Official Luthiers Forum!

Does anyone use reinforcement cloth on the x brace intersection similar to what Martin uses. I can’t find it anywhere for sale.

I've done that on a few guitars and what I would do is cut strips from an old linen shirt and soak it in HHG and form it in place. Wet fingers help. I almost always just use a spruce cap now though.

I guess I’ve never seen a spruce cap before. Searched but cannot find any info on it. Can you please elaborate.

Rather than using cloth to reinforce the joint you use a small piece of the same spruce as the bracing, glued across the open part of the joint facing into the guitar. It need not be thick; 1/8" or less should do. What counts is the length of the patch, to allow sufficient glue surface to keep the open joint from spreading under the down load caused by the bridge torque. I usually make them about 2" long. The top of the braces should be flat all the way across the width of the brace for that stretch, so there's something to glue to. On some guitars the braces are shaped to a tapered cross section all the way along, which leaves a gap on either side at the top of the joint (the part facing into the box), and only about a 1/8" wide surface to glue to on the top.

The danger when the open joint is not bridged, either with cloth or wood, is that the down load from the bridge torque tries to open up the joint, as I said. The sudden change in section at the bottom of the mortise causes a stress riser, tending to split the brace out from that point. This is bad enough, but if there is any run out in the brace stock the split will run across the brace which really reduces the load carrying capacity. The top caves in in front of the bridge. Both the wood and cloth patches help to hold the open joint together. The cloth patch has the advantage of gluing to the sides of the braces, so that it can do a better job when the braces have been cut to section before assembly and there's a gap. The disadvantage is that air exposure can break down the glue over time, reducing the effectiveness. A wood patch is probably better if the design is suited to it.

I use the cloth on joint. Love it. So does Martin. Who am I to rethink that? But there are ways to do it that make it less difficult, less messy. I use tee-shirt cotton. Make a circle about one inch in diameter. Just keep trimming with scissors till ur happy. Then apply some tite bond to the joint on all sides. Let it dry a little. Then put the cloth patch on. Use a sharpish tool to press into place. Then, paint on some more tite bond, till its fully soaked. Pre-soaking never worked for me. Too messy. Too much fuss. The resulting joint is flexible, but solid.

I asked someone at Martin and they told me that they use gun cleaning patches. I use a quarter and a razor blade to cut them and glue them with hide glue.


Sent from my iPhone using Tapatalk

_________________
Good judgement comes from experience. Experience comes from poor judgement.

I used hide glue on cloth side reinforcement until someone mentioned that the large area of hide glue would attract things that would like to eat it. Not sure if that is true, but i switched to Titebond for both cloth side reinforcements and the x-brace cover

Ed

Same thing and I have used those before, still have to trim them down.

I'm not sure about cotton knit for this; too stretchy, I would think. Linen would probably be the best. Strad used linen side tapes on 'cellos, so that's a celebrity endorsement. Silk might be good, too.

Some experiments I've done suggest that hot hide glue works better for cloth side tapes, and probably for a brace patch too. It's a bit stronger, and actually easier to use, at least if you have the top off the guitar. Nothing is particularly easy if you're working in through the hole. Hide glue does break down over time with exposure to air, and I've been shellacking the tapes on my guitars to help prevent that. We'll find out in 75 years or so if it worked.

Alan, gun barrel cleaners are the same material as a tee-shirt. I started with them as I had some around (I like my guns!). Got that idea from Martin. But I wear out a lot of tee-shirts with shop work, so I just started using those. The fabric is nothing but a substrate for the glue. The glue determines the stiffness.

You can pick up gun cleaning patches at places like Bass Pro or Dick's Sporting Goods for about $6-$7 for a pack of 50 pcs.

_________________
Guitar Building = Continuous Improvement Process

IMO the fabric is what's taking the load, the glue just sticks it on. This is a tension load, if you think about it, and glues are not usually considered reliable in tension over time. If they were you could just make sure the box joint was tight, and rely on the end grain glue joint to the side grain of the crossing brace in the 'open' part. We already know that doesn't work. Certainly T-shirt cotton will take some load as compared with no patch: something is stronger than nothing, after all. Cotton-poly is probably better: I've found that the cotton-poly bias tape I use for side reinforcement works pretty well. But that's a plain weave, not knit like T-shirt stuff.

I had always understood that Martin used gun patch material, which traditionally was linen. Maybe I'm behind the times.

I use linen that I picked up for french polishing, but I do hate fighting the threads of the linen while gluing it down. It winds up looking pretty bad. I haven't tried hide yet, I will next time.

