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  1. #1
    Join Date
    Mar 2010
    Posts
    621

    Acrylic machining edge finish…

    I’m trying to edge finish some acrylic and running into artifacts as opposed to a nice smooth finish. I’m running @18K with an .250 oflute bit cutting conventional @120ipm.

    Has anyone any experience and might know what is causing the problem as seen in the pics?

    Thanks for any input.
    Attached Thumbnails Attached Thumbnails 8C87AAF1-2139-42B9-9654-8E52F3F76B8E.jpg   EF12B333-8202-4B38-BE15-BF291482473A.jpg  

  2. #2
    Join Date
    Dec 2003
    Posts
    1206

    Re: Acrylic machining edge finish…

    My guess is a lack of rigidity in the machine or worn spindle bearings,otherwise the tool trajectory should be smooth and wouldn't leave ridges and scallops.You did well not to have swarf weld itself to the sides of the cut pieces.

  3. #3
    Join Date
    Apr 2007
    Posts
    273

    Re: Acrylic machining edge finish…

    Quote Originally Posted by adam_m View Post
    I’m trying to edge finish some acrylic and running into artifacts as opposed to a nice smooth finish. I’m running @18K with an .250 oflute bit cutting conventional @120ipm.

    Has anyone any experience and might know what is causing the problem as seen in the pics?

    Thanks for any input.
    could try to slowing the feed rate down to see if the finish improves? if not i would second the first response and add it could be deflection from the tool as well. 1/4 diameter is not very big and if the tool is long that could give you problems too.

  4. #4
    Join Date
    Mar 2010
    Posts
    621

    Re: Acrylic machining edge finish…

    Thanks for the reply's.

    Could that also be stepper noise? Meaning that the steppers movement is leaving those marks as they are very consistent... this is a rack and pinion machine but very stable, acrylic shouldn't really be a problem for it.

    I've been using shallow passes .125 so no swarf has been even close to welding. Tool stick out has also been minimal since this is only .500 material, so I have to think its not toll deflection.

    Spindle bearings I hadn't thought of, this is a relatively new spindle with very few miles on it but it is Chinese.. how would I check for a bad bearing(s)?

    Also, could I use a .500 multi-flute bit for finishing while I'm running a finish pass @ .008?

    Thanks for the input.

    Adam,
    Gecko G540, Rack and Pinion Drives-X and A axis, 1/2-10 5 Start Acme-Z Axis
    4-THK HSR 25 Linear Slides, KL23H2100-35-4B, Power Supply-KL-600-48 48V

  5. #5
    Join Date
    Jul 2017
    Posts
    347

    Re: Acrylic machining edge finish…

    Send a pic of your cnc machine so we can see if its rigidity issue....that finish looks very good in my opinion...acrylic is "slippery" and tool loves to deflect....if you do a straight line toolpath are the marks still there?

  6. #6
    Join Date
    Mar 2010
    Posts
    621

    Re: Acrylic machining edge finish…

    Here are a couple of recent pics..

    Tool stick out is about 1".. I'm not thinking rigidity is an issue but I could be wrong.

    Will cutting too fast or too slow give this finish?

    Thanks.
    Gecko G540, Rack and Pinion Drives-X and A axis, 1/2-10 5 Start Acme-Z Axis
    4-THK HSR 25 Linear Slides, KL23H2100-35-4B, Power Supply-KL-600-48 48V

  7. #7
    Join Date
    Jul 2017
    Posts
    347

    Re: Acrylic machining edge finish…

    im not an expert..however your machine looks pretty nice..its definitely not a junky cnc router..and looks rigid from what i can see assuming everyhting seems pretty tight etc..as far as acrylic i do know that heat is the biggest issue since the melting point is so low..so it likes to be cut at a faster feed rate than other materials...tool stick out can be an issue too..and could be flexing..i have had bad luck machining acrylic on my junky hobby cnc...to me the finish does not look that bad..but thats compared to what im used to...and from what i understand 0flute and 1 flute are preferred for acrylic to help keep the heat down..but like i said your issue appears to be a rigidity or tool flexing from what i can see..i think climb cutting can create a better finish too...18k rpm is a lot so in order to keep chip load proper you need to feed at a high rate as you are but ..if ur spindle is variable speed .have you tried slowing down both rpm and feed rate a bit?..in my opinion 1 inch stick out is not excessive but could be a bit much for a 1/4 end mill especially when doing acrylic...not sure if you are able to leave a small amount like .005 and then do a finish pass or not but thats an option too.

