Acrylic Edge Quality

[toe_jam]

Okay I'm running into a problem with acrylic edge's. It's okay with other plastics but really sucking with acrylic...I Think. Is this as good as it gets with edges on Acrylic. I have a acrylic Belin Tools bit, 3 actually and tried all 2 of the 3 in case it was the bit. I'm doing this at 15k RPM's at 2.5 inch/sec as they specified. I was exporting dxf files from flexi, but now just did these in part Wizard incase exporting and importing was adding more points. This looks like a sproket to me, I would really have to sand the #%$ out of this to flame polish. All the rollers and bearings seem to be seating right. I have even updated the control software to the latest December update. Any Ideas?

http://www.thesignindustry.com/miscstuff/letteredge1.JPG

http://www.thesignindustry.com/miscstuff/letteredge2.JPG

http://www.thesignindustry.com/miscstuff/letteredge3.JPG

http://www.thesignindustry.com/miscstuff/letteredge4.JPG

Oh and I am using Cast Acyrlic

[paco]

http://www.thesignindustry.com/viewtopic.php?p=171#171

[Brady Watson]

Erik,
You will notice an immediate increase in edge quality if you use a 1/2" bit instead of a 1/4". The 1/2" is more rigid...you also might try using a 3/8" so that you don't eat up as much material.

While this would reduce the # of 'gear' marks on the outer edge, you still will show signs of bit stepdown as seen in your pics. I would recommend offseting (using Allowance) the toolpath so that you leave maybe .02 on the piece. Stepdown and leave a .03 skin on the bottom. Then run a single profile pass to the desired size at full depth to give a nice clean edge.

-Brady

[ron brown]

I'll agree with Brady on a finish pass of .020-.030 high speed on the feed and more rigid bit. Be sure and look at your "waste". If it has a better finish, you are cutting the wrong direction.

Ron

[elcruisr]

Erik,
like the other guys said, bigger is better. I try never to cut acrylic with anything smaller than 3/8" . I've had my best success with climb cutting and onion skin/finish pass tactics described by Brady. The new Onsrud "superflute" plastics tools seem to be giving me the best edge quality so far.

YMMV
Eric

[paco]

Do you think that Erik could have done better than what he got guys (take a look at his pics)? I believe he has got a quite good cut on thoses letters.

[Brady Watson]

Paco,
Yes. For one, he could eliminate the water line marks by doing a finshing pass as directed...and the higher rigidity of a larger bit would reduce flex and give a less jagged edge.

What might be a great cut to some is a horrible cut to others ~ meaning that if you were going to flame polish or buff the edge, the number of grit steps or time post finishing can be dramatically reduced by doing a finishing pass with a larger bit. Again, it depends on the application and customer's needs.

-Brady

[paco]

I agree with all that Brady. I'm somewhat suprised that you believe that the cut could be better... though I'm open to the best! I tend to use small bit (1/8-1/4" and even 1/16") since I often have to deal with details which you "loose" with bigger tool; let say a 4" "W", cut with a 3/8" CED tool bit it would look quite "undetailed"... unless it match the font... But I was to cut circles or squares, I would look for a bigger tool for sure.

[simon]

Erik
It might just be the material that is flexing, and not the machinery. This will be generally worse as the cut gets further away from the clamps.

[paco]

Good point Simon!
Hold down is important on plastic; especialy on thin stock...

[cnc_works]

I'm also in search of improved edge quality cutting 3/8" cast acrylic. Right now I'm using one of the new Onsrud, single flute, spiral upcut bits recommended for acrylic and it seems to do a very efficient cut. I've just made a single pass with no clean-up, I've tried cutting two passes with a .03" clean-up. It is a little better, but not significant. Cut seems to have quite a bit of extra chatter where I Z up for the .12" tabs.

I've checked everything for tight.

I guess one question I have now is whether a spindle would produce markedly better edges than the PC I'm using now, or if the problem lies more in the flexibility of the machine itself. I have a smaller, more rigid CNC with the same PC router that produces edges that are probably 50% better which is a hint that the latter may be the case.

Some of the chatter marks worsen at places (like the tabbing) where there is a certain change in direction or Z, almost like an oscillation sets in.

Ahhhh, the ongoing pursuit...

Donn

[cnc_works]

Of course, I forgot to mention necessary info.

Experimented with feeds from 60 to 100ipm. Run the 1/8" detail bit at 15K, which seems to work just fine. Run the 1/4" at pretty close to full 23K because it seems to bog a little at any slower.

By the way, because of the cuts, for efficiency, I kinda need to stick with the 1/4" bit instead of anything bigger to do the cutting.

Donn

[Flake (Unregistered Guest)]

I have this problem on all rigid plastics, I've tried everything I can think of and have come to the conclusion that this is the best the machine can manage - its down to the vibration inherent to the design of the machine, you only have to put your hand on the frame and feel the vibrations whilst the machine is running to realise it.

Of course I hope I'm wrong

[toe_jam]

It's wierd just yesterday I was cutting acrylic at 15,000 3inch/sec with a Belin 1/4" single flute and the edge came out beautiful, best ever. Ithink it's a lot of factors now batch of cast acrylic, brand, vibration, feed speeds, blah blah blah. I need to try using a 1/2" bit next as recommended by someone. But I do so much lettering that 1/2" is a little to big for my uses. I have a job next week where I will be using one, i'll let you know how it worked out.

[billp]

Is there any chance that the length of the bits in question is a problem? I usually try to use the shortest bit (cutting length)that will work for the job as it does eliminate some of the chatter. Long bits transmit a lot of vibration..

[jay]

Hmm. Thanks, Bill. Very good thought about shorter bits.

[toe_jam]

Hmmmmm. The shortest one I have is 5/8" and thickest acrylic has been 3/8"

[elcruisr]

I can achieve an very good edge in acrylic but there are a number of variables that are involved. First I use the shortest tool possible. Next I use the largest diameter possible. I've had to explain to customers that we have more to consider than maximum yield of parts per sheet if edge quality matters. I've had my best results from the Onsrud superflute tools run withing the factory reccomended feed and speed ranges and using a rough pass / finish pass climb cut strategy. Needless to say the carriage bearings need to be adjusted properly!

Eric