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Thread: Shaping concave surfaces?

  1. #1

    Shaping concave surfaces?

    Newbie with a problem

    Hello, Ive lurked here off and on for some time, and it finally occurred to me to seek help with a long-standing problem here.

    Id like to replace the wood on a cast iron park bench. The metal frame is curved, and the slats ran lengthwise, of course, and fit the curved metal frame. So the slats have a concave surface and a convex surface. Some slats have quite the curve, maybe 5/8 inch depth over 3.5 inch width; some are almost flat.

    The convex surface is not a problem, it can be shaped with a plane or whatever. But Im stuck on how to shape the concave surfaces. There are 10 slats between ~3 and ~4 inches wide.

    Any wisdom out there?

    Thanks!

  2. #2
    Join Date
    Mar 2003
    Location
    San Francisco, CA
    Posts
    9,062
    Stand the slats on edge, and cut the curve with a bandsaw.

  3. #3
    There's a method of cutting a concave surface into a board on your table saw. I've done it in the past. There are videos and articles describing it but I don't remember what it's called so that you can search for it. Maybe someone will posts some links.

    In essence you push your wood across the top of the table saw blade. It requires some jigging and figuring out the angle of the board to the blade. The blade is not very high in this process, just above the table.

    I'm assuming these slats are long, maybe 6 to 8 feet long, to span the length of the bench.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  4. #4
    Quote Originally Posted by Mike Henderson View Post
    There's a method of cutting a concave surface into a board on your table saw. I've done it in the past. There are videos and articles describing it but I don't remember what it's called so that you can search for it. Maybe someone will posts some links.

    In essence you push your wood across the top of the table saw blade. It requires some jigging and figuring out the angle of the board to the blade. The blade is not very high in this process, just above the table.

    I'm assuming these slats are long, maybe 6 to 8 feet long, to span the length of the bench.

    Mike
    I have included a pic of a jig I made for a dewalt portable table saw.
    Utube ogee feet and you should see this technique.
    Attached Images Attached Images
    Last edited by Ron Citerone; 01-10-2020 at 11:12 PM. Reason: Addition

  5. #5
    Join Date
    Mar 2009
    Location
    Red Deer, Alberta
    Posts
    858
    "Cutting coves on a table saw" will probably get you some results as well
    Funny, I don't remember being absent minded...

  6. #6
    I've cut coves on a table saw and it really works. There quite a bit of clean up and hand sanding required after to get a smooth result but it can be done.

  7. #7
    Join Date
    Mar 2013
    Location
    North Alabama
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    439
    There are limitations to the table saw approach, such as the need to clean up the surface, and also the fact that the shallowest curve possible is the diameter of the blade. That may be too much curve in the case of shaping a bench. Consider a coopering plane--with which I have no experience to share, but it is the tool designed for the job.
    Chuck Taylor

  8. #8
    You can cut the concave surfaces by hand with a curved scraper. They also make a curved surform tool that works like a hand plane that will cut a concave surface on your boards.
    surform.JPG
    Lee Schierer
    USNA- '71
    Captain USNR(Ret)

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  9. #9
    I have a friend who makes coopered doors and large concave surfaces using a technique of multiple passes at the table saw. Make a pass, bump the fence over a blade kerf width, raise (or lower) the blade to the correct height, make another pass and so on. In this way you can hog out the curve with the series of saw kerfs and make a lot of sawdust in the process.

  10. #10
    Join Date
    Mar 2015
    Location
    SE Michigan
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    2,428
    Of course a bit more labor intensive, and not knowing what tools you have, Id rough them out with a coping or bow saw and fine tune with rasps, spoke shave, and sandpaper. A sanding drum on a drill press would also work well.

  11. #11
    Join Date
    Dec 2004
    Location
    Cincinnati, OH
    Posts
    699
    The cove cut can be cleaned rather quickly with the curved scraper that is included in many of the sets available. I have just been using mine in a chair seat.

    Another option would be to use narrower boards but you may be limited by attachment to the cast iron frames.
    Rustic? Well, no. That was not my intention!

  12. #12
    Join Date
    Dec 2010
    Location
    Evanston, IL
    Posts
    1,365
    When I had to create a concave surface for a headboard, I made a hand plane to do it. I bought a radius edge Krenov-style blade from Hock Tools (http://www.hocktools.com/products/pi.html) and shaped the sole of the plane to match the radius of the blade. Took a little time, but it works great. You can find lots of tutorials online on how to make a Krenov-style plane.

  13. #13
    Quote Originally Posted by Charles Taylor View Post
    There are limitations to the table saw approach, such as the need to clean up the surface, and also the fact that the shallowest curve possible is the diameter of the blade. That may be too much curve in the case of shaping a bench. Consider a coopering plane--with which I have no experience to share, but it is the tool designed for the job.
    I don't believe that's correct. There are calculators that tell you what angle to address the board to the blade to give you various curves. Your comment assumes that you're addressing the board to the blade at 90 degrees.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  14. #14
    Quote Originally Posted by Charles Taylor View Post
    There are limitations to the table saw approach, such as the need to clean up the surface, and also the fact that the shallowest curve possible is the diameter of the blade. That may be too much curve in the case of shaping a bench. Consider a coopering plane--with which I have no experience to share, but it is the tool designed for the job.
    Quote Originally Posted by Mike Henderson View Post
    I don't believe that's correct. There are calculators that tell you what angle to address the board to the blade to give you various curves. Your comment assumes that you're addressing the board to the blade at 90 degrees.

    Mike

    How could the radius of the cove cut ever be larger than the radius of the blade?

    I can see where different approach angles and/or blade tilts could result in a smaller or asymmetrical radius, but I'm having trouble picturing how it could ever be larger. If it can, please share the technique. Thanks
    Last edited by Edwin Santos; 01-13-2020 at 6:38 PM.

  15. #15
    Join Date
    Mar 2018
    Location
    Orwell, NY
    Posts
    312
    I use a Kutzall disc in an angle grinder for concave (and some convex) shaping when building fiddles and mandolins and such, and it would work to rough out the shape for the bench if you have a steady hand.
    Zach

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