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 Post subject: Brace gluing
PostPosted: Thu Oct 18, 2018 8:34 am 
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Joined: Thu May 24, 2012 8:03 am
Posts: 1096
Location: Chestertown Maryland
This morning I got a innerweb tip from Dan Erlewine at Stew Mac. It was on gluing up a back if the 2 pieces are warped. At the very end he is shown gluing on a brace. He is gluing it on against a flat board.

So my questions is this - will the shaped brace pull the curve into the back so that it is properly domed??

https://www.stewmac.com/How-To/Trade_Secrets/Working_with_warped_wood_flattening_and_joining.html?lac_guid=8f0f5d87-55d2-e811-80e1-ecb1d775572a&utm_campaign=TS0316&utm_medium=email&utm_source=newsletter&utm_content=TS0316_C_20181018


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 Post subject: Re: Brace gluing
PostPosted: Thu Oct 18, 2018 9:25 am 
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Joined: Fri Mar 03, 2006 7:09 pm
Posts: 5884
Location: Hegins, Pa
yes depending on the shape the lower older style may not be stiff enough the higher modern shape will
again it depends on your building technique.
This is where a mold will be a big help , if your free building the warped board may pull the sides and put in camber , a mold will hold them more rigid. Also did you put geometry on the back . If you radiused the back that will help even more.
I do my backs at about .105 of an inch and have seen pretty hairy warps pull in just fine. The better the rim prep the better the success rate for this

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John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
Board of Directors of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com


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 Post subject: Re: Brace gluing
PostPosted: Fri Oct 19, 2018 8:03 pm 
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Joined: Sun Oct 14, 2018 9:30 pm
Posts: 5
Location: Carroltown PA
Actually if you look close he is clamping radius braces to the back in a radius dish. He is just not using a go-bar deck.


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 Post subject: Re: Brace gluing
PostPosted: Sat Oct 20, 2018 7:17 am 
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Joined: Wed Aug 13, 2014 12:14 pm
Posts: 1162
The back will generally conform to the brace shape, but there are limits. I've had big humidity changes warp a finished back from its as-built 15 foot convex radius to slightly concave, so I wouldn't count on the brace shape always being able to hold a form. (When that happened, I decided my shop needed a dehumidifier in the summertime.)

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 Post subject: Re: Brace gluing
PostPosted: Sat Oct 20, 2018 9:12 am 
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Joined: Thu May 24, 2012 8:03 am
Posts: 1096
Location: Chestertown Maryland
Thanks LC - he IS using a dish. I saw the thick particleboard edge and assumed it was the same flat board he used earlier.

The reason I ask is that I have a lot of experience laminating in furniture and boatbuilding and have found that you need at least 3, and better 4, laminations of similar thickness material to hold a shape, so I am imagining that with the taller brace than the back thickness, the end result would be between the flat shape of the back and the curve of the brace.

But not the case - he used a dish

Ed


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 Post subject: Re: Brace gluing
PostPosted: Sun Oct 21, 2018 9:14 am 
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Joined: Sun Jun 25, 2017 3:58 pm
Posts: 275
Location: St. Louis area
Interesting. I glued my top braces to the top using the radius of the braces to pull the top into radius- according to the StewMac instructions. That was not the case for the back thought (used the card stock radius method for the back). I've since bought one radius dish from John, the 15' dish for the back. Anyone else clamp the radiused top braces straight onto the top?

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Karl B


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 Post subject: Re: Brace gluing
PostPosted: Sun Oct 21, 2018 9:15 am 
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Joined: Sun Jun 25, 2017 3:58 pm
Posts: 275
Location: St. Louis area
Interesting. I glued my top braces to the top using the radius of the braces to pull the top into radius- according to the StewMac instructions. That was not the case for the back thought (used the card stock radius method for the back). I've since bought one radius dish from John, the 15' dish for the back. Anyone else clamp the radiused top braces straight onto the top?

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Measure Twice,

Karl B


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 Post subject: Re: Brace gluing
PostPosted: Sun Oct 21, 2018 10:12 am 
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Joined: Thu May 24, 2012 8:03 am
Posts: 1096
Location: Chestertown Maryland
To clarify my earlier statement on laminating, the taller brace can easily overcome the thinner plate and force it to bend and stay that way if glued on in a dish. However, I see a problem in distorting the curved brace to be flat, gluing it on a flat plate, and expecting the brace to overcome the glue joint to recover the original curve.

Ed


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 Post subject: Re: Brace gluing
PostPosted: Sun Oct 21, 2018 7:42 pm 
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Joined: Fri Mar 03, 2006 7:09 pm
Posts: 5884
Location: Hegins, Pa
braces if pushed flat will return to the curve. The stiffness of the brace is in relation to the cube of the height, so even smaller braces will reconfirm to their original shape taking the plate with it.

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John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
Board of Directors of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com


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 Post subject: Re: Brace gluing
PostPosted: Wed Oct 24, 2018 10:47 am 
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Joined: Sun Aug 24, 2014 12:27 pm
Posts: 14
In the Cumpiano book, he arches the brace and uses a thin flat caul and clamps. I found it works quite well when I tried it.


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