I was machining a 2-hour flip machine job and it failed on the very last tool path, seemingly for no reason.
It was doing the cutout contour and had traced out the contour path correctly, cutting about 75% of the way through the material. However, on one of the last few passes the machine started seizing and cut incorrectly into the piece. By seizing I mean it was making the sound as if I had told the machine to jam a bit into the table, that horrible sound of motors fighting against something that won’t let them move.
Any ideas what could cause this? The material is acrylic and the bit was a 0.25" end mill. I did notice chips weren’t clearing from the channel effectively. Could chip buildup stop the motor and cause it to seize?
If you are getting chip buildup, it could melt. After it solidifies, your endmill is now just a blunt rod. Hilarity ensues.
You need to help with chip evacuation. Don’t run toolpaths that are the width of the cutter (make a wider pocket instead), make sure your dust collection is removing the chips, and if not, use air blast to get them out of the way.
You didn’t mention what kind of bit you are using, the usual recommendation is a O flute (single cutting edge) bit.
Answer: yes.
Looks like it is extruded acrylic. I only use cast acrylic. And O-flutes, with low rpm and high feed: low speed of the tool on the cutting edge to reduce friction what could build up heat, and get the bit away from the place asap therefore the high feed, and O-flute since that bit has only one “tooth” with minimal surface. Check whether test runs produce really chips and not just melted plastic. And William’s and Michael’s recommendations are really worth to consider. Took me some years to learn why my thick pieces failed.
Thank you for the recommendations and thoughtful response.
I was indeed cutting a slot as narrow as the tool through a 0.5" thick piece of acrylic. I read your posts about offsets and pocketing and am going to try that this morning. It’s a clever solution to leave a thin outline the thickness of the chip load so that you can do a finishing pass along the entire thickness of the piece. My vacuum extension fits the sweepy 2.0 pretty well but could be tighter, so I might try to improve that later.
I was looking at the chip buildup during the job and they seemed to not be melted, but they were packed in the slot pretty tightly. The machine was moving so fast that I kinda said to myself, “eh, it’ll be fine”. As we saw, it wasn’t.
I was indeed cutting a path the same width as the tool, which I’m going to address today. I was also using a 3-flute Zr 0.25 endmill, because I wanted a nice surface finish on some pockets in the piece, but it was clearly not the right bit for the contour. But I do have a 0.25" O flute, so I’m going to try using that today. I just didn’t want to go back to dialing in speeds and feeds for a new bit oh, well. Thanks for the advice!
Yeah I’m going to take everyone’s advise and use a 0.25 O-flute endmill with low rpm and high feed and use a pocket cut instead of a contour down to the tab height. Thanks for that recommendation.
Can I ask why you use cast acrylic vs extruded? Is there any noticeable difference in the finished products of each? And how could you tell that it was extruded? You’ve a keen eye!
oh, well. Thanks for the advice!