  8. #8
    Join Date
    Mar 2010
    Posts
    621

    Re: Acrylic machining edge finish…

    Thanks.. for the suggestions.

    What feeds and speeds are others using to finish acrylic edging? I am using an Onsrud o-flute bit... just curious.

    Thanks,
    Gecko G540, Rack and Pinion Drives-X and A axis, 1/2-10 5 Start Acme-Z Axis
    4-THK HSR 25 Linear Slides, KL23H2100-35-4B, Power Supply-KL-600-48 48V

  9. #9
    Join Date
    Sep 2010
    Posts
    183

    Re: Acrylic machining edge finish…

    I have had very similar results running "acrylic" bits in conventional mill. I played around and found if you use a spiral O flute for aluminum and run in climb mill you will get good results. I specifically used an amana tool 57342-z sc and a 57347-z sc
    Cutting 1/4 acrylic- 18k
    1st pass- 130 ipm, 60 degree lead. .010" side allowance, .032" bottom allowance.
    2nd pass- 100 ipm 60 degree lead


    Here's the best edge pic I have. In short in the beginning of the pandemic I made hundreds of sneeze shields.
    Hope this helps-
    Attachment 468080

  10. #10
    Join Date
    Mar 2010
    Posts
    621

    Re: Acrylic machining edge finish…

    Many thanks for the reply... I've yet to throw some scrap on the table and run some tests but Intent to as soon as I get another moment.

    The consistency in the "cut marks" being left behind is what is a little perplexing .008 for a last pass should have cleaned that edge up nicely, maybe there is some run out on the spindle?

    Adam,
    Gecko G540, Rack and Pinion Drives-X and A axis, 1/2-10 5 Start Acme-Z Axis
    4-THK HSR 25 Linear Slides, KL23H2100-35-4B, Power Supply-KL-600-48 48V

  11. #11
    Join Date
    Sep 2010
    Posts
    183

    Re: Acrylic machining edge finish…

    I'd give going climb mill with those feeds n speeds a try then see where you are at. I got the same marks until I cut as mentioned in my last post.

  12. #12
    Join Date
    Mar 2017
    Posts
    926

    Re: Acrylic machining edge finish…

    Are you using the edge of the tool or the bottom for the cut? If it's the bottom then it will always leave a visible pattern (unless the tool is the same width or larger than the cut).

    If the pattern can be felt as well as seen then if could be a tram issue. Have you checked if the spindle is square with the table and each axis? If a flat bottom tool engages at an angle then it will cut grooves into the surface.

    Try reducing or increasing the step-over and see if it changes the distance between lines.

    Or, you could just sand out the marks and not worry about it. Acrylic is soft and easy to sand.

  13. #13
    Join Date
    Mar 2010
    Posts
    621

    Re: Acrylic machining edge finish…

    Quote Originally Posted by Goemon View Post
    Are you using the edge of the tool or the bottom for the cut? If it's the bottom then it will always leave a visible pattern (unless the tool is the same width or larger than the cut).

    If the pattern can be felt as well as seen then if could be a tram issue. Have you checked if the spindle is square with the table and each axis? If a flat bottom tool engages at an angle then it will cut grooves into the surface.

    Try reducing or increasing the step-over and see if it changes the distance between lines.

    Or, you could just sand out the marks and not worry about it. Acrylic is soft and easy to sand.
    Edge of the tool.. last pass was for .008. For kicks I swapped out the o-flute bit for a 2 flute and the chatter got worse.

    I'll also take a closer look at the tram of the spindle... but I'm thinking its pretty square to the table since I fly cut the spoil board with a 2 1/2" bit without signs of ridges, etc.
    Gecko G540, Rack and Pinion Drives-X and A axis, 1/2-10 5 Start Acme-Z Axis
    4-THK HSR 25 Linear Slides, KL23H2100-35-4B, Power Supply-KL-600-48 48V

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