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

What we have here is really a composite material with cotton (or other fibre) embedded in a glue of whatever sort. I am not a material scientist but I would think that the tensile strength (which is what we want in this application) is mainly due to the fabric used. The glue is there to keep the fibres in place, as in a fibre glass or carbon fibre composite. So strong fibres linear to the stress would seem the way to go. This would suggest a woven fabric of strong fibres would be best. I tried a quick search but can't see what cotton is like compared with linen but no doubt it could be found.

It does seem to make a lot of sense to use a timber cap since we know that timber is strong in tension and as Alan has pointed out this is issue we are addressing.

No matter what fabric you use, applying a glue to braces, let dry to tack, then fit the patch. Brush on the glue after. No more mess. Soaking a patch then applying it? Tried that. what a mess.

gun cleaning patches are cotton. Some are tee-shirt-like, some are flannel.

I know you're asking about cloth, but a scrap of spruce placed on the top of the x-brace joint is quick and easy. You have all the off cuts and even scrap pieces from scalloping the brace ends just hanging around already. I use med CA and glue the piece on, clamp for a few minutes then chisel/sand to shape. Quick, easy, no mess.
But it's not the Martin way, but that's ok... it works just as well (some say better) at coupling the open brace back together.

google search for x brace cap and you'll see what I mean.

_________________
My Facebook Guitar Page

"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

Is it the glue? Is it the patch? Clearly, neither one of then on their own is going to do it. Its both of them. Just like poly fiber reinforced concrete. as to whether linen is better or cotton? I have no idea. But once u have the technique down, its a super fast and reliable way to support the X brace

I've seen a lot of broken x-brace joints that have cloth reinforcement. The braces crack right at the lap joint. From an engineering viewpoint, the wood reinforcement if far superior.

If you think about it, it only takes a small deflection for the brace to crack at the lap joint and the cloth provides little support to prevent small deflections. But the wood reinforcement would have to come completely delaminated to allow the joint to spread apart causing a crack. That would require a lot of force.

Thanks Barry: that confirms my prejudice on that matter.

Again; any time you glue a piece of cloth down the air can break down the glue, and cloth without glue is not much use. You see this a lot in old bowl-back mandolins, where the cloth or paper reinforcement is just falling off. Wood keeps air away from the glue line. Alternatively, you can shellac the cloth after it's on to keep the air away. Maybe. I've been doing that with side tapes for some time, but, of course, can't speak to the long-term effectiveness.

I am currently repairing a 1974 Martin that had a crack in the X-brace from this same issue. The crack started right at the bottom of the lap joint and progressed horizontally about 3 inches down the brace. I reglued the crack and then I also fit a couple of tiny spruce wedges into the gaps at the X-brace joint that Martin creates due to tapering and rounding the top the braces all the way though the joint before it is fitted. Then I flattened the top of the X-braces to allow a spruce splint to cover the joint. Finally, I covered the joint with a fabric patch to hide the improvements from prying eyes. Ha!

make a lap joint!
Both braces cut to fit tight.

Practice on wood not intended to be your x joint.

Then when you glue the braces on -there is NO reason for a patch.
Mike

_________________
Mike Collins

There is no doubt a wooden cap is superior but traditions man, traditions The wooden cap is a lot easier too! That's the way I go. Actually I've actually cheated in the past and used a wooden cap and put a cloth over it for cosmetics LOL. I'm not one who really cares about what the inside of a guitar looks like but just did it for kicks. The cloth cap has always been a thing of wonderment and mystery for any guitarists out there who peer into the inner working of their instruments.

I'm not at all sure that a tight fitting lap joint is a total insurance against the sort of cracks I've seen. While some contact is better than none, end grain to side grain is not as strong as side to side, and the open side is still under tension, where any glue is weak. A patch, either cloth or wood, is easy to do and solves the problem completely if done well. 'Tradition' is fine, but why would you knowingly do something that's not as good, just because somebody hadn't thought of it a hundred years ago?

Now THAT'S a question designed to spark controversy! Let the arguments commence!

Seriously, though, it is an important question for anyone who wants to build a more traditional instrument. What do we update? What aspects of traditional instruments do we leave well enough alone? The answer varies for me from detail to detail. Some traditional aspects of steel string guitars are best left to the past; others are considered by the players to be essential to the character of the guitar.

Regarding this particular detail (the lap of the X brace and how to strengthen it), I err on the side of improvement, so I do a wooden strip to bridge the "upcut" brace. If someone doesn't like my guitars because they don't have a glue-soaked fabric patch at that spot, that's OK.

So now who is going to do a thru mortise tenon joint on one brace with the other brace scarf joined through the mortise to avoid that clunky looking spruce cap thing. Do we have any takers? Anyone? Anyone?