Martin N-20 Folk Guitar repair

[Barry Daniels]

One of the very interesting features of T.J.'s system is the way the cleat fits into a matching groove on the inside caul. This keeps things from shifting around during assembly. Your veneer cleats, or my double veneer cleats, could be set inside a rectangular groove in some kind of flexible caul. A piece of extra wide plastic binding strip could be laid up with a couple of smaller strips to make a rectangular groove to lay the veneer into. Double stick square magnets on top and Bob's your uncle. T.J.'s magnets are 3/4" square and have a pull of about 20 pounds each. I will have to build a prototype now.

One other observation of T.J.'s kit is that the square magnets act like one big magnet. So when you are using the two cauls, the magnets not only pull the two parts into alignment sideways, but also long ways. It is very interesting and necessary when you are trying to line things up on either side of the guitar. You can't see where the caul is supposed to go when you are working with your arm in the guitar, so this 2-way magnetic alignment effect comes in very handy. I don't think you would get this effect with round magnets, or at least it wouldn't be as strong.

[Clay Schaeffer]

Hi Barry,
My thinking is that since the cleats span the entire length of the crack (unlike diamond cleats) the veneer may be strong enough. I do have some veneer that is probably thicker than .056" in mahogany (?) and some lighter and darker woods, so I could try making some thicker cleats. On the website it said the cleats were cut with the grain at 22 1/2 degrees. I wonder if that makes much difference? I have been keeping the grain perpendicular to the crack (which usually runs with the grain of the guitar -so the cleat is cross grain)
I think I will try mounting some of the small magnets in groups and see how they will act. Some sites do offer square magnets, and they may be available at craft stores, but I'm going to try to use some of the "junk" I've collected and try to put it to good use.

[Barry Daniels]

The 22.5 degree grain direction is as good as 90 degree cross grain in my view. T.J. says on his website that he choose this angle based on vintage tone bar angles.

[Barry Daniels]

I got ambitious and did two cauls at once on a crack that spanned a brace. Three down and one to go.

[Barry Daniels]

I wrapped the outside cauls with wax paper to minimize finish scratches.

[Clay Schaeffer]

Hi Barry,
I played around with the flexible caul idea we talked about. I used some round neodymium magnets I had on hand. I placed them in square corian holders and glued those to a strip of cloth. The first set I left square, but then the light bulb came on (briefly) and I thought I better make the other set have angled sides so they can be "bent". I'm thinking two pieces of low tack tape could be glued together to hold the veneer to the magnetic strip, and could be peeled off after the glue has cured.

[Barry Daniels]

How much clamping pressure did you get? T.J.'s cauls are so strong you feel like you cannot pull them directly apart. You have to twist the outside caul to get it to come free.

[Clay Schaeffer]

Hi Barry,
The magnets did provide a good amount of clamping pressure for clamping the veneer cleat. I'm sure it would vary depending on how far apart the magnets were kept by the thickness of the sides. As a first iteration I am happy with it and I think it will work for most situations. I do see room for improvement. Using the larger disc magnets available and gluing directly to the bias tape might be less trouble to fabricate and provide even more clamping pressure. But I used what I had (I tend to collect junk, so when I can find a use for it, or "re-gift" it to someone who has a use for it, that makes me happy).
The strips do pull themselves into alignment, as you mentioned the plexi cauls do, however placing the second caul in close proximity I think will still be necessary because once some of the magnets make contact it seemed difficult to slide them. Using less strong magnets or having glue on the cleat material (to lubricate the surface) may make positioning them easier. Have you found that glue helps the cauls slide easier? I left the cloth tape a little long on the ends. I can grab it and peel it away from the side if I need to reposition the cleat/caul.
I measured some of the veneer I can use for making cleats. Most of it measures about .030", but some "mahogany like" veneer measured .075" ( I'm almost tempted to try building a guitar with it without laminating!) For spruce cleats I will sand down some of the offcuts from soundboards - another use for wood headed for the bin.
I am interested in hearing your thoughts and suggestions.
I have some plastic edge banding that is almost an inch wide and about 3mm thick that you could groove out to make a caul - PM me your address and I will send it out to you.

