Need help reducing chatter lines in acrylic

[eklug]

17864.jpg
I have been cutting out templates for grinding moulder cutters. They are cut out of .18" acrylic using a .05 straight bit. The machine is running as slow as it will go (10000 rpm and .05 ips). We need the template to be as smooth as glass. Currently our tooler files these edges down with a diamond file. It would be much more efficient if we could get a smoother cut from the get-go. Any suggestions?

[wcsg]

Are you using CASt acrylic? I would use a Spiral O bit by Belin. What is your hold down method?

You can do two two tool paths. Kind of lie to the machine and tell it you have a .28 bit (If your using a .25 CED) then do your second profile as a true .25

A 3/8" or 1/2" bit can be a better edge quality if your part will allow it.

Also their are specialty bits to do a final pass on to help smooth over. But other factors can be mechanical or even software. Too many nodes in your file or improper seating of your bearings on x,y & z axis, etc etc the list can go on.

[elcruisr]

A .05 tool, is that correct? That tool would be flexing like mad while cutting. Use a polished O-Flute spiral designed for acrylic in the largest possible diameter you can then make sure that the machine is tight mechanically. That would make a big difference. Some hand finishing might still be required but not as much.

[richards]

Emily,
Are you using an Alpha? The chatter in your photo looks exactly like the chatter that I always got with my Alpha before installing 3:1 gear boxes on the motors. Even now, with acrylic, I still get some chatter, but now that much.

I'm assuming that the .05 inch bit was a typo, and that you're really using an 0.500 inch cutter. Both Eriks gave good ideas. Personally, I use an O-Flute spiral like Eric Lamoray suggests. I also do my first cut as a climb cut and then repeat the cut as a conventional cut. The cutter seems to flex about 0.020" away from the true path when in climb mode. Following it up with another cut in the conventional mode cleans off that 0.020".

[paco]

I agree with all the above and mostly with the "your photo looks exactly like the chatter that I always got with my Alpha". That's pretty much what you can get with the 1:1 ratio and 3.4.X SB3 control... unfortunately...
A final very slow finishing pass (slow RPM too) might help some more.

[eklug]

No that bit size is correct. A .05 bit. We have to do very tight corners on the templates in order to match the dimensions and profiles for the finished mouldings. we can't use a larger size for 95% of the templates we need. Even a .125 would be much too large. I would slow it down more but i cant go any slower. I do have the PRT alpha and we also have another shopbot with a colombo spindle but that one has horrible scallop marks when cutting diagonal and just as much chatter as the router. (I have to have maintenance look at the pinions on the spindle cnc to see if we can get rid of that scalloping). I cut the entire piece as a climb cut in order to get the cleanest cut on the finished piece and the bad edge on the waste. I screw the piece down to the spoil board. Several screws on the waste and one or two in the center of the template. The acrylic shavings pack the cutout very tightly so the piece is very stable. I think what we will finally end up doing is outsourcing our templates again. We used to go through somebody that had a laser cutter. They only cost $35 each and honestly we are probably spending more doing it ourselves. We have problems sending them our CAD files intact and usable for them (we use turbocad).

[wcsg]

I would have to agree to have this work lasered, way you ge a polished edge too. But you should only be getting charged $100-150.00per hour

[benchmark]

Hi Emily

We have produced cutter templates on the Shopbot. The method we use is a 45D V bit and an engraving strategy using VCarve Pro, this will give you sharp internal corners for the grinder to follow.


Paul

[edcoleman]

Emily:

I agree with Erik - a laser would make quick work of this. Estimating the size of the piece based upon your picture, it would probably take less than 2 minutes on the laser table. If you're supplying a laser ready CAD file (which turbocad certainly is), then the $35 is a bit high. I'd be glad to cut them for you (the only problem would be shipping, I'm in NJ - it might make sense only if you are doing a bunch to spread the shipping cost). Drop me an email (ed_at_colemanwoodworking<dot>com&# 41; if you're interested.

-Ed

[mklafehn]

Emily,

I have a laser machine that could do those pieces for you. I am located in Chicago, so shipping would be minimal. I agree with Ed, that $35 is a little high if everything is set up.

I'm not looking to compete with Ed, but if it saves you money on shipping then maybe I can be some help.
my email is images@digicomimaging.com

Send me a sample file and I will cut you a sample at N/C to see if this is what you are looking for.