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Hi. I arch my braces and then glue them in place using a go bar deck and a matching radius dish. Tonight, when I removed a glued up back plate from the deck, the arch flattened noticeably. In other words, without the clamping pressure of the go bars, the radius of my back plate does not maintain the radius of the dish it was glued in. I'm using brace stock purchased from a trusted OLF supplier, tested it for runout and culled the braces for strength. (Not a scientific method, but I think it's effective at determining the strongest pieces.) I noticed this slight flattening the last time I used this method and forgot about it until just now. Dang. Is it normal for the plates to flatten a bit? Or am I doing something wrong here?

Thanks,

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George :-)

It should definitely hold the radius parallel to the brace (but not longitudinally). If not, it's probably a change in humidity.

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http://www.chassonguitars.com

OK, so this raises an interesting question for me. On my first two, after bringing the backs inside the house from the shop, they curved backward. Oops, humidity problem. So I cut the braces off, made new, and dried the backs in the shop clamped in forms. After everything was dried and new braces installed, I can't say they had the same curvature as the braces themselves.

In other words, I am thinking the back is flat, basically. So it wants to stay that way. If the back braces are flat instead of on edge (half are on mine, according to the drawing I'm building to) they may not be strong enough to hold that original curve. If they don't, does ths mean they need to be thicker? Does the back need to be thinner? Or is it a case of as long as it don't go concave again and has at least some curve, it's OK?

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Phil

http://www.oleninstruments.com

"Those who tilt at windmills are only considered insane by those who can't see the dragon."

just a guess, but if the backs are strong enough to pull the braces out of shape when removed from a clamp setup, then doesn't that indicate some combination of: back too thick/ braces too light...?

If you want to build you must be able to measure and control humidity. If you are not willing to learn how to do that just forget about trying to build acoustic guitars.
Yes, I am serious.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

When gluing braces on a top or back it does not hurt to have the top or back a bit drier then normal. I prefer to do cross bracing at a RH of 35% or so. Less damage happen on wood taking up moisture compared to losing moisture. Arches will then stay in place at the normal RH.
Tom

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A person who has never made a mistake has never made anything!!!

What should the target be for relative humidity in the shop? I am well insulated and vapor barriered and can control humidity within a range of 5-10%. Meaning, for example, 35-45%. As it goes up, I turn on the dehumidifier. If it goes down, I humidify. I suppose that if I tried real hard, I could keep it real close to a target number. I would appreciate your input. Pals, Vic.

Vic: I would be happy working in that range of RH.......! Opinions may vary.
Tom

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A person who has never made a mistake has never made anything!!!

Thanks for the replies, fellas. After further inspection, I think Kent is correct and that what I'm seeing is a slight shift from head to tail, not side to side. When running a straight edge along the overturned back plate, I can see that it has conformed to the arched braces quite readily and when I run my straight edge along the tops of the braces they measure out as flat and level.

So... I think I'm okay here. That is unless you guys are saying that when you remove clamping pressure from your plates there is no flexing at all and the plates remain perfectly seated in their radius dishes in every direction. Is that what you are saying? This is only my fifth instrument and I still have a lot to learn. (There's my understatement of the day.)

For the record, the back plate in question is Claro walnut, with a thickness of .110. My braces are Sitka and measure .32 x .60. Both the walnut and spruce have been stored together inside my home: The walnut for years and the spruce for months. I live in Seattle and my indoor RH ranges naturally between 30% (winter) and 60% (summer). The RH changes very little from one day to the next and has been holding steady in the 40% range since September.

Thanks again for the comments. Your help is greatly appreciated!

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George :-)

That is exactly what it means. And materials being what they are, there will be some springback.

George: A bit different from the top with the X brace. The back gets it's longitudinal arch when you glue the back to your sides......................as long as you use the disc to put the spherical shape on the ribs.
Tom

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A person who has never made a mistake has never made anything!!!

I just completed # 6 and after a lot of research and some experimentation with humidity, I am convinced the bracing tops and backs with very low RH increases the arch when the wood is brought back to equilibrium. 2 identical builds, with braces matched installed with HHG at 35% RH. Both tops were 6% and I heated one top with heat lamps to either side to 130 for about 1/2 hour (let it cool to 120 before gluing, probably 2-3 % RH) and the other for a few minutes at 120. Both tops retained the desired arch directly after removing from the go bar but the arch of the heated one increased by 20% when I brought into 50% RH while the non cooked one stayed the same. I studied Willi Henks thread on UMGF and must say that, while not scientific, sure seems to contribute to the arching. Both were 000 and the heated/seriously arched one sounds like a Larson with crisp fast thinner tone while the other is fuller, richer and more complex. Gluing full rectangular braces and then tapering the ends also seems better than pre-shaped bracing as far as retaining the full arch?? So much to learn.
Tim

Yep, humidity has a huge effect on cross-grain bracing, and the force of the plate trying to swell up will flex the braces and increase the arch.

