Martin Jumbo Kit Back Braces

[phavriluk]

Suggestion to OP: Pick up the phone. Email is seriously easy to ignore.

[tippie53]

they get 1000 of emails a day so till they see it and then decide the proper person to go to
call you will do better

[Selah]

I called them this morning and got through to the lady who puts together the kits. She was very nice. She is going to check up on this and see if it is meant to be that way. I will let you know when I hear more.

[Selah]

I got ahold of Martin this week (via phone) and they looked into this and reported back the following:

“I spoke with production and showed them the photos you provided and they confirmed the radius of what I sent is correct, however you can bring it down if you want to.“

The photos I sent showed the different radii of the braces. They confirmed that they are indeed of different radius for a Martin Jumbo guitar. Now I need to figure out how to shape the rim with braces that have a sharper curve as they get closer to the neck.

From my calculations and measurements, the radius of the braces are (#1 is closest to the tail and #4 is closest to the neck):

• #1: 15 ft

• #2: 15 ft

• #3: 4.5 ft

• #4: 7.3 ft

So, a puzzle on how to shape a back with that varying radii! Any suggestions?

[tippie53]

you can still to a 15 in fact your kit was pre shaped so you don't really have to do much . I still radius all my guitars 15 foot.

martin sands all the guitars in the same machine so you will be fine.
here is the picture of the set up they use

[ruby@magpage.com]

When you are new to this you have an imagined level of precision for each step.

Just think of the the Martin shop pre-1900. The temperature in the shop overnight in the winter has dropped a bunch, then warmed quickly as the stoves are lit, the humidity is changing daily/hourly, they are cutting back wood and leaving the circular saw marks on the inside, conditions could be challenging for the use of hide glue, there was that new apprentice who messed up a bunch of brace stock but they used it anyway, and when they got low o supplies they had to use less than perfect material.

Then think of how the dome on the back changes throughout the year anyway with changes in conditions.

Things don't have to be as exacting as you think - I don't believe they were using micrometers to build these things.

Below is a picture I always think of when I think of precision. During WWII, Gibson's people all went off to war, so they recruited the women who were left behind to build instruments and run the string winding machinery. These are some of the best sounding guitars they ever built despite having mistakes made in construction.

The pictured guitar is one of those and it was in perfect condition when it hit my daughter's repair shop (brooklynlutherie.com). The reason it was not played is that the top had an enormous belly in it making the action extremely high. Turns out the bridge plate was glued in in the wrong place - here it is removed. My daughter flattened the top, put in a new bridge plate, and it was a stellar performer.

The back dome amount is a random sort of thing - different makers use different amount. And Gibson used a cylinder shape for awhile and some modern makers still do. So don't lose too much sleep over these things - your guitar will sound alike a guitar.

Ed

[tippie53]

as Ruby points out don't overthink
the 2 machines shown set up the geometry on the sides and all the guitars go through this process. The top has the mold and sides in . There are stops set up and when the machine is done the top is set up for the top gluing process. The back ( the machine without anything in it ) has a stronger radius than the top. It is about from my eye between 15 foot.
So the backs are all do the same way.
Glue it on and you will be fine.

[tippie53]

here is the old way
NOTE watch for the accurate eye balling.
There is a point where the person is hand planing the geometry on the top. watch around 38 seconds in the vid
https://www.youtube.com/watch?v=Zs9miwi7CKE

[Selah]

So, I made some radiused boards out of scrap 2x4s at 4.5’ and 7.3’. I then attached some self adhesive sand paper to them. I sanded at 4.5’ near the hip of the guitar where the 4.5’ radiused brace is and sanded with the 7.3’ near the neck where the 7.3’ radiused brace is. Finally, I took the 15’ radiused dish and sanded near the tail on the guitar. Looking closely at the result, I think it turned out okay. I’ll be gluing the back on soon.

Sorry for the photos being upside down. I can’t figure out how to fix that with my phone.

[MaineGeezer]

As long as you get a good glue joint all around, and it "looks right" with nice smooth curve transitions, you're fine.