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My only thoughts on that Terrence is that you still want to have the grain orientated properly and that is at least on an off angle to the direction of the crack. A sphere is one of the strongest shapes possible. It's why when jagged rocks erode from wind or water they form a sphere. It is the shape that is the most resistance to the forces of erosion (former Geologist here )

So while your cleats are not spheres per se but more like hemispheres it could be argued they are even better then angular cleats.

EDIT: Oh ok plywood. Yeah you would not have to worry about grain. Might create too much of a node though. IDK

I used a square of 0.9mm ply (both grains at 45 degrees) for an X brace cap and it seems to be holding just fine.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Magnets like that will not provide control of the orientation the grain of the patch. It can turn 360 degrees at will.

How would that be a problem? Or are you joking?

I think that's a pretty good idea and I like it. Anyone who claims to hear the difference in cleat material has been smoking the drapes.

I've been wanting to get some of that modeling plywood too seems like a great material for a number of repair situations.

My rare earth magnets are much larger and more powerful than what we see in the pics. It's not difficult to orient your hand and arm when in the sound hole to have the magnet oriented as I wish and the topside magnet already placed and held in place with my other hand to guide the inside magnet and cleat into place. A quick look with the mirror to ensure cleat orientation is across the grain and it's good to go.

A word of caution, we use very powerful rare earth magnets that we even had trouble buying. The first batch took a year to be delivered to us because the box got stuck to the inside of a mail box we think since it disappeared for 13 months from when we ordered them and they were shipped.

Anyway these are powerful and can injure you badly if not careful. They can jump so understanding magnetic fields is helpful and you need proper spacing between multiple magnets too. So be careful these are not toys. Our's have 42 lbs of pull making them formidable. Wear safety glasses when using rare earth magnets too when they collide from jumping or you let them come together hard the metal encasement can and does fracture and chip and send chips flying, protect those eyes.

We've never had any damage because we are mindful of what I mentioned but if you are new to rare earth magnets please be safe. Hesh's nag over and out

Yes the plywood is good for all sorts of things. I have also used it for cleating side cracks and reinforcing pickup Jack holes. I use the 1/32” the most.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

The magnets in those pics are rare earth ones and I double up on them for extra strength. And yeah they will fly across the room on you. I once lost some for like a year and then found them sticking to a metal breakout box on my ceiling!

I created this jig becasue it was so far back in the guitar it was difficult to get too. So with the magnets you get close and let it fly and it finds its spot perfectly. I like to use HHG so time is important too.

^^ ditto ?

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I got the impression (maybe mistaken) that some thought the magnets could ensure that the grain of the patch would be oriented in the proper perpendicular direction. But the pole direction of round magnets is up and down so it does not control the planer axis. That is all I meant. Maybe everyone already knew that.

Yeah. The daimond pattern in the jig though is controlled in space by the two magnets in a line. So it doesn't matter if it's facing north or south as long as it's in a straight line thereby the grain of the cleat will be across.

After this discussion however I am liking Terrence's idea more and more with the little round patches.

I made a wooden holder for loose feeler gauges for ease of use. I bought 4 or so fan sets of gauges and separated them from the metal holder. The front row is for the even numbers, the back row the odd ones. I marked the 1 mm gauges with a punching tool, they are the most frequently used ones and needed for thick measures. The extra compartment in the back row is used for additional 1 mm gauges.

I let you Americans to figure out how to make a similar holder for your strange measuring system!

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New will change the old,
turn to ashes or gold

http://www.gammelgura.se

Nice gauge holder. I have a magnetic strip on the edge of my setup benchtop with a whole mess of gauges hanging off it, but I like your idea better.

On crack repair: we saw a lot of failed crack repairs come through Greenridge. The worst were square-edged diamond or polygon-shaped pieces of random wood species which appeared to have been sourced from the debris near the repairer's chop saw. A step up, but still subject to frequent failure, were tapered diamond patches, which provided less of a stress riser, but still caused inconsistent stress paths across the crack region.

Dr. Kennedy's very thin aircraft plywood patches seem near ideal for short cracks near structure (e.g., pickguard cracks on all those improperly-stored black guard 1970's Martins), but beg the question elsewhere: why not extend the cleat the entire length of the crack if the material allows? Indeed... Mr. T.J. Thompson's linear tapered cross-grain cleat repair method (as sold by Stewmac) is what we saw as the best performing method for general crack repair. While we saw guitars in for new top cracks with existing repairs of this nature, those new cracks were ALWAYS away from the previous repairs (some owners are apparently very, very slow learners with abundant financial resources to dedicate to providing multiple proofs of Albert Einstein's oft-quoted definition of insanity). Our own repair method was based on a shop-made version of Mr. Thompson's approach and tooling.

In terms of prep and use, the tag ends of red spruce 2x dimensional lumber were cross-cut into 9/16" long pieces, and those that survived the process without revealing hidden cracks were then ripped into 0.080" thick splines. These linear cleats were double-side taped to the edges of 1/2" ply carrier boards and shaped on the 6 x 48 belt sander to 0.060" x 12mm width cleat stock. A light sanding block cleanup prior to use gave us abundant repair stock for spruce tops (we processed scrap redwood, cedar, mahogany, BRW, etc. into repair cleats as needed. Pairs of rare earth magnets offset from the crack by half the width of the cleat provided alignment, with those magnets then moved onto the cleat once the adhesive tacked, with additional pairs of magnets as needed to provide additional clamping pressure.

Using the plywood as a linear cleat like TJ is a good idea. I have done that on side cracks and I think I will try it on tops in the future. I have used larger circular cleats up to 1” when I was really worried about stability but mostly on cheaper guitars.

