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  • calculating cut angle

    I glued up a blank to cut a bowl, but when I went to get my copy of Carol's book to calculate the cutting angle, I couldn't find it anywhere. I guess I never should have cleaned out the garage. Does someone have a handy chart out there for calculating cutting angle based on the thickness of the wood?
    Keith

  • #2
    tan θ = opposite / adjacent
    θ = atan(cut width / wood thickness)

    Also check out Dave van Ess's Angle Calculator.

    IIRC, Carole recommends adding an extra degree to the calculated angle for the kerf.


    --Rob

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    • #3
      I should have paid attention in trig. I tried the angle calculator but it just doesn't seem right or maybe I am just not understanding. The blank I am using took a long time to glue up so I don't want to mess it up by cutting the wrong angle.
      Keith

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      • #4
        Keith,

        Sorry you misplaced the book, but I think I can help.

        It's really not all that complicated once you get the hang of it, but until then, it can be confusing. If you know the thickness of your wood (once it's been sanded, if you've done a glue-up) and the ring width (usually either 3/8" or 1/4") you can solve for the cutting angle. As Rob mentioned, I like to add an extra degree to account for kerf width and miscuts. If your angle is too large, you may have some extra sanding to do on the outside, but if the angle is too small, you may not have enough wood for gluing the rings together.

        The 3/8" wide ring gives more wiggle room than the 1/4" wide ring, but the thinner wall is a little more elegant, and you can use a smaller cutting angle which gives you more shaping options.

        Now, down to business. If you go to Dave's program (AngleCalc), enter the thickness of your wood as a decimal. Then, enter the ring width, also as a decimal. Go to where it says "angle" and click on "solve". The correct angle should appear. Round the number up, if necessary, and add one degree. That should work. However, to play it safe, send me a PM with your specs and I'll compute it for you. That way you can check your own work, and not risk ruining your wood.

        And maybe you'll find the missing book!
        Carole

        Follow me on my blog: www.scrollsawbowls.blogspot.com

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        • #5
          I'll bet you'll get the bowl all made, finished, and everything...
          And then when you go to get a rag to buff it,
          the book will be under the rag...lol.
          Jim

          The limits of the imagination are imaginary.
          No task is too tedious for Art.
          Rock and Scroll

          My Gallery

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          Featherwood Woodcrafts

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          • #6
            I used the book which I think said 20 degrees for 3/4" thick wood with 1/4" wide rings, but it was too thick on the bottom of each ring. The calculator says 18 degrees, and I calculate the same: inverse tan(0.25/0.75) = inverse tan(0.33) = 18.

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            • #7
              Block, if that works for you that's great, but it doesn't leave much margin for error.

              Can you clarify what you mean by the bottom of each ring being too thick? Thanks.
              Carole

              Follow me on my blog: www.scrollsawbowls.blogspot.com

              Comment


              • #8
                I think I was confused - a larger angle would of course be on both sides of each ring (all cuts in other words), so the top and bottom of each ring ought to be the same width no matter what the angle, right? My recollection is mine were wider on the bottom than the top, so maybe the blade was bending...but if it bent for both cuts, it would cancel out somewhat. Gotta head to work now...

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