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  • Need some more help

    Hi all
    Need some more help. I usually make parts one at a time, but now with the cnc I have a project that requires that I make 8 parts from one sheet of 1" HDPE. The sheet is 8x24. I have figured out how to hold the sheet down to the table, but I need some way to be able to flip it over and cut the exact same pockets and outside profile on the opposite side. I do not know how to do this. Any advice would be great!!

    Thanks in advance!

    Walt

  • #2
    Walt, welcome to the wonderful world of fixtures.

    Need to know more about your part to really help much, but let's make some assumptions.

    To do the flip, you will need to accurately locate two edges. This is best done with 3 pins--2 on one side and 1 on the other. Why 3? So that you are not overconstraining the location of the part. All this and more can be found in Carr-Lane's excellent book on fixturing, or really any other book on fixturing.

    Here is an example of a 3 pin fixture:



    It was easy to whip it up. I just used some off-the-shelf dowel pins and some aluminum MIC6 tooling plate I had sitting around. Drill and ream for the pins, Loctite them in place, and you've got your fixture.

    Lots of other ways to do this job too, of course, but that would be one approach. Ideally your part has some through holes that would facilitate clamping to the fixture plate via some threaded holes.

    Cheers,

    BW
    ---------------------------------------------------

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    • #3
      Thanks for that, my part does not have any holes in it, but I could drill holes in the stock. Basically the part is a rectangular peice with 2 pockets on each side, but the location of the pockets is critical as well the outside profile of each piece. I have the part drawn in cad with the outside rectangle representing the stock. Right now I only have the one side drawn, but figured I would just mirror the part to get the opposite side. I am not sure where to drill the holes in the stock for locating, but I would assume on the exact center of the stock, is that correct?

      Thanks
      Walt

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      • #4
        Would this work? Take the rough stock and find the center on one end, drill a hole find the center at the other end and do the same. Use these 2 holes to locate the stock on the table by having alignment pins go thrugh the drilled hole's. Cut the one side, flip it over aligning it back on the pins and then cutting the otherside using a mirrored toolpath?

        Walt

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        • #5
          register pin top left hand corner, dowel will do, 2 sets data, top bottom, job done
          hope you enjoy the headache as mine is mighty
          mark

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          • #6
            If I put the hole in the stock at the top left corner, when I flip it that hole wil be at the bottom left, or am I not understanding something?

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            • #7
              oops, thought you were talking about 2 sheets together, i'll shutup soree
              mark

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              • #8
                No problem! You do not have to keep quiet, all suggestions are welcome!!

                Walt

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                • #9
                  Originally posted by wdp67
                  Thanks for that, my part does not have any holes in it, but I could drill holes in the stock. Basically the part is a rectangular peice with 2 pockets on each side, but the location of the pockets is critical as well the outside profile of each piece. I have the part drawn in cad with the outside rectangle representing the stock. Right now I only have the one side drawn, but figured I would just mirror the part to get the opposite side. I am not sure where to drill the holes in the stock for locating, but I would assume on the exact center of the stock, is that correct?
                  The suggestion I think you were responding to doesn't involve any holes in the part or the stock. Look at the picture, the part being machined is the thin stock not the thick plate, and it is positioned via the edges. Two pins set the translation on one axis and rotation and the third pin sets the translation on the second axis. Machine the edges of the stock you will use against the locating pins when you flip while you machine the first side of the stock. Then you know the position (including rotation) of the new alignment edge relative to the work. If you have indicated in the pins, then you will know the position of the work relative to the coordinate space of the machine.

                  Since your piece is long, you will probably use the same edge on the third pin in both sides. If this edge isn't flat and square, you could have problems unless it happens to be dead center in both orientations. However, if it is offset from center, then you can machine a portion of this edge for reference when you flip.

                  Technically, you don't need to use the pins on the first side of the stock as long as the stock is roughly aligned so all the parts to be machined fit within the space the stock occupies.

                  To align using holes in the stock, use two pins and one hole and one slot.

                  Over 24", thermal expansion/contraction could cause some of your parts/features to be misaligned. Plastic could expand/contract about half a mil per Celsius degree average temperature change.

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