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PostPosted: Tue Jul 09, 2019 6:25 pm 
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Koa
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I see almost everyone does it after they are glued in. My thoughts on this are that if they are shaped first, all the stresses in the wood will be settled in. If you shape after and there are stresses than the top won't have the desired radius.

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PostPosted: Tue Jul 09, 2019 6:48 pm 
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Brazilian Rosewood
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What stresses?


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PostPosted: Tue Jul 09, 2019 7:12 pm 
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Doesn't matter, IMO. Brace wood is such a small quantity that we can afford to be picky and get perfectly straight grain with near-zero internal stress. You do build in a bit of stress when flexing the flat soundboard into its domed shape, so it does fight back against the braces, bending them back toward flat in their less stiff areas. But I don't think carving before gluing would change that, assuming the bottom of the brace perfectly matches the radius of the dish in both cases so the brace isn't flexed during glue-up.

Trying to build a perfect spherical radius is sort of pointless anyway since it will be bent so far out of shape as soon as you put strings on it. It would probably make more sense to try and build with more radius in front of the bridge and less behind, so it deforms toward a perfect dome under tension. I actually do use a flatter radius on the tone bars for this reason (I glue braces using cam clamps like the Cumpiano book). Doesn't make much difference though.


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PostPosted: Tue Jul 09, 2019 7:50 pm 
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I shape my braces to 90% completion before I glue them on.

I can shape the 4 back braces from a rough blank to finished in about 10 minutes.


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PostPosted: Tue Jul 09, 2019 8:06 pm 
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Once you have the radius cut into the brace all of the stress should be gone.

As far as shaping, I use a CF sandwiched in the brace. It is only 0.022" thick but it still is hard to shape with chisels, so I pre-shape the top of the brace pretty much 100% before I glue them down. That only leaves me the sides of the braces to remove material from later.

I should add that, if it is a new body shape or I am looking for a different sound I leave the top of the brace only about 75% cut before gluing in.

Without CF, and you have a brace profile on your plan, I would pre-shape them about 75% then try your hand at using a chisel and tapping as you go.

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PostPosted: Tue Jul 09, 2019 8:13 pm 
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shape then glue in

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PostPosted: Tue Jul 09, 2019 8:47 pm 
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For the top braces and #1 and #2 on the back, no preshaping. Besides being easier to clamp when rectangular, once the initial scalloping and tapering is done it is fast and easy to profile what remains with small plane and carving chisel. With low #3 and #4 back braces, we preshape and finish up once on the back.

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PostPosted: Tue Jul 09, 2019 9:35 pm 
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Thanks for all of your responses. I've decided to go with pre shaping as I'll be using cam clamps the shaping won't affect my clamping much. This will allow me to glue them in my spare bedroom with the ac on. Just need to finish making my clamps. I have one so far.

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PostPosted: Wed Jul 10, 2019 7:54 am 
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Brazilian Rosewood
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It takes only a few ounces to push a top or back into a radius dish, so again I ask, "What stress?" This is really a non-issue. Pre-shape your braces if you want. It really makes no significant structural difference.


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PostPosted: Wed Jul 10, 2019 8:07 am 
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I don't think you have to worry about stress.

There are good reasons to shape before, if you do deflection testing for example you can maybe one day have big enough data to determine what deflection is best for the x-brace for example.

I glue them in as sticks then shape them. The ends of my braces are typically feathered to zero and you cannot do that prior to gluing. I do deflection on the braced top too so preshaping risks going to far.

I'm guessing here though that those who preshape probably do a bit more carving after they are glued? If so then I wonder what the advantage truly is.


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PostPosted: Wed Jul 10, 2019 9:28 am 
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Brazilian Rosewood
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One extreme example of mis-guided, pre-shaping of the braces was '70s Martins. They pre-tapered the X-braces so that when they were notched and joined, there were severe gaps at the top of the joint. And then they hid the sloppy joint under a patch of glue soaked cloth, as if that would make up for this design flaw. Don't do that. ;-)


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PostPosted: Wed Jul 10, 2019 11:59 am 
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I always profile the braces after they're glued on. Keeping in mind that each piece of top and back wood is a bit different why would you expect the same brace profile or stiffness to work for each guitar? Granted, they are often quite similar, particularly if you do something like deflection testing on the plates before they're braced, but if you're looking for the best possible outcome they will need some fine tuning.

In connection with this, I'll note that I don't generally test brace material for density and stiffness along the grain. It just seems easier to me make the braces a little over size, and then trim them down afterward to suit. I do test tops and backs, though. I know there are makers who sort out their brace wood by stiffness and density (or by the tap tone of a standard size blank) and that seems to work OK too.

Mark Blanchard gave a talk at H'burg once time that touched this. He uses the Chladni method of tuning tops, and wondered if it would be possible to work by thicknessing all of the tops for a particular model to produce the same modes (tap tones) prior to bracing them, and then just use the same bracing. He said he wished he had back the time he spent on that one. Pre-brace testing of the tops revealed interesting differences in the modes depending on the length-to-width ratio of the guitar shape and the stiffness ratio of the wood (which he does not measure, so far as I know). Guitars that he liked tended to share certain characteristics in the pre-braced top modes. Now he looks at those modes on a thick top, starting with his biggest and widest shape, and trimming the top down until he finds the shape that works best. The wood tells him which shape of guitar it 'wants' to be.

