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Milling 3mm Perspex (Plexiglass, Acrylic) Disc

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  • Milling 3mm Perspex (Plexiglass, Acrylic) Disc

    I need to make essentially a very large washer (~3" OX x ~1.25 ID) out of clear black acrylic sheet (Perspex 962 Black).

    I'll be using a CNC mill, with a spindle speed range of 80-30,000 RPM and feed to 147 IPM. Cutter will be a new, high quality 1/8" 4FL carbide end mill.

    I would appreciate some advice on speeds, feeds, and techniques to do this without melting or excessive chipping. GWizard suggests ~20,000 RPM & ~147 IPM (.0019" per tooth). Feed per tooth stays the same at lower speeds. If I choose "conservative" on the slider, it drops to .0012 per tooth, but still 18,000 RPM+.

    The recommended speeds seem likely to cause melting (no coolant) & the feed seems likely to chip. I'll probably use 3 passes to get through the full thickness, but not really sure about the rest. It's not a part where perfection is required, but I'm kinda picky.


    Thanks
    Ed

  • #2
    Keep the rpm at the very low end like around 5000 rpm. Feed consistent with less than maximum chip load. If it is at all flexible make sure it cannot climb the cutter or it will. Lube with soapy water for cooling. BTW, I am having difficulty imagining "clear black" plastic.
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    • #3
      Whoa, those speeds are way too high, Even is closer but to be honest I'd veer to a bit lower than that.
      Can you use 2 flute or even a single flute router bit ?

      Secret to milling plastic is to get rid of the chips, recutting chips causes heat and rewelding.

      Just done three small machine windows today in 3mm clear perspex, ran at 20mm / sec but I cheated and used a laser, came out perfect with polished edges - sorry
      .

      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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      • #4
        but I cheated and used a laser
        You SUCK John.

        If cutting dry I would also go lower than 5k rpm. With water it will work fine.
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          Ed,

          I'm currently working on a job which involves cutting some pretty complicated shapes in black acrylic. (Evan, Ed's not entirely wrong about Perspex being 'clear black' - when you mill it to less than 1 mm thickness and hold it up to the light, it's actually dark green and translucent!)

          I did it on a much less powerful machine than yours and went for very conservative feed. I used a 2 mm 2-flute carbide endmill at 12 000 RPM without coolant, with 0.5 mm depth of cut and 8 mm/sec feed. The edges come out very smooth, 'semi-polished', without any hint of chipping. I agree with Evan about the importance of robust workholding.

          My next challenge is to fake up a line bender with some nichrome wire and a power supply, to put an accurate 90 degree fold in it.

          George

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          • #6
            Evan, Ed's not entirely wrong about Perspex being 'clear black' - when you mill it to less than 1 mm thickness and hold it up to the light, it's actually dark green and translucent!
            Yeah, that holds for the dyed acrylic but not the opaque types. Both look the same with any reasonable thickness. I have many sq feet of acrylic and polycarb in a wide variety of colours that I picked up for nearly nothing when a local sign shop went out of business.
            Free software for calculating bolt circles and similar: Click Here

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            • #7
              Be careful with Acrylic !
              You want Lexan or Plexiglass, which are Polycarbonates, and will not shatter suddenly, as Acrylic's will.
              The difference is amazing

              Rich

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              • #8
                Rich,

                Plexiglass is just another acrylic. Makrolon (Bayer Materials Science) and Lexan (SABIC, formerly GE) are the two common polycarbonate brands.

                --Cameron

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                • #9
                  Not quite, Rich. Plexiglass is acrylic, or more specifically polymethyl methacrylate. Lexan is polycarbonate. Yes, plexi will shatter before Lexan, but I'm not sure where that fits in here. I've cut a lot of acrylic and never had it shatter. Shattering would be more a worry of the end product, and the good Doctor never said what that was.

                  I've been cutting and etching a lot of acrylic lately. I use a (is it 'zero' or 'oh'...looks like a flute to me, but that's what they call it.) flute router bit in my router table to cut it. The cuts come out clean, no chipping, but the finish isn't quite mirror like. Osrund makes the bits. I've tried standard router bits, but they just make a big melted mess.

                  I use a 2 flute end mill to etch it, but not without troubles.

                  Best of luck.

                  Edit: Cameron types faster than I do.
                  Last edited by browne92; 11-15-2012, 11:18 PM.
                  Definition: Racecar - a device that turns money into noise.

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                  • #10
                    Be sure to sandwich the acrylic between two stiff/rigid pieces of sacrifice materials such as good solid wood, squeeze TIGHT. Bore through the lot. This method prevents shattering of the fragile material while producing a perfect hole. I have successfully drilled paper, 1/8" acrylic, extremely fragile plastic bottle lids, tin foil and even cellophane food wrap.

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                    • #11
                      I was just looking at a stack of laser cut acrylic tonight - a beautiful job, long thin strips, dead straight, perfect edges, much better than saw or other machined finish.

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                      • #12
                        Originally posted by ckelloug View Post
                        Rich,

                        Plexiglass is just another acrylic. Makrolon (Bayer Materials Science) and Lexan (SABIC, formerly GE) are the two common polycarbonate brands.

                        --Cameron
                        Thanks, I stand corrected.
                        I always use Lexan for exotic parts.

                        Rich

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                        • #13
                          Originally posted by Evan View Post
                          Keep the rpm at the very low end like around 5000 rpm. Feed consistent with less than maximum chip load. If it is at all flexible make sure it cannot climb the cutter or it will. Lube with soapy water for cooling. BTW, I am having difficulty imagining "clear black" plastic.
                          LOL funny perspective. I'm so used to working with my old manual machines that 5000 sounds like the extreme high end! My fastest machine tops out at 5500 and my lathes top out at 1500 ... I would never think to call 5k the "very low end"

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                          • #14
                            Why would you not make this on a lathe? You can stack them and make a bunch at one time. For a washer, it would have been my first thought. Bob.

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                            • #15
                              If it is any help I cut Lexan with a wood router that has a bearing for following a pattern. The key to the cut is clearing the chips! If not they heat up...melt... clump... anf then all bad things happen.

                              I do this for motorcycle windshields. to finish the edge I use a file then buff to a nice class look.

                              Just a thought

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