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litemover
Wed 25 December 2013, 22:46
Hi all,

I'm about to set out on a 48"x96" x 3/4" beveling adventure and was wondering if anyone has any advice on what the best way to bevel 3/4 acrylic would be at 30-45 degrees. The bevels would cut off quite a lot of the edge and I don't want to ruin the workpiece as it was expensive.

Any clues as to feeds and speeds would also be helpful.
Many thanks,
Chris

Surfcnc
Thu 26 December 2013, 20:29
I would cut the bulk off first then come in for a fine clean up pass.
Also I found that a glycol coolant mist spray made an enormous difference in the quality of the finished surface.
Acrylic will also flame polish as you probably already know.

Limit your speed to 12K if dry cutting or you can go up to 20K easily with coolant.
See post http://www.mechmate.com/forums/showthread.php?p=60766&postcount=899

Ross

lonestaral
Fri 27 December 2013, 07:13
My experience of cutting engineering plastics has been,
Low revs and high feed.
Yes coolant helps a lot.

danilom
Fri 27 December 2013, 16:26
Many CNC operators and/or programmers have previous experience in the metal working industry and that can be a detriment when attempting to use similar cutting parameters in acrylic. A typical finish pass in ferrous and non-ferrous metals can be as little as .004”-.005”. When this amount of material is removed in acrylic, it frequently will compress and cause the cutter to actually skip across the surface. This is due mainly to the high rake angles employed in plastic tooling and the aggressiveness of their cutting action. Without at least .015”-.030” of material to remove, most acrylic router bits will not have enough material to bite into and will actually show a deteriorated finished edge over the initial roughing cut

My experience tells the same, when you cut plastics or acrylic you need something that bit can catch on, you can't take a 0.1mm finishing pass on plastics

here is the complete text
http://www.plasticsmag.com/routing.asp?fIssue=Jan/Feb-07&aid=4539

litemover
Sat 28 December 2013, 03:33
Thanks for the advice, I found a 60 degree chamfering bit I'd like to use as the bevel edge. I should take at least .015-.03 of material off then? in one pass Should I mill out most of it in a roughing pass, then finish with the chamfer? Really am not looking forward to chip rewelding. It's an $800 piece of Acrylic and I have one shot at it with no vacuum table.

Any help would be appreciated in the chamfering process.

KenC
Sat 28 December 2013, 10:41
Its nerve wracking working on expensive material. I feel you.

litemover
Sat 28 December 2013, 20:49
Yes indeed. Well, I'll take it slow and test first. We'll see. Anyone know how deep I should go with each pass to keep a nice polishable edge? Cast acrylic 3/4" thickness.

Thanks!
Chris

lonestaral
Sun 29 December 2013, 06:46
Another thought to throw into the mix.

Climb mill or conventional mill ?

My preference would be to climb mill.
I cut my give away gears with a 3.2mm cutter at 15,000 r.p.m and a feed of 500mm/ min climb milling.
Th best of British to you .

litemover
Sun 29 December 2013, 12:04
Thanks Al, I heard for smaller diameters, climb is the way to go, then for larger, conventional is definitely the way to go. My chamfer mill is about 2" diameter with 1/2" shaft and I'm using an ONSRUD 65-025 super O flute, single upcut for this project. It has a 7/8" cutting flute. Though I also have a downcut onsrud O flute, I hear that chip evacuation is crucial at pieces over 1/4" thick. I'm going to profile the piece in 1/8" passes about .060" from the edge leaving .060 on the bottom too, then do one full pass to clean up the edge.

I'm having quandaries about taping the piece down with this tape I use called polyken 105. Hope I will be able to lift it off the table when it is ready without having to use a crowbar.

litemover
Wed 01 January 2014, 15:22
Well thanks to everyone for the help on the thick acrylic. It came out near perfect. I wasn't too thrilled about trying to get the workpiece off and took off most of the tape, which was a smart thing to do. I also made an impromptu vacuum table for the machine which proved invaluable!

It amazes me that just a shopvac can produce so much vacuum. I simply used a 1/2 straight bit to carve a grid into the table, then bought a flange and 1" vacuum attachment and it worked! Well too!

Anyhoo, here are some pics of the acrylic. Thanks Danilo for the links, they proved very useful. I did exactly that.

Cheers,
Chris

http://s27.postimg.org/yza44sv9f/1522284_10202881499467793_2145388198_n.jpg (http://postimage.org/)

http://s27.postimg.org/fvgsogif7/1486829_10202881502707874_447832502_n.jpg (http://postimage.org/)

http://s27.postimg.org/vi821tw77/1559609_10202881503387891_1491539587_n.jpg (http://postimage.org/)