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Bracing a top...getting arch. A problem?
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Author:  brazil66 [ Mon May 30, 2011 4:56 am ]
Post subject:  Bracing a top...getting arch. A problem?
I braced a size 5 top in my shop yesterday. No humidity control in my shop at all. % around 70 or so. It's been damp here for weeks. I (so far) sand my braces (1/4") to the curve..... 25ft. and put in the vac bag on a flat surface for glue up. I noticed at that point there was very little arch after coming out of the bag.....and later on, in the house where the room is 45-50 % it is dead flat.

I plan to sand the top side of the rim/ sides to flat before glueing top on.

I'm wondering , if I'm on the wrong road here. To some degree. Is it better to put the top on a 25' radius dish AND arch the braces to 25' ? Is the top usable being so flat... any thoughts would help.

I sanded the top to .109
Michael
Author:  Arnt Rian [ Mon May 30, 2011 6:33 am ]
Post subject:  Re: Bracing a top...getting arch. A problem?
Your biggest problem isn't that the top now is flat. If the instrument ever goes to a place where the RH is anything like over here during winter (low 20's, sometimes even lower during very cold periods), it WILL crack. Depending on the humidity your instrument is likely to experience during its lifetime, you could be OK if you live in an unusually damp area, but most folks (myself included) recommend assembly at no higher than 50% RH. This gives you a reasonably safe range for the instrument, under normal conditions, for most places. I suggest you chisel the braces off, and find some way to assemble sufficiently dry parts at < 50% RH. Read up on "dry boxes", dehumidifiers etc.

But step one should be: Get a good hygrometer, and learn how to calibrate it. It is very important that you know exactly what you are dealing with...
Author:  Laurent Brondel [ Mon May 30, 2011 7:02 am ]
Post subject:  Re: Bracing a top...getting arch. A problem?
Big problem. Take the braces off your top, it will never be usable. If you decide to go ahead and build your guitar with it (and close the box at 70% RH) it will inevitably sink and crack as soon as RH levels drop.
RH levels in Northern N-E fluctuate wildly, it is true that it's been very humid for a while now, a very common occurrence during spring and summer. Winters are bone dry, of course, and even drier than that depending on how your house is heated.
Now I have both de-humidifier and AC running in the shop, I closed a box yesterday at 35% RH.
Try to maintain at least between 40-50% RH where you build, at least during the crucial times when you brace your plates and close the soundbox. An AC unit is very efficient at removing moisture from the air, so is a de-humidifer when it is not too hot outside. Otherwise wait for dry and windy days to work on the box.
Author:  Dave Fifield [ Mon May 30, 2011 1:23 pm ]
Post subject:  Re: Bracing a top...getting arch. A problem?
Humidity issues already covered, I will comment on your brace gluing method. If I read your posting correctly, you arched your braces, then glued them onto a flat soundboard inside a vacuum bag that was lying flat on a surface, right? There was no other base board or anything else inside the vac-bag? If so, then you will probably have to start over IMO, as the soundboard vs. the braces battle (that is, which one will bend to conform to the shape of the other) will result in no arch to anything - the soundboard will bend the braces straight. The bad part of this is that you now have built-in tension in your braces. Putting a dished base board inside the vac-bag (under the soundboard) will work better, but this method is very clumsy and it's difficult to get the braces lined up etc. A much better solution is the incredibly simple go-bar deck method. Do a search on the archives and you will see many many examples of go-bar decks, homebrew and purchased versions. Make one of these and your glue-up problems will be solved (apart from the humidity issues that is!).

Cheers,
Dave F.
Author:  brazil66 [ Mon May 30, 2011 6:42 pm ]
Post subject:  Re: Bracing a top...getting arch. A problem?
So, if I leave the top in a dehumified room for awhile...and close the box with proper RH percentages / dryness of all parts prevailing at that point. Would I have a viable box then ...ya think?

What I have here is a free back and side set (Maple), and a $10 top. So...I'm into experimenting to learn my chops here.

The caveat of controlling my building space is WELL taken.

But getting that arch. I'm definitely going to stay with the vac sys.(like it)...just need to explore / refine /understand., the best way to get, and to what degree ....the radius of the top.

