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deejayen
02-02-2015, 04:01 AM
I'm not planning on making a ukulele, but I've watched a video in which the builder used a radius dish and sanded the formed sides in a backwards/forwards motion. I've not done any woodworking since leaving school, so I'm struggling to work things out in my head!

Starting with the (very) basics:

What is a radius dish used for?
I presume it chamfers the edges of the sides so that an arched back can be fitted.

How do you make a radius dish?
How do you get the correct radius over its entire surface, and then how do you fit the 'sandpaper' to it?

How do you use a radius dish?
I can't work this out - do you need to move the uke in any particular motion/direction and restrict the range of movement when sanding?

Does the kerfing also need to be sanded to a radius?

Thanks very much for any answers. I suppose my next question will be along the lines of - how do you arch a back?!

Moore Bettah Ukuleles
02-02-2015, 04:36 AM
You're correct in that a radius dish provides the profile necessary to establish the desired radius on the back, and for some of us, the top as well. All builders will have a different approach to this. I use a cylindrical radius form for the tops, sanding in a back and forth motion. A separate spherical radius dish is used for the back. I mount mine a potter's wheel to do that job, it rotates as I apply downward pressure on the ribs. Sheets, strips or large disks (available anywhere they rent floor sanders) are glued on with spray adhesive. I make my spherical radius dishes using a router on a sled who's curve match the radius I want.

lauburu
02-02-2015, 08:49 AM
Apart from 1 archtop, my instruments have always had flat tops. I appreciate that a curved top would probably improve the tone and add strength but there's one thing I don't understand. I haven't been able to visualise what happens to the top at the upper bout (curved - whether cylindrical or spherical) ) when the flat fretboard gets glued to it. Could somebody pls explain?
Miguel

Moore Bettah Ukuleles
02-02-2015, 11:26 AM
Apart from 1 archtop, my instruments have always had flat tops. I appreciate that a curved top would probably improve the tone and add strength but there's one thing I don't understand. I haven't been able to visualise what happens to the top at the upper bout (curved - whether cylindrical or spherical) ) when the flat fretboard gets glued to it. Could somebody pls explain?
Miguel

Yep, that can be a problem. That's why I build in a solera. The dish is flat where the upper bout is and is radiused at the lower bout. My bracing around the sound hole is flat except towards the outside where they are slightly radiused. I'm sure there are other, and maybe even easier, ways to accomplish this but this is how I've always done it.

Beau Hannam Ukuleles
02-03-2015, 06:44 AM
1- I radius the whole top face of the sides in a 28 foot radius dish (standard top radius for luthier build ukes and guitars).

2- I then get a sharpie and mark the upper bout to the waist

3- I sand flat the upper bout flat, leaving the lower bout at 28 foot. (This take only a few swipes, as a 28' radius over such a small distance is very slight.)

Done

PS- I've also sanded Flat the top face of the sides first, then put a 28' radius on the lower bout but i think its harder to do it that way.

Moore Bettah Ukuleles
02-03-2015, 07:17 AM
1- I radius the whole top face of the sides in a 28 foot radius dish (standard top radius for luthier build ukes and guitars).

2- I then get a sharpie and mark the upper bout to the waist

3- I sand flat the upper bout flat, leaving the lower bout at 28 foot. (This take only a few swipes, as a 28' radius over such a small distance is very slight.)

Done

PS- I've also sanded Flat the top face of the sides first, then put a 28' radius on the lower bout but i think its harder to do it that way.

Good point. I have a couple of sheets of 60 grit glued to a marble slab for that purpose.

lauburu
02-03-2015, 08:51 AM
Many thanks, gentlemen. Just what I needed to know.
Miguel

dkame
02-03-2015, 09:27 AM
I'm sure this question has been answered before but what are the main reasons for putting a radius on the lower bout of the soundboard? Strength? Sound? Aesthetics? Seems like commercial high volume producers will radius the back but don't do the top - probably too much effort?

saltytri
02-03-2015, 10:23 AM
Apart from 1 archtop, my instruments have always had flat tops. I appreciate that a curved top would probably improve the tone and add strength but there's one thing I don't understand. I haven't been able to visualise what happens to the top at the upper bout (curved - whether cylindrical or spherical) ) when the flat fretboard gets glued to it. Could somebody pls explain?
Miguel

Rick Turner of Compass Rose solves this problem by using a cantilevered fretboard that doesn't touch the top at all.

Moore Bettah Ukuleles
02-03-2015, 10:53 AM
I'm sure this question has been answered before but what are the main reasons for putting a radius on the lower bout of the soundboard? Strength? Sound? Aesthetics? Seems like commercial high volume producers will radius the back but don't do the top - probably too much effort?

It's a way of stiffening the top without adding any mass.

deejayen
02-04-2015, 09:01 AM
Thanks for all the info. I have a better idea of how it works now. I think it’s amazing that custom builders are able to turn blocks of wood into finished instruments, and with so much precision. It sounds like being able to make various tools and jigs etc is also an essential part of a uke-maker’s craft.

Gary Gill
02-04-2015, 10:19 AM
Thanks for all the info. I have a better idea of how it works now. I think it’s amazing that custom builders are able to turn blocks of wood into finished instruments, and with so much precision. It sounds like being able to make various tools and jigs etc is also an essential part of a uke-maker’s craft.

I've only built 30 ukes thus far, and nearly as many jigs and fixtures. Both tasks are rewarding.

Moore Bettah Ukuleles
02-04-2015, 01:36 PM
It sounds like being able to make various tools and jigs etc is also an essential part of a uke-maker’s craft.

The old joke goes:
Q: How many luthiers does it take to change a light bulb?
A: Five. One to change the bulb and four to make the jig.

:)