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David - Machinist in wood
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Good video and project, Paul. For my needs that's spinning way too fast, though.

I have a couple of radius dishes, 15' and 28', and have yet to lop off the corners but mine is not (yet) motorized. Gotta say it's one of the things I'll probably do if/when I motorize mine.
 

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Discussion Starter · #5 ·
This dish is also a part of my go-bar deck. I've already made another dish that I may use so that I don't have to disassemble the go-bar deck everytime I need to sand. It actually is not moving as fast as the video makes it look. That motor is variable speed so it may eventually be set slower.
 

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i have 5 uke's in my future. i love this idea, wasn't sure how i was going to be handing that issue, might be a good route, thanks for the thought Paul.

ummm, sounds like you owe your wife a cutting board...
 

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where's my table saw?
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Yes, no apparent "radius" for me either. It's more like a chamfer or a bevel. This is because the fronts and backs are crowned surfaces and need to sit on a corresponding angled surface. But, I'm no luthier, I just play chords on a guitar.
The sanding surface has a "dish" to it as he says here:
Whether your are "radiusing them or just sanding them flat .....
 

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David - Machinist in wood
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It's a radius. The braces are also sanded on this radius and that helps to shape the plates for the top and back of the guitar, or uke in this instance.

A 'flat top' guitar typically has a radius of 12' to 18' and the back is usually 25' to 32'. It helps with the sound and also makes the instrument stronger, better able to withstand the string tension.
 

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where's my table saw?
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It's a radius. The braces are also sanded on this radius and that helps to shape the plates for the top and back of the guitar, or uke in this instance.

A 'flat top' guitar typically has a radius of 12' to 18' and the back is usually 25' to 32'. It helps with the sound and also makes the instrument stronger, better able to withstand the string tension.
So that means there's a curve to the top edges or a radius. But it's very slight, with a leg of 12 ft or 18 ft out from the center of that size circle.
The dish duplicates that curve on the top edges of the sides and all around so the front and back will sit flush.
Right? Only a luthier would know this.... LOL.
 
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David - Machinist in wood
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David - Machinist in wood
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Interesting concept, not being a luthier I don't understand the end result completely but can appreciate the ingenuity.
Much in the same way that when you see an empty flat bed trailer you can see it's got a crown to it and it's not really flat. In that way it is better able to handle the stress and load when it's carrying weight.
 

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where's my table saw?
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Interesting concept, not being a luthier I don't understand the end result completely but can appreciate the ingenuity.
A flat top acoustic guitar, does not have a flat top, it has a slightly curved top. So, the mating surfaces on all the edges must conform to that curve/radius.
The sanding dish/disc has that curve "built in", so when it's powered it requires so much less elbow grease.
A violin, cello or bass fiddle also called an "upright bass" or double bass, has a very pronounced curvature to the top. A bluegrass mandolin has a flatter top and bottom, but some others have a large belly on the bottom. I think those are European in origin, my mom had two of them which I inherited.
Concert instruments have formal names but bluegrass instruments are identical and have names like fiddle rather than violin.
I call my upright bass, a bass fiddle.
 

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Does the radius form a portion of a spherical surface or is it an arch from end to end of the fiddle body? If the former, is the MDF square for the sandpaper slightly cupped for that big radius and how is it cupped? If the latter, I don't see how the mechanism does that.
 

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David - Machinist in wood
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how do you apply flat skin to this contured shape?
Guitar backs and tops are in the 0.100" to 0.080" range and easily conform to the large radius. I don't know the thickness of uke backs/tops but my assumption is that they also conform to the specified radii.
 
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