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Thread: Bending poplar using Kerf cuts

  1. #16
    Join Date
    Aug 2009
    Location
    Chocowinity, North Carolina
    Posts
    256
    I do a lot of kerfing in my work. In half inch thick stock (poplar), I kerf 7/16" deep, and never have a problem with strength. However, the bends I'm making are fairly severe. I think making slight bends is more difficult because conventional wisdom says the slighter the bend, the further apart the kerfs can be. If you draw a circle in SketchUp and zoom in on it, you'll find you don't have a circle at all - just a series of straight lines. The more segments you put into the circle, the smoother the curve. Same thing with wood. If you spread your kerfs wide apart, you'll get several straight segments that "curve" only at the kerf locations. To get a smooth curve you need the kerfs close together - something you may not want to do for a slight bend.
    "A lot of people are afraid of heights. Not me, I'm afraid of widths."
    -Steven Wright.

  2. #17
    Join Date
    May 2007
    Location
    Enfield, Connecticut
    Posts
    105
    Hi all,
    I have to admit I don't understand the kerf cutting calculator.
    I wanted to attach a sketch of what I am trying to kerf cut but I can't figure out how to attach a file.
    Can anyone step me through it?
    Thanks

  3. #18
    Join Date
    Mar 2012
    Location
    Napa Valley, CA
    Posts
    916
    Quote Originally Posted by Gerry S. Wojtowicz View Post
    Hi all,
    I have to admit I don't understand the kerf cutting calculator.
    I wanted to attach a sketch of what I am trying to kerf cut but I can't figure out how to attach a file.
    Can anyone step me through it?
    Thanks
    The calculator starts with the radius of the bend. What's your radius?

    To attach a pic, click on the "picture" icon (the middle one) in the bar above your new post to "insert an image"---- then choose the file (in jpg. jpeg. png. or gif. format) to upload.

  4. #19
    Join Date
    May 2007
    Location
    Enfield, Connecticut
    Posts
    105
    Jerry,
    I don't know the radius.
    What picture icon

  5. #20
    Join Date
    Mar 2012
    Location
    Napa Valley, CA
    Posts
    916
    The picture icon is here when you start a post:
    Picture icon 1-17-18.jpg

    We can figure your radius if you give us some info (as requested above)

  6. #21
    Does this board sit on the wall alone? Or does it get trimmed out somehow?

  7. #22
    Join Date
    May 2007
    Location
    Enfield, Connecticut
    Posts
    105
    SCAN_20180116_183405139.pdf

    I have a sneaking suspicion my attachment did not work. It is supposed to be a scan of a sketch I made of what I am trying to do.
    The board in question is 55 3/4 inches long, 5/4 thick and 8 inches wide.
    The 8 inch wide face will be against the wall (which would appear as a half circle on the sketch if it worked) and will receive the kerf cuts.
    The bow in the wall is such that there is a 1/2 inch gap between the board and the wall at either end.

    That is all the information I have. I don't know how to calculate any additional information that might be needed in order to use the kerf calculator.
    If any help can be provided it would be appreciated. If not, I certainly understand.

    Thank you all again in advance for your efforts to help out this hapless soul.

  8. #23
    Join Date
    May 2007
    Location
    Enfield, Connecticut
    Posts
    105
    SCAN_20180116_183754712.pdf

    OK this should work

  9. #24
    Join Date
    Feb 2008
    Location
    E TN, near Knoxville
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    12,298
    Quote Originally Posted by Gerry S. Wojtowicz View Post
    SCAN_20180116_183405139.pdf

    I have a sneaking suspicion my attachment did not work.
    I see a blank PDF. If it's a scan, maybe just attach it as a JPG file.

    I didn't look at the kerf calculator but I assume you need the radius of the curvature of the wall.

    You could calculate the radius of the curved wall from this (see the diagram and equation here):
    https://math.stackexchange.com/quest...t-of-a-segment
    55.75" would be the chord length (l)
    1/2" would be chord height (h)

    Or a little calculator here: https://www.mathopenref.com/arcradius.html
    where Height would be .5 and Width would be 55.75

    I'd offer to take a closer look at the math but my day started early at the doctor's office and is proceeding this evening with my brain modified by pain pills. Calculating two ways the wall radius looks to be about 777 inches but you better check it yourself!

    JKJ

  10. #25
    Join Date
    Mar 2012
    Location
    Napa Valley, CA
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    916
    Calculating the Radius: [(height of arc)^2 + (1/2 chord length)^2]/ 2(height of arc)

    John's got it: at 55 3/4 the radius would be 777. At 56 the radius is 784. No matter, really.

    I don't think the kerf calculator will help you here, because--according to the kerf calculator-- two kerf cuts will get you there--- but you wouldn't have a smooth curve.

    I would kerf every inch or so to get a smooth curve. That bend is pretty slight. Kerfing every inch will give you plenty of flex.

    Another alternative, of course, is to build up the thickness from several thin layers, either in a bending form, or right on the wall. (Or a series of spacer blocks and a final "skin" of thin solid stock).

  11. #26
    Join Date
    May 2007
    Location
    Enfield, Connecticut
    Posts
    105
    John K Jordan,

    Thank you. I got the same figure

  12. #27
    Join Date
    May 2007
    Location
    Enfield, Connecticut
    Posts
    105
    Jerry,

    I think you're right. Besides, I couldn't get the kerf calculator to work. My numbers seemed to fall outsides its range of input parameters.
    I'll try a kerf every inch or so. I agree its not much of a curve.

    Again, thank you all for your help.

  13. #28
    Join Date
    May 2007
    Location
    Enfield, Connecticut
    Posts
    105
    Hi all,

    One final comment. Photos of kerf cut.
    Sorry the second pic is off 90Deg
    Thank you all for all your help and supportkerf01.jpgkerf02.jpg

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