Thank you for listening to me here.....I'm sure I'll look back on this someday and..... :lol:

Michael
Author:  Hupaand [ Mon May 30, 2011 8:08 pm ]
Post subject:  Re: Bracing a top...getting arch. A problem?
It will help your arch if you use a caul or dish while you glue the braces. The braces, by themselves, probably aren't strong enough to hold the top arched.
Author:  Tony_in_NYC [ Tue May 31, 2011 10:43 am ]
Post subject:  Re: Bracing a top...getting arch. A problem?
brazil66 wrote:
So, if I leave the top in a dehumified room for awhile...and close the box with proper RH percentages / dryness of all parts prevailing at that point. Would I have a viable box then ...ya think?


Nope. The top will go concave even if it is not attached to the rim and allowed to dry out.
There are many cheap and easy ways to glue the braces with an arched board underneath. If you do not want to purchase a radius dish, you could follow this method:
http://www.kennethmichaelguitars.com/contourtool.html

That would easily fit in your vacuum bag and preserve your radius of your braces. Plus, you can buy index cards for a couple of bucks vs. a radius dish for $70 or more. Gluing the arched braces on a flat board and hoping they will spring back does not seem to work. Plus, as was said, you are building stress into the braces and soundboard. Not a good thing.
Author:  Laurent Brondel [ Tue May 31, 2011 11:49 am ]
Post subject:  Re: Bracing a top...getting arch. A problem?
The reason why you need to re-brace your top is because you braced it at 70º RH. Wood is hygroscopic, it absorbs and releases moisture, and that is especially true of conifers.
Your top is x and cross braced. When the moisture level in the air goes down your top plate will release moisture and shrink, becoming flat, then concave and when too dry, it will develop cracks because the wood can no longer shrink and is constrained by the bracing. BTW it is as important for the back plate (for a traditional guitar, it is 100% ladder braced).

This is why your top went flat in the house where the RH was much lower than when you braced your top. I suspect that when the RH will go down to 20%, as it must in N-E, this top will spontaneously crack in random places.

In other words, as is, your top is unusable and will only lead to bad issues if you build a guitar around it.

You do not need a dish if you don't want one, and gluing shaped braces on a flat surface is a perfectly valid approach.
I suspect this is how pre-war guitars were built, maybe the OLF Martin and Gibson specialists can chime in. Radius dishes are a very recent addition to the luthier's arsenal, so it is not gospel. Before that shaped cauls were used, or as you did, gluing shaped braces onto a flat surface.

However, IMHO 25' seems a bit drastic for this approach, I would use a gentler radius, like 40' or 60'. A vac bag does not seem optimal either, it complicates clean up and may not apply sufficient pressure. After all, you are building "under tension" with this approach.

Finally, I don''t know about Vermont, but since yesterday the air has pleasantly and drastically dried here in Maine, I've had the shop doors open with a perfect 38%RH inside.
Author:  brazil66 [ Tue May 31, 2011 3:24 pm ]
Post subject:  Re: Bracing a top...getting arch. A problem?
Thanks Laurent, very much !.....25' dish on the way. Gonna stay with the bag, but not sure I see the "building under tension" aspect of that method.
Author:  Mike Lindstrom [ Tue May 31, 2011 10:48 pm ]
Post subject:  Re: Bracing a top...getting arch. A problem?
I can confirm what everyone is saying. A friend of mine braced his guitar last summer. I don't remember if it was really hot and the swamp cooler was working overtime, or if it had been raining a bunch. Either way, it was humid. The next time he came over, it was less humid. We took it out and it was dead flat (braced and glued with a 30' radius). We talked about rebracing it and decided to try it as is to see what happened.

Fast forward to a month ago. He never bought the humidifier I kept bugging him about. His top has a crack you could drive a small car through, and the change in the top's shape is peeling off the bridge. I've been humidifying it aggressively for a few weeks and the crack is now closed. Tomorrow we'll pop the bridge fix everything. I offered to do a replacement top for it, but he want's to try this first. And is now a believer in humidifiers.

And, I must say, it sounded great. We wondered if that could be because the top was already under such tension from the first shrinking.

