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PostPosted: Sun Jan 26, 2020 10:53 pm 
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I am assembling a Martin Jumbo Kit with Rosewood back and sides. I am gluing the back braces and the back braces are of different radii. The one near the tail is at 25’. The one in the hip of the guitar is much “sharper” (probably about a 15’ radius). Is this normal? It makes it pretty hard to glue.


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PostPosted: Sun Jan 26, 2020 11:32 pm 
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I'm sure John Hall will verify my answer, but no, I've never seen nor built a guitar with different back radii. I can imagine the problem you're having trying to glue the braces.


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PostPosted: Mon Jan 27, 2020 12:01 am 
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OP: What's the source of this kit guitar? I am too dull inna head to see how back braces are cut to differing radii.

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peter havriluk


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PostPosted: Mon Jan 27, 2020 7:36 am 
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Location: Hegins, Pa
the back should be glued up on a 15 foot radius also you want to run your cross grained spruce first then relieve the slots with a razor blade.
the radius on the back braces should match.

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Blues Creek Guitars Inc
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PostPosted: Mon Jan 27, 2020 7:39 am 
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Location: Hegins, Pa
you can sand that brace to match. I hope you didn't buy off ebay from a none martin source. Many of the kits on Ebay are miss matched parts from people that buy martin scraps and put together kits .

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Blues Creek Guitars Inc
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president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com


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PostPosted: Mon Jan 27, 2020 9:21 am 
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Make a 15' radius sanding block. Stack up a couple pieces of 3/4" plywood, lay out a 15' radius (180" in the drawing), and cut it with a jigsaw. Smooth it up and stick some #80 sandpaper to it. https://www.stewmac.com/Materials_and_S ... _Roll.html

You'll be able to sand the brace to the correct radius easily.


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PostPosted: Mon Jan 27, 2020 10:04 am 
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Location: Chestertown Maryland
You can also get a 2 foot scrap 2X4 out of dumpster, cut your radius on one of the edges, cut 1-1/2" strips of 80 grit from a full sheet, and stick it on with 3-M 77 adhesive which should be in your shop anyway. I am on #9 and haven't changed the paper yet. You can cut the other radius on the other edge and also stick sandpaper to a flat side and have 3 differently shaped surfaces in one tool. And you can use it to trace the shape of each brace on the material, plane or saw off most of the waste, then sand to final shape. It fits in your vice to.

And like John says, don't forget the cross-grained back strip.. Not too late to fit it in between the braces, just easier to do in one piece and cut out the spaces after gluing it down.

My 15' radius sanding beam in the picture below is cut from the wide face of a 2X4 because I use it instead of a dish for sanding the rims - same idea though. I only build about 1-1/2 a year and am trying to do this well without buying a lot of expensive equipment.

Most tops go from about a 28' radius at the lower bout to flat at the upper bout, so changing a radius on a plate is not that unheard of - just not on a back so much

Your work looks nice and clean

Ed


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PostPosted: Mon Jan 27, 2020 11:03 am 
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LMI sells a double-sided brace sanding jig (15' and 25' radii). I bought one, works nice, and no time spent making it.

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PostPosted: Mon Jan 27, 2020 11:09 am 
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There are good suggestions here. But, I'm going to tell you how to correct the brace that you already have, without making or owning any special equipment, or procuring more wood. This is the "down and dirty" correction.

Please use your imagination. Pretend that the photos that I'm using are of 2 back braces and not a top brace and another brace. Both are of different radii.

All you need to do is match the center bottom of your brace with the 20+ foot radius with one with a 15 foot radius. Trace along the bottom of the 15 foot radius onto the brace with the wrong radius. This is your correct radius.

Just take a piece of 120 grit sandpaper, wrap it around a flat piece of wood, and sand the bottom of the brace that needs correction, until you reach the line that you drew. Keep the brace solidly on its side, and sticking out from the edge of the table, just enough to sand. This will help you keep the bottom at a right angle to the sides.

Alternately, you can use a hand plane to remove the wood from the bottom of the brace.

When you reach the line that you drew, your brace will have the correct radius and can be glued.

Don't forget that you need a center back brace that lays on top of the seam. The brace is thin, and about 3/4" wide. The grain will be 90° to the grainline of your guitar back.

Attachment:
PicsArt_01-27-09.51.13.jpg

Attachment:
PicsArt_01-27-09.51.57.jpg


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PostPosted: Mon Jan 27, 2020 3:35 pm 
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Joined: Fri Jan 24, 2020 5:35 pm
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phavriluk wrote:
OP: What's the source of this kit guitar? I am too dull inna head to see how back braces are cut to differing radii.


It is a Martin Jumbo Kit purchased directly from Martin.


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