[TheHiggsBroson]

Been having trouble getting consistent good edge finishes when cutting acrylic. First time with a CNC Router, 4'x8' - Automation Technologies KL-1325. 1/4" Cast Acrylic.

RPM: 10,000
Feeds: 50 in/min
DOC: .01 in
Bit: 1/8" Onsrud O-Flute Upcut 65-012/65-013
Cooling: Just simple airblast at the tool

Hold Down: Cutting out of 2'x2' square that has masking on the back. I use 3M Super 77 on the back and put it on a piece of MDF then clamp those to the table.

These are the bit/feeds and speeds ratio that I have come to by trial and error. Sample of a good finish I can get is below:
http://www.cnczone.com/forums/attach...d=359354&stc=1
The burrs are because I was trying to leave the masking on but left too much. This is the finish I am aiming for.

These settings were giving me really good cuts up until today. Halfway into an ~2 hr long run the chips just started melting and gobbing and the finish went bad, like the photo below:
http://www.cnczone.com/forums/attach...d=359356&stc=1

Once this started happening I couldn't get it working again until I put a fresh sheet on, but it only lasted a few minutes before it starting melting again. Both of the bits I used today were brand new. I know I can finish the edges with sand and polish but the better it can come out of the router the easier post processing, and those inside windows are hard to polish as well. The goal is to get to production level, using 4'x4', we are a getting a vacuum pump this week so hopefully that will help.

I think my hold-down could be better, there is a lot of vibration in the material when I am cutting, was thinking of screwing my spoilboard into the table with t-slots and then adhering the plastic on. It's just strange the finish changed mid-way through my run. I am wondering if it is maintenance of the machine as well. I had no issues running it yesterday when I oiled and cleaned rails, bearings, collet etc. I didn't do this today but maybe I need to apply oil midway through runs?

I've tried many different feeds/speeds this seemed to be the best combination (best chip-load?). I had some success running it at higher RPM and Feed (18000 * 75) for a few minutes and then everything started melting again. I've heard dish-soap and water could act as a good coolant, is that worth a shot?

Current design limitations have me stuck at a 1/8", I am hoping to convince my boss to change design to allow use of a 1/4". Maybe I can run 1/4" for most of the passes and then finish with 1/8" where needed.

Very frustrated, been at this full-time for the last few days, and overall a few weeks trying to get this going.

Any help appreciated.

[Andreiir]

I am having this problem too , but i used about 4000 rpm for spindle , 1 flute bit , and the fastest speed u can achieve without bit crash .
If u use lover rpm , there will be no more heating problem , or u can use higher rpm , but submerse the part u cut in water .

[jahnj0584]

50ipm is too slow. I cut fine at 16k rpm @ 120ipm or more.

[TheHiggsBroson]

I run at 50 ipm because my spindle speed is at 10000. What DOC and size bit are you using at 16K. I spoke with Onsrud guys they suggest doing a DOC of .125 running at 8000 RPM @ 48 IPM. I've ran some tests today, full depth of cut gives me lots of deflection of my cutter, at .05 DOC @ 8000 RPM and 60 IPM I am still getting some melting.

[Jim Dawson]

I have successfully used dish soap (Dawn) and water. I also have used soybean oil, available from you local grocery store. Just pour some on. A higher chip load and lower RPM is normally better.