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I have a couple of questions. Is a radius dish bigger than the body of the guitar? How is it used to final shape the back? Is it used to shape the braces? How? Once the back is on (and top) is the radius dish used for anything? I hear a lot about puting sandpaper on them... but I do not get it.

Mike

Mike,
Radius dishes are used for several tasks. They are typically 24" in diameter, which is longer and wider than a typical guitar body. Radiusing a top or back is done by radiusing the braces in the dish. For example, if I'm using a 15' radius for the back, I'll sand the braces to the 15' radius using the sanding dish. Then, you place a few pieces of newspaper on top of the sandpapered dish, place your back in the dish, then place the braces in place and glue them in a go-bar deck like the picture shown. Notice the dish is on the bottom, then the top, then braces, and go-bars to glue it down. Hope that helps.

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Tracy
http://www.luthiersuppliers.com

how is a radius dish used to sand the braces? Back and forth with the brace till fits? Or...?

Mike

Mike,

I rough-shape my braces with a plane and then sand to perfection
in the radius dishes.

It helps to tape together a couple of braces and sand them
together (if you do a single brace, its easy to introduce some
side-to-side rocking motion which will cause the edges to get rounded
over).

Works well for me anyway.

Eric

Mike,
Here is a tutorial and photos on building them, adding sandpaper if you choose and the dish size relative to a guitar body.
http://www.infinity-guitars.com/tutRadiusDish.php
chris

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Chris Oliver
Infinity Luthiers
...in the shop.

live every minute...

I looks like its gonna remain a mystery with regards to the building of a radius dish (gotta have the jig plans). But I have heard of "swinging" a router on a rope of the correct radius. In any event, that makes sense, if not impractical for most. I guess the jig is a way to guide the router across the diameter of the dish (as it spins) on a guide that represents the radius (perhaps this was made using the long rope too).

But, fastening several braces together to avoid "rocking" makes sense except the outboard pieces will be at a slightly smaller radius since they are not sanded along the centerline of the dish (sorry, I'm an engineer). In fact, since a dish is curved in two directions, the bottom of the brace will be curved longitudinaly as well as laterally. This is why a belt sander against a curved platen makes more sense to me...

Am I simply missing something here? Over analyzing the problem?

Mike

Mike,

>>>In fact, since a dish is curved in two directions, the bottom of the brace will be curved longitudinally as well as laterally. >>>This is why a belt sander against a curved platen makes more sense to me...

>>> Am I simply missing something here? Over analyzing the problem?

Remember that the glue-up of the braces to the back/top also happens in the dish. That is,
the back/top is forced into a spherically radiused shape, not a cylinder. So, the braces
should spherically radiused, also. (I haven't done the math, but I suspect that the difference is
only significant when sanding a glued-up X-brace). Your belt sander technique would be
appropriate for a cylindrically radiused design.

I've read many, many times, that making radius dishes is horrendously messy. I cheerfully bought
a pair from one of our favorite sponsors. But see http://www.youtube.com/watch?v=tvGemvizrz4
for a video from Chris Paulick on how to make them.

Eric

PS Disclaimer: anything I say is subject to being contradicted by someone who is more experienced
and knowledgeable than I.

thing is because of the way certain braces sit against the top/back of the guitar, they dont need to be sanded down the centerline....

read this tutorial by Hesh

http://luthiersforum.com/forum/viewtopic.php?f=10117&t=16058

about 3/4 of the way down kinda explains what i mean

and there are pics of him sanding braces

(cheers Hesh )

anyway, as i read it, braces need to be glued parallel to the force exerted by the gobars, they are not glued parallel with the radius of the dome

hopefully this makes sense to you, because i think ive confused myself lol

edit: i got beaten to it lol

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Mike, after sanding the radius in a dish, I will lay my braces on a shooting board and use a short plane (low angle) to ensure that the curved edge of the brace is not rounded and a perfect 90 degrees to the height axis of the brace.

http://bluescreekguitars.com/blog/
here is another use for the disks. They are one of the most versatile tools ina luthiers arsenal
john hall
blues creek guitars

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John Hall
blues creek guitars
Authorized CF Martin Repair
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You Don't know what you don't know until you know it

Mike,
Check out the youtube video. I did a jig according to Chris' video this past weekend and it worked slick.
Sure there was a pile of chips and dust on the floor, but hey... what would making a guitar be like without the chips and dust?

The one modification I'd suggest would be to make a fixed frame and mount a sled to the router on drawer slides (ball bearing type.) This would negate the need for a big movable frame. When you change radius, move one end of the drawer slides.