I've been tracking those modes for some years now, and looking at correlations with the measured stiffness ratio of the top. As you'd expect, tops with higher cross grain stiffness tend to work better with Jumbo type outlines, while guitars like a 12-fret size 1 that are relatively narrow don't need as much cross stiffness to work well. There is a pretty wide range of stiffness ratios that will work for any given outline, but, as usual, if you work 'to the wood' you tend to get better results with less effort. But not no effort: each piece of wood varies enough to affect the outcome, and needs to be treated individually at some point in the process.


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PostPosted: Wed Jul 10, 2019 2:53 pm 
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by all the posts I think it shows that it is more a preference . I have done it both ways and just prefer to pre shape. I do allow a bit for voicing.
Martin braces on the line instruments are pre shaped custom are hand carved.

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PostPosted: Wed Jul 10, 2019 3:40 pm 
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Wow that's really interesting if I understand it correctly, find the guitar shape that best matches any given top? Never thought of that angle before.


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PostPosted: Wed Jul 10, 2019 4:07 pm 
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Other than the radius I do all my shaping after the braces are installed. Easier to use go-bars and. I like the shaping process. Makes me feel like I am making each top unique as I carve and tap.

In reality most of them wind up very close but not always.

When I have visited Jim Olson I have seen drawers full of pre shaped braces, scallops and all.

A million ways to build a great guitar eh?

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PostPosted: Thu Jul 11, 2019 12:50 am 
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banjopicks wrote:
I see almost everyone does it after they are glued in. My thoughts on this are that if they are shaped first, all the stresses in the wood will be settled in. If you shape after and there are stresses than the top won't have the desired radius.


It's a lot easier to clamp without slippage when the brace is just a square block of wood. Try clamping braces when it's tapered and profiled and see if you don't run into problems...

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PostPosted: Thu Jul 11, 2019 3:12 am 
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I enjoy brace carving, no apologies, but it's also easier to glue a unshaped brace.
I only pre-shape the brace ends when/where they are going to be tucked, usually scalloped on the end of the belt sander with a luthier's friend type attachment.
For the "slippery gluing brace" problems, I just place small pieces of wood/oilboard around the brace first, holding them down with a go-bar rod.
That brace ain't going nowhere!

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PostPosted: Thu Jul 11, 2019 3:05 pm 
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Tai Fu wrote:
banjopicks wrote:
I see almost everyone does it after they are glued in. My thoughts on this are that if they are shaped first, all the stresses in the wood will be settled in. If you shape after and there are stresses than the top won't have the desired radius.


It's a lot easier to clamp without slippage when the brace is just a square block of wood. Try clamping braces when it's tapered and profiled and see if you don't run into problems...


I have no problem clamping my pre-shaped braces with my go bar deck.
I leave a small flat on the top of the brace for the go bar to sit on.


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PostPosted: Thu Jul 11, 2019 4:08 pm 
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bluescreek wrote:
shape then glue in


Me too. Once I have a design down, I trace it out, save it, and use to preshape. Fine tune after.


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PostPosted: Thu Jul 11, 2019 4:12 pm 
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Colin North wrote:
I enjoy brace carving, no apologies, but it's also easier to glue a unshaped brace.
I only pre-shape the brace ends when/where they are going to be tucked, usually scalloped on the end of the belt sander with a luthier's friend type attachment.
For the "slippery gluing brace" problems, I just place small pieces of wood/oilboard around the brace first, holding them down with a go-bar rod.
That brace ain't going nowhere!


I do not find it anymore challenging to glue either. But I understand the point from gluing the starting braces. I bought a sioux city pencil grinder with a right angle head. Nothing simpler for tuning. Glue in rough shaped (90%) braces, hit with pencil grinder to tap, and done.


Last edited by Mike OMelia on Thu Jul 11, 2019 6:44 pm, edited 1 time in total.

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PostPosted: Thu Jul 11, 2019 4:36 pm 
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Double post

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Last edited by Colin North on Thu Jul 11, 2019 4:40 pm, edited 1 time in total.

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PostPosted: Thu Jul 11, 2019 4:38 pm 
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Colin North wrote:
Mike OMelia wrote:
Colin North wrote:
I enjoy brace carving, no apologies, but it's also easier to glue a unshaped brace.
I only pre-shape the brace ends when/where they are going to be tucked, usually scalloped on the end of the belt sander with a luthier's friend type attachment.
For the "slippery gluing brace" problems, I just place small pieces of wood/oilboard around the brace first, holding them down with a go-bar rod.
That brace ain't going nowhere!


I do not find it anymore challenging to glue either. But I understand the point from gluing the starting braces. I bought a sioux city pencil grinder with a right angle head. Nothing simple for tuning. Glue in rough shaped (90%) braces, hit with pencil grinder to tap, and done.

Cool!
I have used a dremel with a small sanding drum once for diffucult to reach places when a chisel wouldn't get in.

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PostPosted: Thu Jul 11, 2019 6:19 pm 
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This is the tool. It's not cheap. But I absolutely love it and recommend it. https://www.browntool.com/Listview/tabi ... fault.aspx



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PostPosted: Fri Jul 12, 2019 2:42 pm 
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I use violin maker's finger planes for most of the profiling of braces. It's easy in any event, since I'm doing all of that before gluing the top to the rim, when you can get at everything.


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PostPosted: Fri Jul 12, 2019 8:17 pm 
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Alan Carruth wrote:
I use violin maker's finger planes for most of the profiling of braces. It's easy in any event, since I'm doing all of that before gluing the top to the rim, when you can get at everything.



Me too. It might just be my favorite thing to do in lutherie.

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