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I'm sorry if this has been discussed before, but the search did not find much...

I suppose most (95%?) builders brace their backs, but some are gluing the braces to the rim, and then the back.

I see some advantages to this approach: the doming can be created rather easily: shape the braces to a certain radius, and leave the center one higher by app. 3mm than the outer 2.  This way you can get a doming in all directions.

Also, no need to worry much about the braces fitting into the linings....

Thing is I am joining the back tonight and tomorrow might try to close the box. I don't have a dish for backs and i don't feel like making one these days :)

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For many years I braced the backs off the body and then fit.

This past summer at the Romanillos course though I decided to try it the other way (very traditional Spanish a' la' Torres). It is fairly easy to arch and fit the braces into the linings and add the little brace blocks under. The overall arching (doming) was done using a radius dish (awkward) or as I did, a sanding stick around 30'' X 4" formed into the correct arch. The tricky part beyond just the usual fitting is to relieve the center reinforcement strip accurately so that there are no gaps, and to apply just the right amount of glue to minimize unsightly accumulations (which you are stuck with) along the back braces). Also it was neccessary to "prop" under the braces to prevent their being deflected under clamping pressure.

I think part of the reason for the early Spanish makers doing it this way was their clamping method. The use of cordage to bind the back to the rims not only puts downward pressure on the back but pushes inward on the rim. The presence of the braces at this stage certainly helps to prevent deformation of the body. We used a cross-like device (a center stick with three cross bars at the back brace positions with notches at their ends) to concentrate the pressure at the brace locations. Another thing required by this method of clamping is to precisely trim the outline of the back. Any overhang will lever the edge up and spoil the joint as one of the class members found out in his haste to close up his guitar.

All in all this is a valid way of doing this operation but it presents it's own requirements. Personally I went back to my usual methods on my next guitar but was happy to have had the experienceof doing it the traditonal way.      

  

Thanks! 

The props are a great ideea. I might want to use gobars instead of mere sticky tape. I have recently switched to L-V fish glue and it seems to need a lot of pressure.

I have a couple braces saved for an old guitar, and I will use them as radius templates when planing the bottom of my own braces.

I thought I should make the center brace stand up a little higher then the rest, and have a completely flat rim. 

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One questions, did you make the braces 1mm short to allow expanding along the grain?  I think Shawn told me the top transverses were being made 1mm short on each end, but I did not hear/read about the back braces before.

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I actually mentioned this practice in the thread on old brown glue, it was as David said the usual method with the old Spanish builders and also the Parisian and London builders in the 18th and 19th century. In fact I've just done this on a Torres 'copy' I'm building. I do however profile the rims first with the sanding diah, then install pre-radiused braces, pencil witness lines on the rims and braces then sand with the dish until all pencil lines are gone.

Accurate cutting of the centre strip is the only problem, but I get over this by cutting the slots in the back strip first then taping the braces onto the back and using that as a template for cutting the notches in the linings.

Colin

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I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

Yes, the back braces were made so that they cleared the rim by about 1mm. Fish glue is a good choice in that it will give you more open time to work.

The issue of back arch/doming is one where I think a long as you have a fair curve in all directions (it does NOT have to be spherical....) and you will get a good fit it is fine.

Hi Colin,

Which Torres have you made a copy of?

David,

I wish the LV glue would have *less* open time.  It proved really difficult to glue dentellones in place - they did not set for a very long while.

I am building a Torres inspired guitar too. I used the Nostberg plans but it ended up being about 1mm larger all around: 275 UB, 361 LB, 231 W, 475 long.  Depth 90mm.

Top Italian spruce and b&S cypress.

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As David and Colin said, it was tradition to do it this way. In my classical guitar building DVD I show the "traditional" method using a handplane to then shape and radius the braces but then use a radius dish to do final sanding and true everything up. I find this method more difficult than the steel string method of radiusing the braces and putting them on the back then fixing the back to the rims. The braces are then tapered to approx. 3/32nds and tucked through the kerfing when gluing to the rims. Many times I have considered changing this in my classical DVD to make this step easier for my students but haven't done it.
Of course, there are structural and tonal issues to think about in both methods.     

