting edge so that it is parallel with the length of the drill bit or a little negative angle. You don't want it grabbing or digging in like a regular grind will do.

edit: McGuyver nailed it in the last post.

Brian

browne92
01-05-2013, 11:24 PM
Note: The same technique is also useful for drilling out a hole in metal only slightly larger...like drilling from 1/2" to 17/32"...keeps the drill from digging in and binding.

elf
01-06-2013, 01:24 AM
Thanks everyone for the replies. Now I know the proper way to make a "dull" drill :o

p.s. The pictures really helped.

EVguru
01-06-2013, 06:03 AM
I've always known it as 'dubbing' a drill.

John Stevenson
01-06-2013, 06:19 AM
Problem with using a normal jobbers drill on thin acrylic is that the point breaks thru well before the lands and then looses the centring effect of the point.
The two lands are then free to threepenny bit around and this is what leads to cracking.

With the wing and spur which is basically a wood grind the point in is the cut right up to the last. Lets face it perspex behaves more like wood than metal so why use a metal drill.

dalesvp
01-06-2013, 08:14 AM
Sandwich the acrylic between two pieces of wood. Squeeze the sandwich tightly. Drill through the the top piece of wood and through the plastic. You can drill any "un-drillable" material this way and have a good clean hole. I've successfully drilled acrylic, various fragile plastics and even paper this way. It works!