# Electrifying a Radius Dish Sander



## Paul Montgomery (Jan 28, 2018)

This is a luthier tool and I know it’s not for everyone, but I’m thinking that there might be some clever people here who just might find their own use for it.


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## epicfail48 (Mar 27, 2014)

30 seconds spent cutting off the corners will save a lot of recovery time when one of those corners smacks into a finger


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## John Smith_inFL (Jul 4, 2018)

excellent addition to your shop: (I agree, those knuckle buster corners should be removed).


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## difalkner (Nov 27, 2011)

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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## Paul Montgomery (Jan 28, 2018)

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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## TimPa (Jan 27, 2010)

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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## Half Fast Eddie (Jan 12, 2022)

So ... why is it called a dish sander, and what are you radiusing? Seems like you are sanding it flat, level … i don’t see a radius.


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## woodnthings (Jan 24, 2009)

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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## difalkner (Nov 27, 2011)

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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## woodnthings (Jan 24, 2009)

difalkner said:


> 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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## difalkner (Nov 27, 2011)

woodnthings said:


> So that means there's a curve to the top edges or a radius.


Yes sir, and it's easy to see when you look at a guitar from the side.


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## FrankC (Aug 24, 2012)

Interesting concept, not being a luthier I don't understand the end result completely but can appreciate the ingenuity.


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## difalkner (Nov 27, 2011)

FrankC said:


> 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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## woodnthings (Jan 24, 2009)

FrankC said:


> 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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## ducbsa (Jul 6, 2014)

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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## Paul Montgomery (Jan 28, 2018)

This isn't my video, but it is one on YouTube that shows how the radius dish is made.


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## difalkner (Nov 27, 2011)

Here's a video on radius dishes I made about 6 years ago (video is admittedly too long - sorry) -


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## FrankC (Aug 24, 2012)

I think I understand step 1, sand edges to get contours, now how do you apply flat skin to this contured shape?


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## difalkner (Nov 27, 2011)

FrankC said:


> 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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## FrankC (Aug 24, 2012)

Thanks David, now this starts to make sense.


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## Paul Montgomery (Jan 28, 2018)

The ribs are sanded to the same radius. The top or back is laid into the dish, the ribs are set in place, and with downward pressure are glued to the back/top. The back/top conforms to the dish and after drying you have a back/top that fits the sides of the instrument.


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## woodnthings (Jan 24, 2009)

Paul Montgomery said:


> The ribs are sanded to the same radius. The top or back is laid into the dish, the ribs are set in place, and with downward pressure are glued to the back/top. The back/top conforms to the dish and after drying you have a back/top that fits the sides of the instrument.


That's what David said:


difalkner said:


> 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.


I was convinced I understood, but now I'm confused. I get it that the surface is a radius or curved, BUT in this video the sled David is using forms a concave surface and I was thinking it would be a convex surface to allow the convex curve of the underside of the top to mate with the sides.





Where did I go wrong?


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## Paul Montgomery (Jan 28, 2018)

woodnthings said:


> That's what David said:
> 
> 
> I was convinced I understood, but now I'm confused. I get it that the surface is a radius or curved, BUT in this video the sled David is using forms a concave surface and I was thinking it would be a convex surface to allow the convex curve of the underside of the top to mate with the sides.
> ...


When running the sides over the plate it creates a bevel that is convex (high on the inside, lower on the outside). When placing the top/back in the dish and gluing the braces in , you create a dish shape that is concave on what will become the inner side with the braces (and convex on the outer side). So the cave back mates with the convex sides. Don't know if that makes sense or not...


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## woodnthings (Jan 24, 2009)

Paul Montgomery said:


> When running the sides over the plate it creates a bevel that is convex (high on the inside, lower on the outside). When placing the top/back in the dish and gluing the braces in , you create a dish shape that is concave on what will become the inner side with the braces (and convex on the outer side). So the cave back mates with the convex sides. Don't know if that makes sense or not...


Yup, the concave surface creates the mate for the tops and bottoms. That's what I thought, just didn't think it through. It's like stacking plates upside down, all the curves mate together. But that's only for sanding the body of the guitars and mandolins and ukes, not the tops or bottoms. Right?


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## difalkner (Nov 27, 2011)

woodnthings said:


> But that's only for sanding the body of the guitars and mandolins and ukes, not the tops or bottoms. Right?


The backs and tops are already at their desired thickness and conform to the radius dish. So yes, it is for sanding the edges of the sides so they match the radius of the back/top. But these are also used for sanding the braces to make certain they are at the proper radius.


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## m.n.j.chell (May 12, 2016)

I, certainly, wouldn't need the "radius" part of the dish sander ... but I like the idea of a large diameter flat disc sander! I might try to make a similar device with a flat top, to sand box sides even before gluing on a bottom, etc.


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## woodnthings (Jan 24, 2009)

m.n.j.chell said:


> I, certainly, wouldn't need the "radius" part of the dish sander ... but I like the idea of a large diameter flat disc sander! I might try to make a similar device with a flat top, to sand box sides even before gluing on a bottom, etc.


You have a great idea, making it horizontal rather than vertical! 
I don't know if I would power it with a drill motor, but maybe?
There aren't a lot of variable speed motors so maybe a single slower speed would work, a belt drive jack shaft rig?
As for sanding discs, I think they come in large diameters for floor sanders:








20" Cloth PSA Sanding Discs


20" CLOTH PSA SANDING DISCS These 20" Pressure Sensitive Adhesive discs consist of Aluminum Oxide abrasive material that is bonded to a x-weight cloth backing. The resin bond is a strong bonding agent which keeps the Aluminum Oxide grains on the cloth backing while providing an element of heat...




www.maverickabrasives.com


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## tadol (Nov 5, 2021)

A master luthier I know just has his radius dish mounted to a lazy susan bearing, clamps to his drill press table, and then drives it with a rubber sanding drum mounted in his drill press chuck - very simple and easy -


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