[Barry Daniels]

Glue on the cleat helps a little bit to slide the cleat into position. Sounds like you have a good setup. I used HHG on the three cracks I am dealing with and the rapid positioning and no need for clamps made this possible.

[Bob Francis]

This is a very interesting discussion.

[Paul Breen]

So for the TJ System, using 3 magnets in a row, they would have to be configured: +,-,+ on one clamping side and -,+,- on the counter clamp. This would allow for the cauls to be aligned and attract one way but also allow for one of the cauls to be rotated 180 degrees and still attract. In the images Barry has with two sets of 3 magnets on a caul, the sets would repel one another, assuming the TJ system has the magnets all programed the same way.

If on the other hand, you had 4 magnets in a row; +,-,+,- you would have to observe the magnetic polarity, the caul and counter caul would only attract oriented in one direction. Five in a row would again allow for one caul to be rotated 180 degrees and still attract, so an odd number configured together would have that advantage.

I see that Clay has 6 in a row. Are they all glued in the blocks, same polarity up (+,+,+,+) and taped down repelling each other or did you configure them +,- ect. but then need to place the row of 6 magnets paying attention to polarity? Neodymium magnets are quite strong and be a bit of a trick to manage. Did you have trouble applying them on the tape strips like that? This idea definitely has some merit for side crack repair, curved surfaces are tricky for crack alignment.

I have been using magnets for several years now but have only been using them placed singly and in repulsion to their neighbors. When using these strong magnets, I always clear the bench of clutter and especially anything steel. The magnets are carefully arranged in ready order on my bench, clamp/counter clamp and also in order of pairs that repel the neighbor pair. These magnets can damage instruments or human flesh if not handled with great care and some planning. The TJ System is appealing to me because of the way it can be placed as a single magnet.

Placing single magnets lets me stack them or not for increased or decreased clamping. Once a pair is set, I use an opposite polarity set, which can be set pretty close and still feel like I have control.

These are jigs for gluing loose braces, set to repel their neighbors.

Magnets work great with sticky back sandpaper for any sanding inside of an instrument too. I use masking tape or thin acetate on the outside of the instrument to protect the finish from scratches while moving the counter magnet around.

[Barry Daniels]

T.J.'s kit comes with three sets of cauls, one set with 2, 3 and 4 magnets each. I did not realize that they would have alternating magnet pole configurations, but I just checked and that is exactly correct.

That is a cool setup, Paul, for clamping a loose top brace.

[Clay Schaeffer]

Hi Paul,
The magnets on the strip are all oriented the same way and are repelling each other. The magnets on the second strip are oriented to be attracted to the first strip (but again repelling the other magnets on their strip). I used clamps to hold the magnets in place while the glue dried. I can see where adding magnets to the back of the existing caul could add additional clamping force if needed. By adding them individually and selectively they could add force where needed and also be less trouble to remove when the time comes to do so.

[Barry Daniels]

It took me awhile to complete the repair but need to close with some final pictures. The finish polished up pretty nice, but I didn't want a full gloss because the neck was left in its natural state. So I sanded up to 6000 grit then hit it with a lambswool buffing pad and some caranuba paste floor wax.

[Barry Daniels]

Photo of face-on of the back, close up of the re-glued binding on the side, and repair on the top. Finish touchups with Glu-Boost CA.

[Paul Breen]

Nice work Barry. I had to go back and look at the original images you posted of the damage, I can't see any sign of repairs to the back or the binding. The work to the top is just barely visible in the image. The owner should be quite pleased I would think.

Did your planning with the magnets pan out as planned?

[Barry Daniels]

Thanks Paul. The repaired back cracks are almost invisible. The finish repairs along the bindings did not turn out quite as nice, but are not too bad. In the end, I am very happy with the TJ Thompson back crack repair kit.

[Darrel Friesen]

Looks great Barry. Way beyond my repair skills. I learned a lot from this thread.

[Barry Daniels]

Thanks Darrel.

[Bob Orr]

Very interesting, enjoyed following this. Lovely work.