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16th April 2021, 07:33 AM #1GOLD MEMBER
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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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16th April 2021, 07:54 AM #2SENIOR MEMBER
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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
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16th April 2021, 07:55 AM #3Senior Member
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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.
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16th April 2021, 08:04 AM #4Senior Member
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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.
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16th April 2021, 09:01 AM #5GOLD MEMBER
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How would a round bottom spokeshave go on the concave surfaces?
regards,
Dengy
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16th April 2021, 09:10 AM #6SENIOR MEMBER
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16th April 2021, 10:29 AM #7
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.Jimcracks for the rich and/or wealthy. (aka GKB '88)
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16th April 2021, 01:38 PM #8Senior Member
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16th April 2021, 05:14 PM #9
Gooseneck hand scrapers can also deal with concave surfaces ,not easy to find these day's but they are about.
Johnno
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17th April 2021, 04:04 PM #10
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!
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17th April 2021, 04:28 PM #11
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.
- Andy Mc
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17th April 2021, 06:33 PM #12GOLD MEMBER
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The secret in all cases with these machines is a very light touch!
Going to try your drink bottle tip tomorrow, Skew, great idea.
Any snow down Oberon way yet?regards,
Dengy
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18th April 2021, 03:24 PM #13
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21st April 2021, 12:49 PM #14GOLD MEMBER
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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.
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21st April 2021, 01:08 PM #15GOLD MEMBER
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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 Rod’s post, but will be doing a lot of practice first
regards,
Dengy
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