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An experiment, with some success (pics)
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Author:  robertD [ Mon Aug 06, 2007 9:16 am ]
Post subject: 

Hello All,

Some of you may remember, the “Humbled by Humidity” topic I started, a little while back.
When I showed what happened to my top and back, after they sat around in a high RH environment.
I got to thinking about what JJ Donohue said. He suggested to “slightly dampen the surface and
go-bar clamp it into the appropriate radius dish for a day or 2" JJ, if your listening, I didn’t do it
exactly as you suggested. However, your idea gave me one. I got to thinking that the top itself, is
only part of the problem, and probably a small part. I thought that the major reason for the top to
become concave is due to the braces inverting their radius.

So, I devised an experiment! I thought, why not let gravity do the work, by introducing a little
weight, and see what happens. I didn’t get pics of every thing that transpired, but, I think I got
enough, so that you get the gist of it.

The following pics show in order the progress, that took about 3 weeks to complete.

Pic #1- Top with a 25' radius gage.






Pic #2 - The jig I made, that allowed pressure to be placed at the edges of the top, and
approximately the waist area. The total possible deflection is a little more than a .25"
which I believe is a little more than what you would have, considering the width of the top,
at the original 25' radius.






Pic #3 - Blocks, strategically placed.





Pic #4 - The weights applied.





Pic #5 & #6 - initial results after a few days under pressure. At this point, I thought for sure I had
achieved complete success, however, after a couple of days, it returned to its concaved position.
So, back on the weights did go! This time I left them on for a week.








Pic #7 & #8 - these pics were taken after a week of sitting around in the shop with no weights.
I’ve almost got the full 25' radius back, in relation to top and bottom. However, it didn’t get any
better than flat, in relation from side to side. (The opposite side of the gage is straight). This time
I think they’re gonna hold.

So, I guess, I have had some success, since a flat top is not all that unusual. And, it sure is a heck
of a lot better than a concaved one!









Thanks for looking!

Robert
Author:  JJ Donohue [ Mon Aug 06, 2007 9:50 am ]
Post subject: 
Congratulations in saving the top, Robert. Nice work.

More importantly...you should now get the top glued to the sides ASAP (immediately) so that it stays at its intended radius.
Author:  Bill Greene [ Mon Aug 06, 2007 10:34 am ]
Post subject: 
Not bad, not bad at all...for a Georgia boy.      Glad to see you're going to use that top, too.

Say, what was the jamming/festival thing in Elijay this past week? John Kinnaird said he was down your way at "a jam".

Bill
Author:  robertD [ Mon Aug 06, 2007 9:17 pm ]
Post subject: 
JJ, I’m going to do just that, as soon as I bend a mahogany side set with the new hot pipe bender
I built. Also, I’m thinking, I need to shave down the braces a bit.

Bill, can’t tell you how glad I am too! I had some concerns there for a while, but, now I’m feeling
pretty good about this top. Got a few more adjustments to make, and some decisions to make
with binding/purfling, but, it won’t be long now before I start to build the box.

Bill, I wish I could say I knew about the jam/festival, but, I hadn’t heard anything. Of course, I
was in Florida for vacation July, 22 - 28. So, I might have missed it. Maybe John will chime in,
and tell us about it.

Hesh, thank you my friend! As far as, civil war history goes, I know about as much, as what I
see on the history channel. Those books belonged to my late grandfather, who really was a “civil
war history buff”. He had a small library, which I inherited. It just so happened that they were
the heaviest things I could find at the moment. Maybe one day I’ll get around to reading some of
them. For now though, they make really good weights!   

Robert
Author:  Mattia Valente [ Tue Aug 07, 2007 1:38 am ]
Post subject: 
Robert: in future, bracing top and back should be some of the last things you do; I aim to have less than 3 days between gluing the first brace onto the top and gluing the top on to the rim set.
Author:  JJ Donohue [ Tue Aug 07, 2007 6:52 am ]
Post subject: 
[QUOTE=Mattia Valente] Robert: in future, bracing top and back should be some of the last things you do; I aim to have less than 3 days between gluing the first brace onto the top and gluing the top on to the rim set.[/QUOTE]

Mattia...sounds like a good objective.

