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  1. #1
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    Mar 2008
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    Default Smoothing concave and convex curves

    I have cut a series of elongated S curves on a 25mm thick radiata pine board 140mm wide . The curves vary in radius, but are around the 300-400mm radius.
    I am having a lot of trouble getting the curves smooth, on both sides of the cut.
    I have used my Makita jigsaw with a Festool timber curve cutting blade, and also used a bandsaw, but the end result after sanding by hand with a sheet of sandpaper folder in four is a series of bumps.
    The most difficult is the concave cut ( the inner side of a U cut)
    Can anyone please advise the best way to get the concave and convex sides of this cut very smooth?
    regards,

    Dengy

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  3. #2
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    Default

    Hi Dengy,

    I would think a bobbin sander would be the way to go.
    you can buy bobbins for a drill press, but you could also make your own with a lathe, or some creativity.

    If it's a 1 off job, you could glue your sandpaper to a homemade bobbin.
    If you don't have a drill press you could use a hand held power drill rigged up so that you only have to hold the workpiece,
    which is probably easier than clamping the workpiece and holding the drill.

    Hope this gives you some ideas.

    Regards, Jeff

  4. #3
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    Jan 2019
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    Aus
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    If it's a one off large curve, I use the flat side of a cabinet makers rasp on an angle along the curve, followed by sanding. This evens out the bumps from cutting the curve.

    If it's not pine, I'll use a scraper to clean up the rasp marks and maybe do a final sand if needed. Otherwise just sand to remove the rasp marks.

    If I'm making a lot, I'll make a template out of 6mm MDF or similar, which is easy to shape. Then screw onto the workpiece in a place that can be cut off later or that won't be seen. Then rough on a bandsaw, then finish using a template routing bit either on the router table or on a handheld router.

    If you can shape a sanding block to have a curve a bit smaller that the radius you require, that can help a lot.

  5. #4
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    Jan 2019
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    Just to add, I have a set of bobbins that Jeff was talking about, which work great. I've used them to clean up curves on Australian hardwood that was a bit too tough on my nice rasp!

    I like the ones that you can use normal fabric backed sandpaper with e.g. https://www.timbecon.com.au/sleeveless-drum-sander-set. They generate a lot of dust though, good if you can bring your DC nice and close to the drill if possible.

  6. #5
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    Default

    How would a round bottom spokeshave go on the concave surfaces?
    regards,

    Dengy

  7. #6
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    Oct 2010
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    NSW
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    Quote Originally Posted by Dengue View Post
    How would a round bottom spokeshave go on the concave surfaces?
    That's what I'd use.

  8. #7
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    A Wadkin JTA is what I use for this sort of work.
    You could obtain or turn up a piece of suitable diameter stuff a bit longer than a sheet of sandpaper and using double sided tape or contact attach a suitable grit of paper to it.
    Use this in a sliding/twisting motion on the internal curve whilst holding the piece in your bench vice.
    Similar for out side curve using flat paper holder these are used by panel eaters/spray painters and probably even stocked at bunnies.
    H.
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  9. #8
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    Jan 2019
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    Quote Originally Posted by Dengue View Post
    How would a round bottom spokeshave go on the concave surfaces?
    With those large radii you can use a normal flat one as well - don't really need a curved one!

  10. #9
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    Default

    Gooseneck hand scrapers can also deal with concave surfaces ,not easy to find these day's but they are about.
    Johnno

    Everyone has a photographic memory, some just don't have film.

  11. #10
    Join Date
    Apr 2006
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    Hi Jeff

    My preferred approches are as follows:

    Machine smoothing convex surfaces: Disk sander or horizontal belt sander.

    Machine smoothing concave surfaces: Bobbin sander or bobbin end of a horizontal belt sander. You can get inexpensive bobbin sander mandrels to go on a drill press.

    The secret in all cases with these machines is a very light touch!

    Hand sanding: Spokeshave on both surfaces. And/or :
    • Sandpaper on cork bloch on convex surfaces, and
    • Sandpaper on flexible batten (plywood) on concave surfaces. Wooden boat builders fair hulls with sanding battens over two metres long!


    In gaging smoothness, your fingers are probably more sensitive than your eyes. You must be really careful in sanding not to bevel or round the surface - usually you want it flat and perpendicular to the sides. It is hard!

  12. #11
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    I've also found that sandpaper wrapped around a suitably sized plastic bottle (filled with water or compressed air) can be surprisingly effective for concave splines.
    I may be weird, but I'm saving up to become eccentric.

    - Andy Mc

  13. #12
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    The secret in all cases with these machines is a very light touch!
    Yes, you hit the nail on the head Graeme. Never having used a bobbin sander before, I pushed hard and ended up with a series of lumps and bumps along the edges.

    Going to try your drink bottle tip tomorrow, Skew, great idea.

    Any snow down Oberon way yet?
    regards,

    Dengy

  14. #13
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    Quote Originally Posted by Dengue View Post
    Yes, you hit the nail on the head Graeme. Never having used a bobbin sander before, I pushed hard and ended up with a series of lumps and bumps along the edges. ...

    Me, too ! First ten times ...

  15. #14
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    I used to make a lot of octagonal small side tables with curved legs and I tried all sorts of sanding options with varying degrees of success but always dusty. One day I pulled out the trusty old spokeshave and in a few swipes a smooth even curve with only shavings to deal with. As stated previously flat foe outside curves and rounded for tight inside but a straight will do a great job on gentle inside curves (just a bit more blade projection)and is much easier to control for beginners than a rounded.
    Much more satisfying than sanding.
    Regards Rod.
    Rod Gilbert.

  16. #15
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    Have been able to borrow a flat and round bottomed Stanley 151/152 spokeshaves. Just need to sharpen the blades and get into it. All keen after reading Rodís post, but will be doing a lot of practice first
    regards,

    Dengy

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