Acrylic pockets and contours are chipping

hello. ive noticed recently that im getting some chips in the material when pocketing and contouring cast acrylic. since getting the HDM i’ve made many changes to my CAM and rather than go back and redo everything to see what the culprit might be i figured I’d ask here in case its some thing simple.

3.12mm (1/8") flat bit 2 flute upcut (onsrud 66-309)
1mm doc
11500 rpm
2800 mm/m
1mm stepover
plunge ramp

thanks in advance

Have you seen:

from the #MaterialMonday series?

Could you post a photo?


perfect. exactly what i needed I’ll go from there and see where i end up

maybe dont use an upcut bit? I cant see how that would be beneficial. Maybe go with a single flute for all the roughing out and hit a finish pass with a higher flute count to prevent as much melting as possible. Using a downcut will hopefully keep the edges from chipping. Cast acrylic is pretty brittle, well, all acrylic is pretty brittle in my experience.

Maybe also inspect your endmill with a loupe. It’s easy to miss a chipped flute, a worn cutting edge even more so.


I’ve updated my CAM to:

3.115mm (1/8") flat bit 2 flute upcut (onsrud 66-309)
2mm doc
18000 rpm
1829 mm/m
1mm stepover
plunge ramp

and everything is working great. I went ahead and moved to a new bit just in case but the increased DOC has me cutting faster than before even with the slower feed. Beautiful surface finish and no more chipping.


Upcuts are essential. Downcut and even straight cut will cause a lot of melting plastic as the heat needs to com up and out. :slight_smile: But single flute upcuts work sooooooo much better.


So we can’t really tell if it was the feeds and speeds or a worn out cutting edge :wink:

Try the old tool with new F&S on a scrap just for giggles

Source please Mr. Jedi

Pretty sure it was there tool. I was using those speeds and feeds for awhile without the chips. That said I’m glad to have adjusted the speeds and feeds because i feel the quality is better and the runs are over twice as fast. My previous machine had an anemic 500w spindle that my previous recipes were cooked up to suit. Been so busy with production I haven’t found time to play around with optimizing for the HDM.

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Source for the single flute endmills?

I generally use the ones Carbide 3D sells, or off the shelf Amana endmills.

I used a Single flute SPE but for most of my acrylic work. These are about 7mm and I leave small .25mm tabs on the final pass.


.125 Single Flute:
SpeTool O Flute UpCut 1/8" X 1/2" Carbide End Mill Single Flute Spiral CNC Router Bit Aluminum Acrylic Woodworking Cutter

.25 single flute
Yonico Solid Carbide Single Flute Upcut End Mill Router Bits CNC Spiral O Flute 1/4-Inch Diameter 1/4-Inch Shank 31015-SC


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