I like the magnet alignment idea a lot but when you switch to clamping magnets after tack up don’t they have to be pretty close together? How do you keep them from popping over and sticking to the adjacent magnet?

Have you used plexiglass cauls with small alignment magnets and clamps like the Thompson system StewMac sells?

Thanks Woodie.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

The TJ Thompson back crack repair system is the bomb. 3/4" square magnets adjacent to each other, the entire length of the crack. The inside and outside cauls align perfectly and quickly enough to allow the use of HHG. I have an extensive thread over on the MIMF, if anyone is interested.

N-20 Martin Repair

Where we don't want a continuous line of magnets along the cleat, we either space appropriate to magnet strength (this can be tricky...each size of magnet and combination of adjacent sizes means a different minimum distance to be observed) and work from far to near re: sound hole, or use a UHMW PE spacer block with a tiny rare earth magnet or piece of cellophane double-sided tape holding it in place.

We found it useful to have some of the less aggressive, adhesive-resistant, plastic-covered magnets to fill in between the end magnets... seasonally, we had 70-80 of these available on each bench for just that sort of thing. They would come out sometime in Feb and stay on the bench until we saw the last of the winter's RH damage in June.

One downside to those linear cleats is ease of removal - they can be a trial where a subsequent bridge plate replacement or other structural repair requires removal of the cleat for access (Taylor bridge plates...ugh!!!!). I had a nice set of miniature-sized scrapers with the ends safe'ed and edge slightly radiused for that sort of clean-up task on soft top woods... I really should retrieve them from Mr. Morelli now that he is in the process of a move and before they are exported to Tennessee or some other faraway land.

On the question of Mr. Thompson's cauls, etc., no - we generally have not found them to be needed, but only because we use magnets as discussed above to align and directly clamp the cleats.

I should mention that I always find your posts delightfully informative and often thought-provoking, Dr. Kennedy - thank you.

Thanks for the reply Woodie and also the link Berry. I have been gluing the crack first using about 5lbs of air pressure to blow the glue in the crack and then clamping with cauls. Cleats are placed as a second step. Is that the way you guys do it or all at one time?

I have been using small magnets on the cauls for alignment so I think I could do the same for the linear cleats.
Here is my most elaborate creation so far preassembled outside the guitar and after crack clamp up. Almost too elaborate but I got it to work

Off to read your thread Berry. Lots of good ideas

EB6EA5BF-20C5-4117-B153-344185CA54C2 by Terence Kennedy, on Flickr

B0DC5F20-93C4-45E3-BD5E-1B90BB11A8FC by Terence Kennedy, on Flickr

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It's not what you don't know that hurts you, it's what you do know that's wrong.

One advantage of the TJ system is that everything happens at once. Rub some HHG into the crack then wipe off finished surface. Place the outside magnetic caul into position and tape down. Put some HHG onto cleat that is double-sided taped onto the inside magnetic caul and place it inside the guitar. The magnets pop it into the proper position. Done.

This is not my invention (read about it in one old American Luthierie), but it's by far the best improvement in my shop lately. It's a toothbrush on a stick, very useful when cleaning up hot hide glue. Hot hide glue goes very sticky fast, it is hard to clean up in the small corners even with a damp cloth right after the gluing is done. The toothbrush have hard and sturdy bristles and makes the job easy, one dip in water make the job even quicker. The long handle (an 8 mm birch rod) is great too.

The toothbrush is cut, the remaining plastic handle is thinned down to a triangular flat point, a matching "dovetail" cut is made in one end of the birch rod. The flat point is cut parallel to the top flat side of the brush. Superglue and some electrical tape completes the joint. The spring in the plastic handle in the toothbrush makes it work even better when doing the cleanup.

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New will change the old,
turn to ashes or gold

http://www.gammelgura.se

I just watched the video on the TJ Thompson system on Stew Mac and I'm not sure I agree with the all at once method. I always apply clamping pressure on the crack itself by squeezing the sides in. You can actually see glue squeeze out when doing this. THEN I add cleats. Or if I was gong to to the TJ Thompson I would then add the strip. If it's a really tight crack then clamping is not necessary but more often then not I clamp up.

I have used the tooth brush to clean hide glue for a long time but not the long handle. That is a great idea.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I do much the same for my feeler gauges, in my case used to check relief on my classical fretboards. I use carbon strips (no truss rod) so the necks are too stiff to get relief from string tension. I also plane in a twist for more string height on the low strings. It's a bit complicated, and will probably be more so when I start adding a radius.

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formerly known around here as burbank
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http://www.patfosterguitars.com

Cool holder!
We have a thing in US called decimal inches (mils).
1mil = 0.001”. My gauges and scales are marked in mils.
My work-related engineering jobs are done in metric, but our Asian vendors convert our stuff to inches.
Then I get to convert their stuff back into metric.
At home, I only use fractions for cutting pies.

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wah
Wah-wah-wah-wah
Wah

Thanks, that's another thing to know! The most weired thing about the US system is the fractions. I have to divide the numbers first, then multiply with 25.4.

Incidentally, a Swede (like me) by the name Carl Edvard Johansson made a precision measure set in cm and mm that was so good that England and USA could standardize how long an inch actually was. From Wikipedia:

"The inch measure used in England and the United States differed by a few thousandths of a mm, but by C.E. Johansson's efforts, England and the USA could finally agree that the conversion factor between inches and mm, would amount to exactly 25,400 mm = 1 inch. This was established as an international standard in 1933 by the ASA (American Standards Association).

Illustrative of C. E. Johansson's accuracy as a human being was that he kept a diary throughout his life. During all the years he was employed by Ford, he did not come to work late a single day except one day, when he got a puncture on his car, which was carefully noted in the diary."

_________________
New will change the old,
turn to ashes or gold

http://www.gammelgura.se