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Thread: Concave and convex curve cutting

  1. #1
    Join Date
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    Concave and convex curve cutting

    Actually trimming. A version of a Morris chair will be built, one for which the side pickets, each 5/8" x 2-1/4 x about 9", butt to top and bottom rails with curved surfaces. See the image.

    Top ends are very slightly convex and bottoms concave. I'll be doing these with paper patterns stuck to the stock with spray adhesive.

    How to best trim the cuts, which I will make with a crosscut saw, to the correct curvature?
    Attached Images Attached Images

  2. #2
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    There are a few ways. One would be a spokeshave. There are ones with a radius to the sole for working concave surfaces. A regular flat sole spoke shave will work on a convex surface as will just about any plane.

    There are also circular planes:

    In Use.jpg

    The one above is a Stanley #113. Stanley made two other versions; a #20 and a #13.

    One of the tricky parts of working a curved surface is working with the grain reversal at the highest or lowest part of the curvature.

    Sometimes it works to slice across the grain.

    jtk
    "A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
    - Sir Winston Churchill (1874-1965)

  3. #3
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    I would use a gouge with the right curve or close. Trim to your paper template right up to the tenon. if I didn't have a gouge use a straight chisel, almost as easy.
    Jim

  4. #4
    My approach would be build a jig out of MDF that spaces the slats, and then use a router and pattern bit...

  5. #5
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    Another read of the original post made me realize there are both end grain and flat grain to be curved.

    As Jim Pallas says, a gouge of a similar curvature would be the easiest way to trim the shoulders on the styles.

    One way around that is to make the ends of the styles housed into the surface of the rails.

    jtk
    "A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
    - Sir Winston Churchill (1874-1965)

  6. #6
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    Gene, You may have to explain a little more. I would hope that you are not planning to use a butt joint between the slats and the upper and lower rails. End grain glued to long grain, even if toe nailed, is weak. I suggest you tenon those slats into the rails. You could house the slats in a groove and block in between possibly.
    Jim

  7. #7
    Quote Originally Posted by James Pallas View Post
    Gene, You may have to explain a little more. I would hope that you are not planning to use a butt joint between the slats and the upper and lower rails. End grain glued to long grain, even if toe nailed, is weak. I suggest you tenon those slats into the rails. You could house the slats in a groove and block in between possibly.
    Jim
    +1 Very weak and toe nailing is likely to split the slats. Still a weak joint.

    I like the groove and blocking approach for simplicity. You don't need to worry about the curvature as the ends will be buried in the groove. If carefully fitted, no one would ever know.

    A proper mortise and tenon joint would be very good, but awfully tedious to make and you'd still have the issue of those curves, just twice as much for the shoulders on each side of the slat, top and bottom. Very difficult and very error prone as a hand tool process. Busy tooling to do it with a router, too.
    Last edited by James Waldron; 07-13-2018 at 1:21 PM.
    Fair winds and following seas,
    Jim Waldron

  8. #8
    Quote Originally Posted by James Waldron View Post
    A proper mortise and tenon joint would be very good, but awfully tedious to make and you'd still have the issue of those curves, just twice as much for the shoulders on each side of the slat, top and bottom. Very difficult and very error prone as a hand tool process. Busy tooling to do it with a router, too.
    This is why God gave us the floating tenon (or Domino for the lazy man.) Then all you have to care about is matching the curves.

  9. #9
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    Joints will all be done using dominos. The curvature is so minimal that sandpaper and blocks might be best.

    Or maybe a final-cut rasp.

  10. #10
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    Gene, I am about to start one of these, is this your own design or some one else's?
    Chris

    Everything I like is either illegal, immoral or fattening

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