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Thread: Machining both sides of material

  1. #1
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    Default Machining both sides of material

    Hey everyone,

    I have a PRSAlpha and I'm having difficulty aligning circular cuts on both sides of a sheet of MDF. The process thus far has been cutting the circles on one side, setting a jig to the outside corners of the material, flipping the material, and then cutting the same circles on the other side. For some reason the circles aren't lining up and I'm getting a small lip on the cut as if the circles are shifted offset from one another (see attached photos). The strangest part is that the circles towards the middle of the material have the largest lip, while the circles on the outside only have a tiny lip. I've checked the drawing file to make sure it was symmetrical, I've made sure our table is square, we've tightened the x motors to ensure that they aren't slipping, and the material is being placed in the jigs in the exact same place when flipped but no dice. If anyone has any other suggestions I would really appreciate it.

    Thanks,

    -Pat
    Attached Images Attached Images

  2. #2
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    Let me ask a stupid question. Why are you using a cutting toolpath like that? Why not cut it all the way thru from one side?

    Get the right type of bit so as not to blow out the bottom (thats a question for some other expert...).

    What is the waste material here, the circle cut out of the center so the important part is the piece with the hole? You could get some sacrificial material under the plywood and cut the hole deeper then the material and with the right bit you should have no problem.

  3. #3
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    I was going to ask the same thing. I think it is the roundovers but it's faster to cut them on a router table or with a handheld router.

  4. #4
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    How about instead of a jig which holds the sides of the material you make a jig with a few circular upstands which fit into the partial holes you've cut?

    John

  5. #5
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    Thanks for your responses. Jack, we're trying to achieve a roundover profile on each circles edge on both sides. Sorry I forgot to mention that in the original post. We considered using a handheld router but would like to accomplish this on our CNC machine if possible. I'm sure we will encounter the same problem on future projects that require machining from both sides so we would like to come up with a solution on this particular project.

    Thanks,

    -Pat

  6. #6
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    Unless your material is exactly square and the parts are exactly centered and you place the piece in the exact same spot on the table you are going to have problems. Even the swelling of the wood will make your cuts off.I have cut some stuff that the change in temp during the cut caused the cut to be off.

  7. #7
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    When you flip the sheet it comes off a different edge you may have to mirror your piece to get it to line up correctly, Ihave done this successfully but I set up block and cut the edges then mirrored the cut and placed it at the rights place to make it work. Good luck.. it does take a little to get your mind around it to make it work but it is possible with a little tweaking, only worth the time if you have a few to cut though.

  8. #8
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    A relatively simple way to do this is to make your jig slightly oversized, say 1/4" wider than the board. Place the board against the right side of the jig to cut the first side. Then flip the board and place the same edge that was against the right side of the jig, against the left side of the jig - that way you are always registering from the same reference surface.

    The best way to make the jig is to place it on the table and have the Shopbot machine the sides that the workpiece will register against. That way you know the jig is referenced properly on the table.

    You will have to offset the second side vectors by the 1/4" to make up for the workpiece being shifted. I hope this makes sense. Once you correlate the jig locations and the vectors for both sides it's easy to cut multiple parts without having to put extra holes in the workpiece for registration, or use a different jig for the other side.
    David Buchsbaum
    Beacon Custom Woodwork, Inc.
    dba Atlanta Closet & Storage Solutions
    404-309-9146
    david@atlantacloset.com

    atlantacloset.com
    beaconcustomwoodwork.com

  9. #9
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    Quote Originally Posted by cnc_fabricator11 View Post
    Thanks for your responses. Jack, we're trying to achieve a roundover profile on each circles edge on both sides. Sorry I forgot to mention that in the original post. We considered using a handheld router but would like to accomplish this on our CNC machine if possible. I'm sure we will encounter the same problem on future projects that require machining from both sides so we would like to come up with a solution on this particular project.

    Thanks,

    -Pat
    the thing is it takes far more time to do it all on the cnc. If you just cut the full sheet out and while another is getting cut you round over that one you cut the cut and setup time in less then half. That lets you get the part made faster.
    the easy way to do this is the cut the part slightly large and drill holes in each end. your jig will have matching pegs. you cut one side flip over and use the page to center it again. The problems with this is holding the part in place well. You can do one sheet by rounding over one one full sheet drill the holes and chop the sheet up then do one at a time to do the second side and all the slots.
    But any method will cost you time and drive you nuts in trying to get the material perfectly flat for a good roundover.

  10. #10
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    Harvey Tool has back corner round endmills (I'm sure other manufactures do to) It may take a few tries to get the toolpath right but they allow you to round the bottom edge of a part with out flipping it over. I think you would have to add a "tail" to your profile cut and allow clearance for the backround to go into the tail before moving up against the part. It probly would take a little longer to do it on the bot then going old school and using a router but if the bot is doing the work your fingers don't need to approach the whirling blade of death, and you can get other things done so your not really wasting time.
    Last edited by twelchPTM; 07-18-2012 at 05:36 PM. Reason: after thought

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