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Old 01-10-2010, 02:01 PM   #1 (permalink)
 
Join Date: Dec 2009
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Default Drilling Acrylic Problem

I'm a new pen turner and am branching out from wood to acrylic. I have found that when drilling a 10MM hole in an acrylic blank that the bottom is breaking out even when I go very slow. Any helpful hints?
Thanks!
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Old 01-10-2010, 02:05 PM   #2 (permalink)
 
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Hi'
I use a backer block under blank & ease up on pressure near bottom. i do not get much blowout. I get very little when first entering blank.
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Old 01-10-2010, 02:06 PM   #3 (permalink)
 
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Leave the blank long, stop the drill before breaking thru, then trim the blank back to the hole.
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Old 01-10-2010, 02:09 PM   #4 (permalink)
 
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Acrylic can be a bit tricky, especially as you finish the drilling. What many/most people do is to leave the blank a bit (3/8 to 1/2 inch) longer than necessary. Then, when they have drilled far enough to accomodate the tube (they mark the bit to when they have drilled far enough), they stop and cut the blank the correct length and the hole is drilled all the way through.

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Old 01-10-2010, 02:15 PM   #5 (permalink)
 
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Thanks, guys..I'll give it a try.
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Old 01-10-2010, 02:41 PM   #6 (permalink)
 
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Default Had the same problem...

...and implemented the solution above: leave the blank longer than the finished hole, and cut it back after you drill. I've made several big pens now using this technique and haven't had a single blow out.

Also, I use a much slower drill head speed: around 500 rpm and I back out every 1/2 inch. Also, I've noticed that some of the slag left in the bottom of the hole tends to heat up, get soft, and then harden. So, I use my shop vac to keep sucking the slag out of the bottom of the hole as I drill.

Yes...I know. It takes me a couple of minutes to drill a big PR blank, but after having blown out several expensive ones, I decided the time was worth it.

Good luck.

Regards,

t.
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Old 01-10-2010, 02:58 PM   #7 (permalink)
 
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I believe ( and said before ) that there are a couple of things going on here. There are a couple of solutions already mentioned. But if you understand WHY it happens then it's easy to remedy.
A drill bit acts like a screw as it exits the end of any material but particularly brittle stuff like hard plastic. Now the 'thread' of this screw takes over and pulls the drill bit the rest of the way while taking up the slack ( backlash )in the drill press or lathe quill. Bang, you have a blowout
You can grind the bit to suit plastics or you can buy special bit for plastics and don't use them for anything else so you don't have to sharpen them.
Another method is to dull the drill bit first. This is a common trick with guys that make signage. When they get a new standard drill bit they dull it by drilling into cement or a brick
There is normally less of this slop in a lathe quill or a very good drill press. If you drill on the lathe, then partly tighten the quill clamp.
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Last edited by skiprat; 01-10-2010 at 03:00 PM.
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