I'll add that I keep the braced plate gobar clamped in its appropriate radius disc when not in use and try to have it glued to the rim within 1-2 days. I glue the braces on day 1 and on day 2 I shape and tap until I'm happy with what I hear. I glue to the rim on day 2 or 3. I learned this the hard way.

Author:  Blain [ Tue Aug 07, 2007 10:29 am ]
Post subject: 
Very cool.

Thanks for sharing.
Author:  PaulB [ Tue Aug 07, 2007 2:14 pm ]
Post subject: 
I'm at the mercy of the weather. I brace when the humidity is at around 45%, Carve the braces when the humidity is about 45% which might be a week after gluing the braces on, then glue the plate to the rim when the humidity gets back to around 45%.

The whole process for me might take a couple of weeks or a month or more. Sure, the plates move around a bit with humidity changes, but they do come back to the right radius and sit nicely in the dish when the humidity returns to the same conditions.

When I'm storing them waiting for the humidity, I keep them upright - usually leaning against the posts of the go-bar deck so that there's even exposure to air on both surfaces. Inconvenient, but no big deal. My new shop will have humidity control.
Author:  robertD [ Tue Aug 07, 2007 10:11 pm ]
Post subject: 
Hesh, thanks for making a very good point! I have also wondered about this very same thing. I do
tend to agree with you Hesh, in that as long as a proper RH environment is maintained, a braced
top should be OK not attached.

Let me second that, I would welcome any comments on this issue too!

Robert
Author:  JJ Donohue [ Tue Aug 07, 2007 11:05 pm ]
Post subject: 
About a year ago, I sent a picture to Mario of a top I had braced. After commenting on much of what I was after, he "strongly suggested" that I get this glued up ASAP(immediately). It was his recommendation to make up the sides and have them ready to accept the plates. His practice is to glue the plates immediately after voicing...and he's very disciplined about keeping his shop at 45% all year.

While it had always been my intention to glue them up within a week of voicing, my 1st guitar's top lingered freely for several months in 40-50% conditions. Today, that guitar is noticeably flatter than the others. Subsequent guitars have had their plates glued up within a few days.

This is in no way intended to provide evidence that statistically proves a point and I was surprised that it was viewed as a novel idea. To me it just makes common sense and it's the way I'll continue to build.
Author:  Mattia Valente [ Wed Aug 08, 2007 12:12 am ]
Post subject: 
Hesh: don't underestimate the forces on a top that dries out (and shrinks, gross grain) and their ability to pull fairly light bracing (which doesn't shrink) along with them. Ask me how I know ;) And yeah, it's likely all about humidity control.

Once a top is glued down, the rim set (intrinsically stiff in the direction the top's going onto it) limits things further, but the top still can and belly and shift about, obviously, but far less dramatically.
Author:  PaulB [ Wed Aug 08, 2007 10:09 am ]
Post subject: 
I should add that I bake my all my tonewood in the big lab oven at work to stabilise them (at least I've read that it stabilises them. I get stuff shipped to work for this very reason, then they might sit stickered on a shelf for a year or two before I use them. So I'm working with pretty stable wood that does move back to it's starting point after shifts in humidity.
Author:  Steve Saville [ Sun Aug 12, 2007 4:56 am ]
Post subject: 
[QUOTE=Hesh1956]
This notion that a braced top MUST be installed on a rim fairly quickly keeps coming up here and I would like to ask for other opinions on this.

My thinking is that if your shop and materials are kept under constant, acceptable RH control that a top need not be installed on a rim at once, in three days, what ever..... I see photos of production shops like Huss and Dalton where tops are kept for months or more braced but not on rims.

It seems logical to me that if your braced top cannot hold it's shape/dome in a constant RH environment then your bracing is not doing the structural job. Granted there will be some give and just because it was braced to say a 25' radius does not mean that this is what you end up with always. But serious deformation occurring simply because you didn't attach the top in three days when proper RH control is in place....... I don't buy it.
[/QUOTE]
Hesh,
I agree with you. With my last two guitars, I had the tops braced for well over a month before I attached them to the sides. I had no issues as I controlled their environment.
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