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Internal corner 'fluff' when 2D machining of Polypropylene

Evolmit

Plastic
Joined
Aug 8, 2023
I cut stencils from 1.5mm to 3mm thick PP sheet, using a carbide 1/8" single flute 'spoon' type cutter (5/32" for the 3mm). On average, about 10000-18000 rpm (thin material at slower rpm), 500mm/min plunge and 5500 mm/min cut speed.
It cut's beautifully, but I have a couple of issues with the cut finish.
Firstly, in each internal corner, there remains a little flake of swarf, 'stuck' on, and requiring vigorous blowing off or picking off with a scalpel. This is very time consuming as each character has 2 to 5 internal shapes with 3 or more internal corners. I can cut hundreds of characters each day......
The machines linear and centripetal acceleration/deceleration settings are set quite fast to allow for another job we have in production on the same machine. Having said that, I have played with these settings to little or no avail.
Secondly, where the cutter unloads at the end of the cut, what remains is a little bunch of swarf, bunched tightly on the cut edge that requires picking of with a tool/fingernail. I have tried lead ins and lead outs. Both linear and arced.
I have even tried a linear lead out at 180deg to cut line, essentially going back over the end point to 'knock' it off. This kind of works, but still leaves a small clump that needs picking.
se-points-and-crns.jpg
If anyone has any suggestions as to how I could fix (or even improve) these issues, I'd be much appreciative.
Cheers.
 
I think the cutter is leaving the corner before it can finish cutting a full chip. One option would be to slow down the feed rate in the corners, if your software has that option. Another option is to use a cutter that's small enough to move through an arc in the corner instead of making a sharp 90° turn. Failing that, maybe try switching between conventional and climb milling (which ever you're doing, try the other.)
 
I think the cutter is leaving the corner before it can finish cutting a full chip. One option would be to slow down the feed rate in the corners, if your software has that option. Another option is to use a cutter that's small enough to move through an arc in the corner instead of making a sharp 90° turn. Failing that, maybe try switching between conventional and climb milling (which ever you're doing, try the other.)
Thanks mhajicek.
Your 1st suggestion will be the one to muck around with, I think. Option 2 might be an option, in conjunction with Opt 1, but my machine likes to go quite slow round tight radii, and this seems to leave a worse version of the problem due to the cutter rubbing at these slower speeds. On top of that, we are trying to minimise radii where the corner should be 90deg. Going smaller in cutter size causes the material thickness/cutter width ratio to be quite high, and to go slow enough to not break the cutter, I get said same rubbing which leaves a bad edge.
I have tried conv and climb milling. Only one direction gives ma a clean cut. Yes, the other direction gives slightly better corners, but the edge ends up horrible.
I have also tried up and down spirals, but these either lift the job, or push swarf underneath. Neither of these outcomes work well with a vacuum hold-down, so I'm stuck with single flute straight cutters.
I'm off to play with option 1. I'll report back with results. Again, many thanks. :)
 
What I would try....

Do a small pocket toolpath for your entry points. Then... At the corners, run a helix hole toolpath to get the corners cleaned up so there is really nothing there to cut when you do your final finish pass. OR... Maybe just a finishing pass. Not sure your production can take double the part time because these will add time. But I imagine cleaning this "fluff" adds more time than tweaking your tool paths.
 
Wait.... Finishing passes are your friend, Just run a .01 finish pass with the overlap lead i lead out and that will clean up.
 
At 10K with a .125 bit you have 327 SFM
I run about 230 SFM
If you can, drop Rpm's but keep the feed.
I think you're "heating" the material.......need to adjust rpm and feed to find the happy spot.
Also cutter must be sharp.........are you using a quality cutter?
 
Thanks to everyone for your input. I am going to try all your suggestions. I'll report back with the results.
 








 
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