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  1. #1
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    working with perspex, looking for some information

    Hey G'day folks,

    Some advice please..

    I have a number of 9/11 mm perspex sheets varying sizes average sizes 12 x 14" thereabouts.

    Thought I'd give a go at doing some reverse bas reliefs in the medium. I did this using a tapered ball nose 1mm radius. (I thought I'd try the tapered)

    It was my first try conventional offset, started at the middle of the piece and ended up with a nice piece. Its come out good enough for me to use as a mold and try my hand at Cold Casting a couple extra. (something else on my bucket list to have a crack at)

    Now I have tried unsuccessfully to do some other work, using single spiral end mill 2mm and 3mm, (cutting some pieces for other small projects that I have) with little to no success!

    Spindle speed, 18000 according to doctor google but that didn't do me much good, the bit was not ejecting the shrapnel well enough, particularly doing a profile cut.
    Faster spindle feeds, only melted the perspex which then stuck to the bit cooling and making problems. The melted bits got progressive larger and well you can imagine the result.

    Tried slower feeds and speeds, ended up snapping a couple of bits.

    Using a 6mm endmill produced somewhat better results but its too large for what i need it to do.

    Trying to peck some holes for threading the bits tend to melt their way through rather than cut.

    If anyone has worked with perspex, a tip on the type of bit to use, speed and feeds would be great.

    At the moment it seems more an accident that I got a nice piece rather than by design.

    Dr Google isn't helping much.




    Cheers,
    Steve


    Attachment 420782

  2. #2
    Community Moderator ger21's Avatar
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    Re: working with perspex, looking for some information

    What feedrates, and depth of cut?

    I'd lower the rpm, and take shallower cuts.
    Gerry

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  3. #3
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    Re: working with perspex, looking for some information

    Well the item I finished was done in a single pass, and it turned out fine with the tapered bit . 150mm/sec

    The others cuts I did, at 1.5mm / 2mm / 3mm depth of cut 60 mm/sec. I don't know how much slower to go, without causing a problem with rubbing and creating more heat.

    Problem I had in doing more passes and shallow depth was the chips were not coming out being hot they were adhering to the trench and change to a brittle property which the end mill was having problems working through.

    rpm you think to high, ok can always go lower.. I can afford to loose a few more bits.

    Cheers and ty for your reply.
    Steve

  4. #4
    Community Moderator ger21's Avatar
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    Re: working with perspex, looking for some information

    Cutting slower is the opposite of what you want. If it's hot, or melting, you usually want either higher feedrates,or lower rpm, or both
    It's hard to clear the chips when using small bits. What you really need is an air blast, or something else to help cool and get the chips out
    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  5. #5
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    Re: working with perspex, looking for some information

    I just finished reading an article suggesting the same thing.. go as fast as practicable to avoid heating..... ty for your reply

    Steve

  6. #6
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    Re: working with perspex, looking for some information

    Try using a 50-50 mix of dish detergent and water as coolant. Just brush it on
    Dave
    In the words of the Toolman--If you didn't make it yourself, it's not really yours!
    Remember- done beats perfect every time!!

  7. #7
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    Re: working with perspex, looking for some information

    I may give that a go, seeing as my wife has already objected to putting a couple of pieces in the freezer overnight.

    Cheers,
    Steve

  8. #8
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    Re: working with perspex, looking for some information

    Quote Originally Posted by curiosity22 View Post
    Hey G'day folks,

    Some advice please..

    I have a number of 9/11 mm perspex sheets varying sizes average sizes 12 x 14" thereabouts.

    Thought I'd give a go at doing some reverse bas reliefs in the medium. I did this using a tapered ball nose 1mm radius. (I thought I'd try the tapered)

    It was my first try conventional offset, started at the middle of the piece and ended up with a nice piece. Its come out good enough for me to use as a mold and try my hand at Cold Casting a couple extra. (something else on my bucket list to have a crack at)

    Now I have tried unsuccessfully to do some other work, using single spiral end mill 2mm and 3mm, (cutting some pieces for other small projects that I have) with little to no success!

    Spindle speed, 18000 according to doctor google but that didn't do me much good, the bit was not ejecting the shrapnel well enough, particularly doing a profile cut.
    Faster spindle feeds, only melted the perspex which then stuck to the bit cooling and making problems. The melted bits got progressive larger and well you can imagine the result.

    Tried slower feeds and speeds, ended up snapping a couple of bits.

    Using a 6mm endmill produced somewhat better results but its too large for what i need it to do.
    Hi,

    18,000rpm is very fast. I cut perspex at around 1,500 - 3,000 rpm with a 1/8" single flute cutter. I have a fog buster and use it to blow air with no coolant. The air helps to clear the chips and cool things down.

    You need to adjust things to find the sweet spot. If you break bits, it could be that the feed rate is too fast. If you slow it down, then the plastic might melt, so you then need to take shallower cuts so the feed rate can be increased.

    https://www.homanndesigns.com/index....roducts_id=131


    Cheers,

    Peter
    -------------------------------------------------
    Homann Designs - http://www.homanndesigns.com/store

  9. #9
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    Re: working with perspex, looking for some information

    Hey Peter G'day,

    Thank you for that information, ok that's a big difference in spindle speed. I can see the merit in that, I will give that a try in the morning. When you say shallow cuts are we talking less than 1mm ? so far I have been taking about 1.5 mm but I can take less if need be.

    I will start with your tips and see how I go from there.

    Thank you for taking the time pitch in with a reply.

    Cheers,
    Steve

  10. #10
    Community Moderator ger21's Avatar
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    Re: working with perspex, looking for some information

    But what feedrate at 3000rpm? You should get similar results at 18,000 rpm with a 6x higher feedrate.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  11. #11
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    Quote Originally Posted by curiosity22 View Post
    Hey Peter G'day,

    Thank you for that information, ok that's a big difference in spindle speed. I can see the merit in that, I will give that a try in the morning. When you say shallow cuts are we talking less than 1mm ? so far I have been taking about 1.5 mm but I can take less if need be.

    I will start with your tips and see how I go from there.

    Thank you for taking the time pitch in with a reply.

    Cheers,
    Steve
    My expertise is more in electronics rather than machining. I think my cuts would be 1mm at most.
    Also, it is my understanding that the depth of cut should be no greater than 30% of the cutter diameter.
    It’s all about the chip loading on the cutter, then compromises for the material machine rigidity etc.

    Peter

  12. #12
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    Re: working with perspex, looking for some information

    Well Im off to the workshop to try all this out, if 30% of cutter diam is meant to be depth of cut Im pretty safe there then. So I am either going too slow or not fast enough. Or the spindle speed is to quick or will be too slow.. Will have some results later in the afternoon.

    Cheers and ty for pitching in.
    Steve

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