The whole thing was a worthwhile project. I've been cobbling together frames and weights and all sorts of weird stuff to achieve a decent radius on the top/back and linings. This was a great investment of time. The materials were all scrap.


I asked Chris Paulick about it and he uses adhesive backed sanding discs (available from a variety of sources including some of our sponsors.) I'd like to keep in line with the "free" radius dish and use some sort of adhesive to attach the sandpaper to the disc. The question I'm waiting to see if someone will answer is.... What did they do in the olden days before adhesive backed sandpaper? Did they scrape the old stuff away after it was worn out? Burn it off with a torch? Use cheap glue so it would release? Toss the dish and buy a new one? Hopefully someone can shed some light on this one.

Good luck,

Dave

Where is the video?

Mike

This is how I do it but I want my linings in place to give the stiffnes to the rim assembly

The video is referenced in emathre's post above.

http://www.youtube.com/watch?v=tvGemvizrz4

dave

Ok, I found that video. I really think I have the process down now, at least the way I want to do it.

Now, I am interested in the long compass technique. I found a website describing it, but the site did not say how to specify a particular radius. I get the fixture, but now its math time.

Mike

Here is the math for the long compass. Thought this was kind of cool, how to draw a large radius arc with a simple tool (and some math). For those who have not seen this technique, take your board that you want to cut an arc in, mark the end of the chord (c) points, and use the equation to figure h at the center of the chord. Next, "adjust" your long compass (two long strips of wood, each being longer than the chord length) as seen in the picture (put pins at the chord ends and the top of h). Fix the angle in the two strips (screw, glue, etc). Remove the pin at the top of h. Put a pencil in the vertex, and rotate the long compass about those two pins on the chord, always keeping contact with both pins. Thats it.

For the calculus inept like myself, check out this link to help anyone doing the with the math.
http://www.1728.com/circsect.htm

This circle calculator has helped me for a couple years in figuring all kinds of cool stuff related to the various radii on the guitar.

Dave

Good find Dave ... its in my favorites list now. Oh, I did check my equation against that site and it is correct!

Was wondering if everyone understood my description of how to use the long compass once it is fixed into the proper configuration.

Mike (btw, its not calculus, its geometry )

Has anybody ever made one of these dishes using hook & loop paper? (as opposed to glue). I am thinking the added thickness of the velcro layer may not be deisrable.

Mike

Hello everybody,
Firstime builder here. Working on a 00 size guitar.
How do you handle glueing the flat bridge to a radiused top?

JimD

Hmmm, till I started posting here, I never thought of the top as being radiused. But it is. There has been some discussion about "flat tops". I'm sure the answer has to do with "shaping" the bridge, but I have never heard anybody discuss this before.... so I too would like to hear the answer.

Mike

Mike,
For the record. You can dome sand the rim top or you can flat sand the rim top and linings flat for the top and still use domed bracing. A domed 28’ top radius is so much closer to flat than a 15' back. You can attach it to the rim with out adding excessive strain on the top to rim glue joint.

I use to be a stickler about dome sanding tops but have changed my mind 180 degrees on this for a couple reasons. It was very important to me back when I would inlet brace into the rim. But now days my braces zero out at the lining so this is not the issue it once was for me.

Yes there are true flat tops out there. But the dome provides better structural integrity and resists the inward rotation of the area between the bridge and fretboard extension much better than a flat top.

Jim,

Welcome to the OLF.

To answer your question, tape a sheet of sandpaper to the top, where the bridge will sit, and sand the bridge bottom on the paper until it has a matching radius on its bottom.

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Brad Tucker

Bump on this question:

Has anybody ever made one of these dishes using hook & loop paper? (as opposed to glue). I am thinking the added thickness of the velcro layer may not be deisrable.

Mike

Mike,

As you have thought already, you don't really want to add extra thickness under the paper. Blues Creek John and myself both sell pressure adhesive paper for these dishes. Tracy at LutheirSuppliers may as well. Once you have the paper on it last a LONG time and you can cover it with a sheet of thin plastic or news paper when you want you the dish in your go-bar deck.

Another way to radius the bottom of your bridge is to sand the radius of top dish onto a scrap of wood, like a chunk of dry 2x4. Now you have the the convex radius of the concave radius of your dish. Glue a piece of sandpaper to that piece of sandpaper to that chunk of wood and you now have a small sanding block to concave radius the bottom of your bridge.

I have included a 3A top with your bracewood billets, I will send an invoice soon!

Thanks

Shane

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Canada