But not even just moisture from the air in involved. Water absorbed from the glue can cause the plate to swell just underneath the braces. Then the glue sets up, and then the wood dries out and creates a visible indentation on the show face. Mainly when working with very thin plates, which can react very quickly. I don't really mind the look of it, but some people glue braces with epoxy to avoid it, since that doesn't have any water in it to swell the wood.

I prefer bracing on the dry side, around 30-35% if I can get it, but max 40%. The air gets so dry here in the winter with heating, and I hate having to have everything in cases and constantly filling humidifiers.

One of the things I love about redwood and western redcedar is that they move very little with humidity change. They're more brittle than spruce, so they crack after about the same RH drop, but they won't have sucked in as far. And that means the poofing up effect is less as well, so you can build at lower humidity for winter resistance, and not have action problems or glue joints popping on a humid summer day either.

A newly braced back (black walnut) and top ( Port Orford Cedar) were resting in my shop today when a very dry (22% RH) Santa Anna blew through San Diego. The top bowed at least 1/2" at the edges, top side bowed up and the back with the braces up still had quite an arch in it from the braces (15 ft. circ radius.)

I turned them both over and within an hour and a half the top had bowed the other direction and the back had flattened, almost going the other direction, against the curve in the braces. Wow.

I wonder if hanging tops/backs would be better than laying them flat to accommodate humidity changes more evenly between the two surfaces?

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Thank You and Best To All

I lean my braced tops so that air can circulate around them until I glue them to the sides.

I'd hate to glue them immediately after they'd been laying flat regardless of the humidity.

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Old growth, shmold growth!

To those who are deliberately over drying there components before glue up here is something to consider. When the wood returns to normal moisture content at say 50% RH where most are likely to keep a guitar, the plates are now expanding against the braces and inducing a fair amount of stress to the glue joints. Joints that are constantly under stress are more prone to failure. As such I believe any less that 40% shop RH is too low for construction.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

A little bit of springback is to be expected. The top and back, obviously, have their own stiffness, so they will fight the braces and give the free plate a topography that is not totally dictated by the curve of the braces. But, the radiused sides will help bring things closer to the intended arch.

One way to get the free plate closer to the intended arch is to sand the braces to a slightly smaller radius than the target arch, or sand them to the target radius, but then glue them in a dish of a slightly smaller radius.

Another step I take, to control the final arch of the back on the completed guitar, is in how I glue the back to the sides. I do this in a radius dish on my go bar deck, with the back on the bottom. I put a go bar or two in the middle of the back to press the back down into the radius as it's being glued to the sides. I don't/can't do this with the top, because at that point the back is in the way. But, if you glue your top on first, you could do it with the top, and then you could do it with the back, too, by putting a go bar or two through the sound hole. (If you want to get a little more elaborate, you could do this on both the top and the back using vacuum clamping or magnets to hold the plate down to the radius dish.)

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

As a follow-up to my original post, I have to admit that my initial concerns were unfounded. With the center strips in place and the back plate completely braced, I beleive all is well. Thanks once again for all the replies and comments.

Todd Rose - I haven't noticed you posting much of late. Your thoughtful and expert input is a huge part of what makes the OLF such a valuable resource and wonderful community. It's great to have you back! Any chance we can see some of what you've been building lately? Your work is always amazing and an inspiration.

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George :-)

George, many thanks for your kind words. Much appreciated. I am starting to go in some new directions with my instrument making, which I'm excited about, and I'm presently nearing completion on an instrument of a whole new design. If all goes well, I should be able to post some photos of that instrument within a few weeks.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

As others have noted, some spring back is normal. To adjust for this when gluing the top or back to the rim, I hold the top or back in the appropriate radius dish with vacuum and bring the rim to the top or back for gluing. This way the intended radius is held and I believe that I get a better glue joint between the top or back and the rim. If I've done a good job of sanding the rim in the dish, I get a little glue squeeze out all around the joint with just light hand pressure at any point on the rim - i.e. a (near) perfect fit between the plate and the rim. Thanks to Charles Fox for this method.

Chuck

Mind telling how thin?

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

I agree here. I just keep my shop 45% year round and never have a problem.