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Thread: Dimensional Accuracy

  1. #21
    Join Date
    May 2014
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    Dublin Ireland
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    After more tests, its seems that the cut is consistently out by 2.85mm on a range of cuts 18mm, 20mm, 22mm, 24mm, 26mm, 28mm, 30mm, 32mm, 34mm and 36mm.

    I have figured out that if I convert lines to either arcs or lines in rhino, then bring a DXF into VCarve that the kinks don't appear in the VCarve drawing but still appear in the CNC'd wood. Doesn't solve dimension problem, as this is a second problem, hope I don't confuse anyone.

  2. #22
    Join Date
    Feb 2004
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    Sawdust and Noise, Brampton Ontario
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    110

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    As the difference is consistent regardless of the size of the part the problem is not the Unit Values as these would give larger differences with larger moves.

    Your drawing shows that the part is being cut with a climb strategy. Any deflection using this strategy will result in oversize parts (undersized spaces) and the difference would be consistent regardless of the distance moved. If you changed to a conventional strategy you may find that the gaps become oversize (parts undersized) because of the same deflection. A strategy to produce the most accurately sized parts (if the problem is deflection) is to cut most of the way through the material (80 -90%) using a climb cut and then finish cutting in the conventional direction.

    As mentioned earlier an undersized bit would also cause an oversize part but changing cut direction will not have an effect.

  3. #23
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    May 2014
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    Dublin Ireland
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    Climb Vs. Conventional
    http://imgur.com/atOvtUd
    Both are messed up, but differently.


    I need to get something built, so figuring out if it needs to be +3 or -3 in different places and is working. Not an ideal way to have to make something, but a temporary solution. Hope to get this fixed as soon as possible! Thanks for all the great support so far!

  4. #24
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    May 2014
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    Dublin Ireland
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    http://imgur.com/ywjB0II

    This was intended to be 60mm long, but cut the hole 57mm (-3mm) and the leg 63mm (+3mm). So I remade a top that had the holes as 66mm and got holes as 63mm (-3mm) which fit the leg perfectly. So there is some game of three's going on.

    On the right side of both top and bottom joint, the corner has been snipped off the leg creating a slight gap. I don't think this has anything to do with hardware as the corner is snipped off every similar shape in the same position every time with the same dimension and angle of the kind of bevelled snip. The same is happening with the holes of the top. The top left corner of the bottom hole being snipped and if you rotate that hole 90 degrees anticlockwise to position of top hole, same corner snipped at same angle.

  5. #25
    Join Date
    Sep 2008
    Location
    San Jose del Cabo based since 1997
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    1,244

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    what are you generating your toolpaths with? post the drawing
    Vector Studio 22

  6. #26
    Join Date
    Jan 2004
    Location
    Novato CA
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    223

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    Quote Originally Posted by Simops View Post
    My bot is setup in imperial and I use imperial tools only.......I design many of my requirements in metric and occasionally in imperial and always use the inch PP......never a problem as SB3 does the conversion automatically and cuts are always spot on regardless whether imperial or metric.......great feature of SB3.

    Cheers
    Great to know thanks.

  7. #27
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    May 2014
    Location
    Dublin Ireland
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    I use Rhino5, then VCarvePro that came with my ShopBot, then SB3 to play file.

    Image attached that show various points of setting up VCarve. I export them to Shopbot file in MM.
    http://imgur.com/a/iiN2L

    ---------------------------

    Will try as suggested changing file to inches, SB3 to inches as bit is imperial 1/4 inch bit that came with the ShopBot.

  8. #28
    Join Date
    Apr 2013
    Location
    Kennebunkport, Maine
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    4,419

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    James, Newbie here, but wondering why you didn't "create fillets""dogbone" to .25" directly in VCarve? Both my Onsrud starter set .25's" are undersized. .246 and .247" respectively.
    scott P.
    2013 Desktop/spindle/VCP 11.5**
    Maine

  9. #29
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    May 2014
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    Dublin Ireland
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    Just taught myself a new trick, didn't know that feature was there......I'm very new.

  10. #30
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    Apr 2013
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    Kennebunkport, Maine
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    Did you measure your bits at the tip? Won't explain all your problems, but might help a little? Measure at least 5 times, Throw away high/lows and average middles(or keep going until you get 3 identical readings). Something is funky. Your pictures help a lot!
    scott P.
    2013 Desktop/spindle/VCP 11.5**
    Maine

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