Noticeable surface height differnce
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  1. #1
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    Default Noticeable surface height differnce

    Hi there,

    I'm having an issue with noticeable surface finish height between different ops.

    I'm working with a part made of cast acrylic. It's a 3D shaped part but the problem I have is with a flat surface (highlighted in the second image).
    Due to the shape of the part I have to "face mill" this surface with various passes and some of those passes overlaps (have a look at first image). There is a noticeable surface height between those passes even though in code it's always at the same Z height.

    All of the cuts are done with the same cutter (3mm single flute with polished edge from Datron) with the same feedrate and at the same Z height. All of them are climb cuts.

    I'm loosing my mind here! I'm sure that it's not in the machine because there is no height difference between passes within one op even when there are retracts in-between.

    Please have a look at the second picture to see what I mean. I quickly spray painted it black to make it more visible.

    Feeds and speeds for all operations on this surface:
    24000 RPM
    1500 mm/min

    Does anyone have any idea?
    I can post the code if needed, but it's rather long..

    Cheers,
    Bartosz
    Attached Thumbnails Attached Thumbnails pic-1.jpg   pic-2.jpg   pic-3.jpg  

  2. #2
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    Default

    I don’t know diddly about cast acrylic but the step over from the picture with the tool path looks much larger around those three round bosses and the triangular one. It is possible the heavier cut in those areas is making your tool deflect more. Can you match the step over in those areas to keep a consistent chip load on your tool? Good luck!

  3. #3
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    Is your workholding strong? I'm thinking that the part is possibly lifting with heavier cuts.

    What would happen if you ran a part and then without removing the part, ran the program again? That might tell you some things. If nothing cleans up, you could lower your Z height a small amount to see if the surface is made flat at a very low depth of cut.

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    Thanks for the replies.

    That's true that the stepover varies between the cuts. However I don't think that this would cause any tool deflection. The part is previously roughed out and there is only 0.2mm of material left for the finishing pass. The feed per tooth is only 0.0416667 mm with a very sharp cutter.
    That being said, it never hurts to run a few tests and try to make the stepover similar in all passes.

    Part is cut out from a bigger sheet which is held down on a vacuum table. This thing is not going anywhere.

    I will try to re-run the surfacing tool path, thanks.

    I could of course play with different Z heights for each op to make it match but I would rather prefer to understand why is it happening in the first place.

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    Looks like a program problem to me. Attached images are far too small to see the toolpaths clearly, but I can see that there is a single linear toolpath clear through the middle of the part that corresponds with the "gouge" in the third picture.

    My guess is the final floor pass is incomplete. Don't know what CAM or strategy you're using though, so can't suggest anything more than that.

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    I would agree with the part probably lifting, plastic is not easy to hold securely. Also, is the part warming up or cooling down between operations? The part may be growing or shrinking. And yes, stepover can make a dramatic difference in deflection, I believe 33% and 66% stepover deflects mostly along the direction of travel, giving the least amount of sideways deflection. And I have used downcut tooling with some success on parts that wanted to lift really badly, they're a useful option for materials that flex easily or can't be clamped tight (or vacuum fixtures).


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