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Good morning:

Finished my first build a while ago and I'm getting ready for my second. I'm thinking of trying out the radius dish/go bar deck setup for gluing up the back and top bracing, but I'm not sure I'll like it.

I was tempted to buy 1 radius dish just to see how it goes instead of getting multiple dishes (one for the back, and one for the top). Is there a commonly accepted general radius that will work well for both the back and the top? I can't see why having the same radius for the top and back would be a bad thing, but most folks seem to have different radius dishes for everything.

What sayeth the group? Can one dish do the job (at least for a test guitar)? And if so, what's a good middle of the road radius? 25', 20', etc.?

Thanks again,

Jason

I know makers that use 25' or 28' for both

But check out our dish prices likely you can afford a pair.

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Ken Cierp

http://www.kennethmichaelguitars.com/

Most builders want a greater radius on the back than the top. Also having separate dishes for the back and top allows you to use one while the other is out of service during glue up.

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Michael F Smith
Goat Rock Ukulele
http://goatrockukulele.com

Hi Jason, fwiw, I build with a 25' back radius and a flat top, and it seems to work fine. Some excellent, high-profile builders use a flat top.

If you don't like it, you can always try a radiused top on subsequent guitars.

Good luck

Steve

A 28 foot radius will work for both the top and back. If you arch the top too much it makes it harder to fit the neck.

I have to confess that I use the same radius for both top and back. Though you should remember, I am far from an expert. I made an adjustable radius dish a long time ago because I thought I would want to do all types of different radii. The dish worked well enough but the only problem was it was never totally obvious from a glance what radius it was set to (since it does multiple radii you can't just label it). I worried that I would forget to adjust it or change it back moving from top to back and whatnot. I have been just using the same radius I use for the top on the back. In fact, I don't even know what radius that setting actually is. A long time ago I made a radius template based on the spline curve Cumpiano recommends for tops in his book. I set the dish to that and haven't touched it in a long time. I'm interested to hear the responses on this, who knows, I may be persuaded to go back to a more pronounced radius for the back. . .

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I use two different radii, 25' for the top and 15' for the back. If you decide to use one radius for both, I think it makes sense to use a radius closer to what people use for the top for both, i.e., a 25' or 28' radius dish. If you go the other way, and use a 15' radius for both the top and back, the top is going to be pretty round, and that can cause all sorts of problems. Using a 25' or 28' radius on the back just means you have a flatter back than other folks, and that's no big deal. Having a rounder top than other folks can be a big deal.

I also use the 25' top and 15' back - that's a pretty common configuration but by no means the only one or necessarily the best either. I would not hesitate to use 25' for both top and back. I would not use 15' for the top as that would require increasing the neck angle more than I would like.

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Steve Smith
"Music is what feelings sound like"

If you contact the Martin factory they'll proudly tell you that Martin makes flat top guitars. However the main braces are contoured to 52' and the finger board extension area is sloped 1.3 degrees, note the top edge of the rim is sanded flat and sloped on a two stage sander. Currently Martin backs are contoured to 15' braces and rim edge sanded to match.

You can find more detail and images here:

http://www.kennethmichaelguitars.com/neckangle.html

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Ken Cierp

http://www.kennethmichaelguitars.com/

I'm another who uses 16' for the back and 25' for the tops, however I only own one radius dish (the 16 footer). When I glue the top braces on I pre radius them (on a belt sander) and simply clamp against a 25 foot caul (piece of wood that I've sawn to that radius).

The 16' dish does come in handy when sanding the rim to fit the back

And for assembly there's this method:
http://kennethmichaelguitars.com/contourtool.html

Which we use in our vacuum clamp since some braces are flat not contoured while the main braces have a 40' contour -- kills two birds with one stone so to speak.

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Ken Cierp

http://www.kennethmichaelguitars.com/

Great discussion. This forum never ceases to amaze. Kenicerp, it looks like I'll be buying 1 (or 2) dishes in the near future.

Thanks again everyone.

I have used Ken's card idea ( a stack of business cards works well) for the top - recommended.

Wow! 52' radius?
Is that even measureable over a 20" span?
Very interesting, Ken!

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wah
Wah-wah-wah-wah
Wah

Yes you can see the 52' contour and even the 65' that Taylor uses for their top of the line models. Of course it not much. Its best to have some level of doming -- since a domed plate actually has more surface area covering the perimeter than a perfectly flat plate. The sound board braced to form a dome has a little better chance of surviving low humidity shrinking rather than cracking.

We sell plenty of both sizes -- I should note that Taylor also uses perfectly flat braces on these top end models said to have slightly more perceived bass response.

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Ken Cierp

http://www.kennethmichaelguitars.com/

As Usual, I did it different.
My tops were 10' cylindrical and backs were 10' dish.

When I toured Michi's new shop I was surprised to find that he uses 50' for the tops.

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https://www.instagram.com/fostinoguitars/
https://www.facebook.com/PuraVidaUkuleles/

With a radius of 50' plus, is there a need to flatten the upper bout / soundboard for the fretboard extension any more?


Sent from my iPhone using Tapatalk

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Keep in mind that, as an example, the process at the Martin factory sands the top edge of rim flat and the extension area is also sanded flat -- the important point is the extension area including the neck block are sloped 1.3 degrees.

In other words -- at least Martin (and I agree) see no value in sanding the tiny bevel on the rim edge that would be produced with a 52' contoured sanding dish.

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Ken Cierp

http://www.kennethmichaelguitars.com/

We use a different approach in the shop to fix the neck geometry, but I've had some pro/con discussions on milling in upper bout wedges with other builders. I would think that approach would be the most consistent way to ensure a fixed geometry, but it seems as though Martin still needs multiple bridge thicknesses to get to a reasonable saddle height and has more variation than I would expect to see. Is the variation just the usual bell curve of stiffer/softer bracing, top, etc., or due to a reluctance to expend labor in fine-tuning the the neck angle in the manufacturing process? Or perhaps just the normal distribution at work with materials and factory processes generating the 1/32"-1/16" differences in overall distortion of the guitar at string-up?

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A constellation only takes shape when one maps the whole.
- Beth Brower

Ken

Thanks for the index card tip - I will use it on my next instrument. Fascinating topic

Ed

I pulled 30' for the top and 15' for the back out of my hat for no good reason 13 years ago and stuck with it. I have to angle the upper bout a little with a sanding jig.

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It's not what you don't know that hurts you, it's what you do know that's wrong.