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I have two guitars in the shop that need lower bout side repair. Cracks and such. Something that would apply internal and external high pressure. Was thinking about those big magnets with a shaped caul. Ideas?

Possible. Can you post pictures, Mike?

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Hopefully Clay will chime in with something he posted on MIMF. If I remember correctly he made a flexible strip of material with a row of square magnets inside that were mated with a similar row on the outside. Might be in the Tool section over there. There was a commercial version too.
Here's a link to some magnets on the bay https://www.ebay.com/itm/182044406789.

I like tuning machine clamps for side repairs. The machine clamps are used to align the the halves together while also gluing a cleat in place and spool clamps are used to clamp them shut.

The flexible magnetic cauls were inspired by the cauls made by TJ Thompson. Since his cauls are ridged I decided it might be nice to make some that would flex and work for side cracks. Rather than premade cleats I cut veneer strips that would be cross grain to the side cracks. I used masking tape to hold the veneer together, which can be removed after glue up (or left in place).
Neodymium magnets have become quite popular for aligning and gluing cracks and work quite well.

We had a few hundred rare earth magnets in a variety of sizes, and never saw a side we could not align and glue.

A few notes:

- Work across the side where you have multiple cracks - there will usually be a clear progression of crack glue-up evident with just a little experimentation

-Avoid reinforcing the side before working all the way across - side reinforcement will usually get in the way of the next crack to be glued, so be patient

- Stack magnets for additional pressure, and go to the larger ones (1" diameter) where alignment is an issue

- Sometimes fish will be the answer on gluing, and sometimes thin CA...and sometimes a combination of both

- You can use thin CA to tack weld the bits and pieces in place on rosewoods, but avoid use of the material as a filler for missing wood on mahogany, maple, etc.

- Face half of the magnets with peel'n'stick UHMW plastic from Woodcraft (sold in sheets...get the thinnest stuff available) - keeps the ones used on the finish from scratching and potentially sticking

- Luthier's cam clamps are ideal for applying gentle pressure across the side once all pieces are aligned

- The slight ripple that may be apparent after a multi-crack side repair may be minimized with an internal cross-grain graft...just handle the transition to the linings intelligently by avoiding the creation of a stress riser

While I learned the old-school technique of using tuning machines to close the gap, other active repair people will readily identify guitars so treated for their stitch-like scars from the process. I did a fair number of side repairs at Greenridge and never saw the need to do the tuning-machine-assisted repair once I mastered the rare earth magnet process. And no Frankenstein scars!

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A constellation only takes shape when one maps the whole.
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Spool clamps are good too for closing side cracks after aligning with magnets.

1/32” RC model airplane plywood is flexible and makes a good cleat if cosmetics are not a factor.

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Hi Mike, Yep that's what we use and not the wimpy, wimpy, wimpy rare earth magnets that you see at StewMac but independently sourced ones that have about 40 lbs of pull. They are a bit dangerous to know this in that they can jump and pinch the sheep... out of you if you are not careful. But I've used them for 14 years now on hundreds of repairs.

Get a cutting board and then slice it up in to 1/8" thick pieces and these are excellent flexible cauls for the curves and Titebond original will not stick to them. No benefit to HHG here in that you need time for magnet placement this repair is not supposed to ever need servicing in the future.

Funny story, we had to order them twice, the first time they never arrived. The second time they arrived fine. They a year later the first batch arrived with a note from USPS saying the box was magnetically stuck to the inside of a mailbox for a year.... .

Old school baby and kind of like a dent puller in a body shop. Good idea. Herb (David) used to do it this way we use rare earth magnets.

Hahahha! I had something like that happen to me. I lost some of those magnets and found them a year later sticking to an electrical outlet junction box between raters of the ceiling of my shop. I have no explanation at all what so ever as to how they got there. The darned things are expensive too.

Yeah your's made it to the ceiling. We are constantly losing ours putting them down somewhere and then they leap to a screw head in the bench and stick to it. Uppty things and yes they are expensive too.

Dave made a bunch of various cauls and fixtures to expand our ability to use them. WIth his permission I may post some pics here if I have not done it in the past. We have one we call "Big Bertha" that is massive, weights 14 libs, has a metal housing and a jack screw crank that lowers the massive magnet to the teflon caul. You place it and then screw it down. He used a crank because the clamping force on this one exceeds our ability to lower it slowly without it trying to jump out of our hands.

He's done another one that I use a lot which cantilevers over a loose brace, a notch in the teflon registers on the brace top and a mating magnet on the back, front of the instrument clamps the loose brace down. Very clever and works great.

My use of them is primarily for cracks with a teflon sheet on the top side to protect the guitar. I typically put some waxed paper on the magnet faces inside the box so I can have them back when I am done and the glue dries Don't ask but there is someone in our area with a 40lb of pull rare earth magnet under their lower bout still. It's hard to get good help