Back braces on #1

[kencierp]

Oh -- and the rounded bottoms on the braces also reduces the traction of the glue surfaces and the braces tend to move around more during the gluing/clamping process --

[nkwak]

Hehe - yes, I did see that. I even have a pack of 3x5 cards that I was going to try but I was confused on how to actually determine the arc. I'm learning as I go, but next time I think I'll give it a try. I see now that my plans for this guitar included a pre-rendered arc along the bottom of many of the brace diagrams so I could always trace them and make a template to follow.

[kencierp]

The idea is to match the contour sanded on the bottom of the brace, you slide the stack of cards under the brace -- this give you a perfect match to what you are using for any given guitar. If you sand the brace off a bit say, to a 15.5 or 16 or 14.5 radius it won't matter and the caul you make from the stack of 3x35's will still match perfectly -- make sense?

[kencierp]

You can make a pretty accurate contour gauge using a long piece of string, picture frame hanging wire is better and some guys just use a tape measure. Tie a pencil on the end measure out "X" feet to match what radius you want, have someone hold the it at the mark, then draw the arc on a piece of what ever material you have handy.

[tippie53]

There are so many ways to do this. I use a Gauge block so the radius works you can make one using your disk just trace out the arc through the center and cut and true it out. Apply sand paper and you can make perfect braces . Don't over think this , it isn't that difficult . If you do use the dish just use the center . The amount of arc is negligible but do be aware not to rock the brace.
The little square block will help to avoid doing that . This is about getting it reasonably close. Once you glue them to to the back you make a hydrometer and this will be in flux as the RH changes.

[Darryl Young]

Hmmmm.......I'll throw another opinion out there for consideration.

I agree with Ken in theory......in practical application, I can't see how it could make a difference if one sands in a radius dish or not. If I did the math correct, using a 1/4" back brace the distance from chord to arc using a 15ft radius is 0.0000434" (4/100,000 of an inch). Someone please correct me if I calculated that incorrectly (Length=0.25", Arc radius=180" or 15ft). I doubt anyone could hold 1/1,000" tolerance if hand sanding so if that math is correct, it seems the difference between sanding in a dish and sanding on a flat, radiused surface is less than the tolerance one could maintain (or even measure). Again, assuming I calculated correctly, not sure how sanding in a radius dish could affect the quality of the glue line.

As for building stress free, it seems there would theoretically be less stress after gluing the back to the rim if one sanded in a radius dish then glued the back braces to the back in the radius dish (again, a mute point as the difference is negligible).

Finally, I wouldn't want my braces to all be parallel as shown in the diagram above with the exagerated radius. I would want my back braces to be angled radially toward the theoretical center of the radius. If you make them parallel to each other, what reference point or surface do you choose to make them perpendicular to? And again, this is a theoretical discussion as the difference in angle would be so tiny I'm guessing no one could even measure it much less control it. In John's case, if his brace appears to lean it's likely an issue with leaning the brace while sanding which is an issue if sanding on a flat or radiused surface.

Please correct my math if I made a mistake and I will gladly admit the mistake. If the math is correct, I can't understand how this is an issue.

[kencierp]

Stress, perhaps was a poor choice of words and really not of any concern --- lets just say that on average with flat sanding there is much less force required to get the braces to conform to the contours while clamping the braces to the plate and the plate to the rim. Who knows stressing the plates or the rim might be a good thing? Good fitting glue joints --- that's what I'm talking about.

[tippie53]

I have done it both ways , I don't think it matters . Your math is correct , the chord is so small as to be negligible . All that matters is that when you are done it makes a guitar .

[kencierp]

Neil, As a final comment I would sight that Kinkead, Cumpiano. Young, Willis, Brosnac, and Sloane to name a few, all outline a similar back plate construction process as to what I have suggested here --- you came with a problem and again I am presenting my thoughts as to the cause and this popular strategy with a high level of confidence you will next time have better results.

[nkwak]

FWIW I appreciate everyone's input on this matter. Ken, I truly think that I am going to try your technique next time. My back was allowed to sit in the go-bar deck for a week (because I do most of my work at my friend's place) so there was plenty of time for the glue to set and the pieces to adjust to their new shapes. It looks like there's a good bond between all four braces, though the #3 brace isn't perfectly straight across the width of the plate. Oh well.

I'm on to shaping them by this point. I use a mini plane to get the profile and then a 3/4" chisel to taper the ends.