Tonal issues from using a different construction method?

Alan,

If you use the "traditional" method then your braces are approx. 6mm thick on the ends. If you use the steel string type method then your braces are 3/32nds on the ends. In my humble opinion this will cause tonal differences. Also, the traditional method doesn't use a radius dish and the sides are the same width from the end block to about the waist then taper to the neck block. The radius is only across the back in the lower bout not along it. Once again I think this can make a tonal difference.

Robbie,

I assume the center strip is glued using a flat surface?

Thanks!

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[QUOTE=AlexM] Robbie,I assume the center strip is glued using a flat surface? Thanks!
[/QUOTE]

Yes, I glue it on a flat surface.

Robbie wrote:
"If you use the "traditional" method then your braces are approx. 6mm thick on the ends. If you use the steel string type method then your braces are 3/32nds on the ends. In my humble opinion this will cause tonal differences. Also, the traditional method doesn't use a radius dish and the sides are the same width from the end block to about the waist then taper to the neck block. "

OK. I agree that the height of ther brace ends on the back probably has some effect on the toner, although it's hard to say how much. To me it seems as though neither of those is intrinsic to the method, so any tonal differences don't follow of necessity. There is no reason why you could not leave the brace ends higher when they are glued to the back first, although certainly there is good reason not to trim them down too much if the back is put on after they are inletted and glued to the rim.

I find that on my own guitars, which use fully domed backs built in a dish, the side height is virtually constant from the tail block to the waist, although this depends on the degree of taper. I would guess that if you laid a straightedge along the center seam of the back on a traditionally made instrument you'd find that it was very close to having a constant lengthwise curve, as well as the crosswise one. I have not seen instruments by 'masters' that had the sorts of dips and humps one would see if the curve varied too much from place to place.

I do, also, question whether one could find any consistent tonal difference that could be confidently attributed to the exact shape of the back dome, so long as that was not too weird. I'm sure I'll get lots of argument about that one, though!   

This is all very timely as this is the exact spot in the build that I'm at.  So, taking advantage of the opportunity, I decided to check out a few things.

First, tapering the body from the waist to the heel, and then doing the necessary adjustments, produces almost exactly the same result as placing the body on a 25" radius dish with the waist centered over the center of the dish.

Second, shaping the back braces 3mm for the top and bottom, and 4mm for the waist, pretty much matches the 25' radius.

So, given that, I decided to glue the braces to the back first, rather than into the body.

Alex, Without a dish, you can make a robust batten which can serve as a pattern for the braces, a clamping caul for gluing in the braces, and a sanding batten for sanding the rims.

Tonal differences?  I don't think that I'll ever be able to tell.

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Before I got radius dishes I made an adjustable checking stick for shaping the back edges. Let's see if I can explain it.

If you clamp a rectangular stick to the bench extended out horizontally and hang a weight on the end it will bend down. The bend is sharper toward the fixed end, and straighter at the weighted end. If you use a tapered stick that is thicker toward the clamped end the bend there will be less, and it will be more pronounced toward the weighted end. I made a WAG, and decided that a straight taper, with the fixed and about twice as deep as the free end, would give something pretty close to a circular curve.

I took piece of striaght grain, flat cut wood about 24" long, by maybe 3" wide, and dressed off one of the sides so that it was really straight. Then I drilled two holes 2" apart in the center of the length, with the outer edges 1/2" from that dressed edge of the stock. I used the band saw to make a straight cut that started 1/4" from the dressed edge and ran up to the outer edge of the drilled hole on either end. Then I wedged the cuts apart at the ends until the dressed edge had the right amount of curve. This edge could then be chalked and used to mark high spots along the linings. The nice thing about this is that it can match any radius curve you happen to run into with out the ned to make a new dish form.

When used carefully the results were pretty good. I really could not tell the difference between the outcome of using this stick and the dish. The dish is a bit quicker, of course, and can also be used as a caul when gluing the braces in the go-bar deck, which also speeds things up.