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Thread: Dimensional accuracy when cutting plywood

  1. #1
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    Default Dimensional accuracy when cutting plywood

    Just got my Desktop last week and started out by cutting some modeling parts out of 1/4" aircraft grade plywood (Midwest brand). The issue I'm having is if I, for example, cut a 3.000" diameter circle it measures about 10 thousandths over once cut. I tried adding an extra final pass and this hasn't seemed to have any effect. Is there a "springiness" issue with wood or am I expecting too much from the machine?

    I've tried it with multiple bits, both 1/8" and 1/4", and there is no difference. I'm thinking of setting the bit diameter 20 thousandths smaller but before I do that I thought I'd ask here to see if you guys have any ideas.

    Thanks,
    Randy

  2. #2
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    Randy...
    Are you cutting in "climb mill" direction? Try cutting "conventional". This may result in part dimensions that are too small. Half the difference of the size variation is your deflection and/or bit diameter inaccuracy.


    OR, you can simply set the bit 10 thousandths smaller. That should get you a lot closer. Of course wood can expand and/or contract when cut, and bits seldom cut the exact diameter they are sold as. (Do the math)
    Gary Campbell
    CNC Training & Technology
    GCnC411(at)gmail(dot)com
    The Ultimate Woodworking Machine
    http://www.youtube.com/user/Islaww1


    "We can not solve our problems with the same level of thinking that created them"
    Albert Einstein


  3. #3
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    I'm cutting in climb. I was thinking of reversing the direction of the extra last pass to cut in conventional to see if that helps. I'll report the result.

    Randy

  4. #4
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    IF the bit measures exactly the same as entered in the tool database, and results are the same with different bits...
    then perhaps simply use the allowance offset till it's as close to perfect as wood gets?
    DOUBT it's Desktop's accuracy
    scott
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    2013 Desktop/spindle/VCP 9
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  5. #5
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    I've run into the same problem cutting metal parts. I solved it by cutting slow, making sure the bit was the actual size it was advertised as and if that wasn't working i'd dial it in by giving the bit an adjusted diameter.

  6. #6
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    Quote Originally Posted by rcbrust View Post
    I'm cutting in climb. I was thinking of reversing the direction of the extra last pass to cut in conventional to see if that helps. I'll report the result.

    Randy
    It will make a difference. Climb cut tends to pull fibers out. Conventional tends to push them in. First thing the guy I bought my Bot from taught me was to use climb first, leaving an onion skin. Then come back and finish with a one pass conventional cut all the way through. For several reasons, it also give a smoother final cut.

  7. #7
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    Yeah, I'm definitely going to try leaving about 5 thousandths and then run a final pass the other way.

    Dimensional accuracy of the bits has been brought up. I was assuming that the bit diameters were fairly accurate, like within 1 thousandth or so. Are there any brands that are known for their accuracy over other brands?

    Randy

  8. #8
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    In many cases it is not the bit "actual" diameter that matters. It is the size of the cut that bit imparts on the material. In every case, when sub .010" accuracy is required, I would suggest making a 1"/sec cut 1/8" deep in the material you will use and using a digital caliper measure the actual slot that the tool cuts. That way you will know what diameter your material thinks the bit is.

    You also don't mention what feeds/speeds/pass depth you are using. On a small machine like the desktop aggressive cutting will deflect the cutter, towards the vector when using conventional direction and away from the vector when using climb. This shows up the most on hard materials
    Gary Campbell
    CNC Training & Technology
    GCnC411(at)gmail(dot)com
    The Ultimate Woodworking Machine
    http://www.youtube.com/user/Islaww1


    "We can not solve our problems with the same level of thinking that created them"
    Albert Einstein


  9. #9
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    When cutting plywood, that I needed really tight accuracy on, I first cut in a climb direction leaving a 1/32" skin. This will push the bit away from your cut line. I then do a conventional cut in one pass thru the skin. This will pull the bit to the line. I've had really good luck using this method. The beauty is in the software you can save both toolpaths in the same file, if the bit being used is identical for both cuts. When doing a large sheet of parts, I do the first pass leaving the skin on all the parts before doing the final conventional pass.

    Another thing to consider is that wood, even plywood, will relieve internal stresses, when cut, that can cause the plywood to move dimensionally. I know of no way to eliminate this issue for .001" accuracy in a wood part, which the machines are capable of. Wood, even plywood, is a moving medium to work with. Just the nature of wood.
    Don
    Diamond Lake Custom Woodworks, LLC
    www.dlwoodworks.com
    ***********************************
    Life is not a journey to the grave with the intention of arriving safely in one pretty and well preserved piece; But to skid in broadside, thoroughly used up, worn out, bank accounts empty, credit cards maxed out, defiantly shouting "Geronimo"!

    If you make something idiot proof, all they do is create a better idiot.

  10. #10
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    I had a number of emails regarding my last post. So to clarify, I did not mean that the bit was deflecting. For all practical purposes, they don't. I meant that the machine was deflecting which forces the cutter into or away from the vector. "Failure of the machine to withstand the forces generated by the bit, allowing it to vary from the cut vector".
    Gary Campbell
    CNC Training & Technology
    GCnC411(at)gmail(dot)com
    The Ultimate Woodworking Machine
    http://www.youtube.com/user/Islaww1


    "We can not solve our problems with the same level of thinking that created them"
    Albert Einstein


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