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holding down acrylic

Discussion in 'CNC Mills/Routers' started by tommy arno, Aug 2, 2017.

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  1. tommy arno

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    Any tips on the best way to hold down the spoil board on a cnc router, ive tried cutting a sheet of perspex and the sheet seems to run up the bit, the acrylic is screwed down to the spoil board but the board isn't fixed down, I don't know if the acrylic is coming up or the whole board but the the Z distance is not staying the same
     
  2. Jonathon Duerig

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    You definitely don't want your spoilboard to float. The best way to connect it to your table is with L-brackets like these:

    L Bracket

    One end can screw into a T-nut and the other end can screw into a spoilboard. If you use MDF, then just use a wood screw that is just small enough not to penetrate the surface.

    I cut acrylic regularly. Getting the right feeds and speeds is tricky. You want to lower your spindle speed quite a bit. And you need your feed rate to be fast enough to prevent melting. Using single-flute up-cut bits designed for cutting hard plastic is best. I typically cut much shallower than in other materials as well. I use a 2mm radius bit and cut 1mm of acrylic on each pass.

    Note that if your Z-axis isn't 100% accurate or your board isn't 100% level, you will be cutting deeper on the first pass than you specify no matter what. So you might try making your first pass extra shallow to compensate. For example, let's say that because of an uneven spoiler board, some of the material is 0.5mm higher than other parts. This means that if you specify a 1mm cut depth, the first pass will sometimes be cutting 1.5mm deep in the material, a 50% increase in cut depth, and melting will be more likely. So if you set the first pass to 0.1mm and the other passes to 1mm, then you can be more certain that no pass will cut too deep.

    As far as clamping, I find that the best way to clamp is to get my stock in pieces twice as large as my working envelope. I have a 600mm x 600mm working envelope, so I get my stock cut down to 24" x 48" at my supplier (they use US units). This way, each time I cut I have a long 'tail' that extends out from the cutting area. I clamp a 2x4 across this tail for a quick and completely secure side. Then you can use less drastic clamping methods on the other three sides, either tape or screws or low profile clamps or whatever. When I am done cutting half of the 24" x 48" stock, I turn it around and cut the other half and clamp the skeleton of the first half.

    -Jonathon Duerig
     
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  3. tommy arno

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    thanks for the reply. I have fixed the board down and managed to cut a small circle. but the edges are really rough, ive used an 8mm bit to cut a 80 mm circle in 3mm acrylic, with a speed of 22600, I slowed the feed rate down to 1800 s anything wrong with this set up?
     
  4. Jonathon Duerig

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    I'm not sure what the best settings are for you. I have never used a bit that large to cut acrylic. For my 2mm bit, I use a speed of 12000 RPM and a feed rate of just 450 mm/min. So if you slow everything down, you might get better results.

    Keep an eye out for melting. The first clue for melting is often the smell. Partial melting might lead to rougher edges as pieces are partly melted then broken off. Though it also sticks to the bit and eventually causes a bit to break when it melts in my experience.
     

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