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  1. #1
    Hi,

    I'm trying to cut some parts from 5mm black acrylic and what I'm finding is that cutting on the short axis I get a decently smooth cut, but along the long axis I get a kind of streak of roughness, but it is not across the whole cut depth. Can anybody suggest what is causing this? I did cut in multipul passes and then did a finish pass a well. I'm using a 3.17mm cutter and went at half the speed GWizard suggested, halving both the mm/min and the spindle RPM.

    The rough part is slightly inset from the rest of the cut, not proud, is that means anything. Also the rough part is at the top of the cut so it seems like things improve as the cuts go deeper.

    Here are a couple of photos.



  2. #2
    Ahh, I just looked at the sheet I cut the part from and it seems to have a better finish on it. I used conventional milling as GWizzard suggested but it looks like climb is better?

    I'd still like to know why it is okay on the other axis though and what causes it to look bad just on one of the passes.

  3. #3
    I was going to suggest climb milling, not sure what the reasons are but on the signmaking forum I use when the question comes up people always say you get a better finish with acrylic using climb rather than conventional milling.

  4. #4
    isnt slowing the spindle speed down a bad thing for acrylic? i'll be honest though that looks like crap on the bit, it's not melting the protective sheet to the router bit or anything daft like that is it?

  5. #5
    Slowing the spindle down shouldn't be an issue if I keep the feedrate proportional, right? The chipload is the important thing I think. Still, maybe it will cut smoother if it is faster, I should try.

    I might also simply need to try another cutting bit, perhaps this one is slightly damaged.

  6. #6
    Quote Originally Posted by Tenson View Post
    Slowing the spindle down shouldn't be an issue if I keep the feedrate proportional, right?
    I think that's right but dont see it would help for this, i could be wrong.

    Quote Originally Posted by Tenson View Post
    The chipload is the important thing I think.
    Yea its a good indication that things are working well, i.e you want chips not dust, chips not spirals, though this is probably different for different materials.

    Quote Originally Posted by Tenson View Post
    I might also simply need to try another cutting bit, perhaps this one is slightly damaged.
    Are you only getting this in soft stuff "plastics" or what, it could be the deflection/accuracy thing seen in climb vs conventional.

    .Me
    .Me

  7. If your chip evacuation is not sufficient then this is normally the cause of this. Do you use a decent vacuum to remove the chips as they are being cut? What tends to happen is if you start to recut the same chips they become hot and rub and "smear" around the cutter and are deposited on to the material edge and result in what you see in your picture!.. thats what ive come across in the past.

    Failing that use some wet+dry like 1200 grit to clean the edge then give it a lick with a blowtorch preferably using MAP Gas as this burns hotter and has less oxygen content so less contamination of the plastics edge.

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  9. #8
    I do have a vacuum but maybe it's not working well enough in that direction of cut. I'll have a closer look tomorrow and also try another bit.

    This needs much more than 1200grit, even with 150grit it takes a bit of work to remove the recess where the rough part is!

    How much of a finishing pass do you chaps recommend for acrylic? I was using 0.3mm but maybe if I used more, it would sort it out?

  10. #9
    From the images in the first post it looks like the machine slightly lost position after a few passes, then continued as normal. Tricky to explain, but what I think happened is at the depth of the poor finish the machine lost position and moved further away from this cut, but not by much, such that on the subsequent passes the cutter didn't quite touch the first bit that was cut, but did stick hot swarf to it, hence the poorer finish. Meanwhile the cutter would have cut more from the waste piece, over the full height - hence the better finish on that part. I've seen this happen on my machine when a part hasn't been clamped well, so it moves ever so slightly giving the same effect. Could be the same here...


    Quote Originally Posted by Tenson View Post
    Slowing the spindle down shouldn't be an issue if I keep the feedrate proportional, right? The chipload is the important thing I think.
    Spot on...

    Precisely what feedrate, spindle speed and depth of cut were you using?
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  11. #10
    Hi Jonathan,

    I had it pretty well clamped I think. Please remember the roughness doesn't seems recessed from the rest of the cut, so possibly not just re-cut chips embedded in it?

    I was using:

    700mm/min
    11,000RPM
    2 flute 3.17mm cutter
    1mm multiple passes
    0.3mm finishing pass

    I'm not sure how it would have lost position at such a low feedrate. I guess I might need to check the rotating ballnuts to make sure the cogs aren't working loose.

    By the way I got my vacuum extraction working just the other day.


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