Mike
Author:  Laurent Brondel [ Wed Jun 01, 2011 6:06 am ]
Post subject:  Re: Bracing a top...getting arch. A problem?
brazil66 wrote:
Thanks Laurent, very much !.....25' dish on the way. Gonna stay with the bag, but not sure I see the "building under tension" aspect of that method.
I am confused!
You really do not need the dish, but you need to dish the vac bag so to speak… If you read my post again you'll see that IME gluing shaped braces onto a top (or back) on a flat surface is a valid method. You'll end up with a shallower arc on your plate, but it will be arched and "under tension". Under tension here means that because you glue shaped braces on a flat surface, as you release the clamps, the plate will arch slightly as the braces try to spring back to their original shape. Again, it is a very valid approach, no dish is needed and it results in very stable braced plates (as long as you work around 40-50% RH).
However you'll need either a go-bar deck or assorted cam clamps to get sufficient pressure on the braces. As Todd points out, the vac bag may result in thick glue lines, which means possible failure with time, especially at the braces ends. You don't want that.
Author:  brazil66 [ Wed Jun 01, 2011 4:48 pm ]
Post subject:  Re: Bracing a top...getting arch. A problem?
My bag has an effective clamping pressure of 1800 lbs/ sq ft. I'm not really worried about thicker glue lines. Anywhere. And in my particular case.....I'm not a Go Bar kinda guy.
Movin on....

I'm going to try the sanded to 25' braces set in a 25' dish to get the full 25' arch, and see where that gets me. As opposed to the flatter version (somewhat less than 25" arch) of sanded 25' braces on a flat plate.
A 40' or 60' arch sounds like a good idea...I'll try that. I just happen to have a 25' set up...so that's where that come from....not wedded to it.

To avoid any arch, in the neck area...I think I'll need to suppress/ circumvent the arching forward of the soundhole to make the neck fitting less of a chore. In some way.

I thank you for the advice relating to my shop environment problem. That"s obvious now. Don't know what I was thinking.
Author:  WaddyThomson [ Wed Jun 01, 2011 7:45 pm ]
Post subject:  Re: Bracing a top...getting arch. A problem?
brazil66 wrote:
My bag has an effective clamping pressure of 1800 lbs/ sq ft. I'm not really worried about thicker glue lines. Anywhere. And in my particular case.....I'm not a Go Bar kinda guy.


So, 12.5 lbs per sq inch, max!
Author:  Jeff Highland [ Wed Jun 01, 2011 8:06 pm ]
Post subject:  Re: Bracing a top...getting arch. A problem?
Vacuum pressure only acts on the surface that is presented to it.

In the case of a brace 15" longby 0.3" long you have a total force of 56 lbs (at 12.5 psi) which is enough if you have good fitting joints such as a shaped brace on a plate in a dish.
It is not enough to deform parts to fit together.
Author:  TomDodson [ Thu Jul 21, 2011 7:33 pm ]
Post subject:  Re: Bracing a top...getting arch. A problem?
Funny, I have had almost the exact opposite at the moment. The soundboard has arched more than I'd like. I think it is down to the braces changing shape slightly as wood is taken off but I could be wrong. I'm fairly sure that it will be fine upon gluing the sides and soundboard and I think that it will settle down soon enough. As for your problem, if its consistent from soundboard to soundboard then my first thought would be to compensate by radiusing your braces more to achieve your desired arch after gluing the braces :)
Author:  Rick Davis [ Thu Jul 21, 2011 8:19 pm ]
Post subject:  Re: Bracing a top...getting arch. A problem?
What most everyone said: remove braces, clean up (De-glue-goo is your friend if you want to keep the same top thickness), and re-brace using either a dish or flexible cauls BUT DO IT AFTER THE HUMIDITY HAS BEEN CONSISTENTLY BELOW 50% FOR A COUPLE OF DAYS OR MORE.

And the "building under tension" concept has come up again. It's true that gluing up on a flat surface will result in something other than the 25' radius you sanded the braces to, but that's true if you flex a flat soundboard to match the brace radius, too. Something's gonna flex, some compromise will be reached between brace and plate, and everything's good. I have never seen or read any evidence that "building under tension" produces an inferior instrument. It's not good to have gaps in jointed surfaces or so much tension in, e.g., the sides that you have a hard time clamping them to the blocks, but that's not what's happening here. If there's a bit of spring back in bent pieces or you flex the back and soundboard to match a radiused brace, so what?
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