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I'm fixing the little crack that started in the 13" archtop. HD cedar top. I took the back off. Not a terrible job. Not an easy one. An added upper block split while taking it off. Not too much more carnage. Hot water in a syringe, heat gun, palette knife.

The crack couldn't be seen without string pressure. As I expected it is right next to the tone bar. Here's a list of to dos:

1 remove tone bar on bass side, and glue the crack.

2 make a stiffening patch where the opening is under the fake humbucker. Maybe 1-1.5" NS, and going over the crack. Cross-grain, and a few mm thick tapering to nothing.

3 make a smooth transition over the patch and out a new bar in.

4 add some stiffness to the sides. They are thin, and iffy at the back with the little tentelones?

5 Maybe add a 3/8" hollow brass rod between the blocks to take some stress off the plates. The belly does rise a lot. The back is thin. It surprised me when I took it off! It's only 175 grams.

6 glue it back together

7 glue on another piece of plastic binding. Never again. I like wood.

Anything I missed? Any suggestions?

As Johnny Carson used to say, it was built light. Right at the edge of wetness.

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Why be normal?

Is the crack visible in pictures? I know you said it was not unless under string tension. Is that a left-hand instrument?
How is the repair going?

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Measure Twice,

Karl Borum

The crack is right next to the bass side tone bar; coming from the very corner of the cut out for the fake humbucker. A definite point of stress. That's why I want to have the stiffness patch go past the corner and under the bar. It will go past the corner on the other side without removing the bar.

It looks like you can see that it got wet from the water in the photo.

It is right hand.

I think I'll work on it first in the afternoon, and then stop and do something cooler, like building! Today I'd like to plane the bar away, and glue the crack.

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Why be normal?

Ken it looks like to me that there is a major deficit of much needed bracing and supports and that is likely why it cracked and will crack again elsewhere. The loads are not being addressed and I think that you need to rethink this design and/or add supports, bracing and reinforcements.

I would also go with CF long before I considered brass which is not used in the guitar support world to my knowledge at least not in a production sense.

Archtop design has evolved some over time but what we see these days is largely for one reason, it works. Not trying to be unkind but this did not work and again I encourage people to learn all we can about convention and then and only then is when I would start any departure from convention.

I could not see the crack either and wanted to comment and I suspect that's why you have not had many comments.

Hesh,

This is not an archtop in the sense that most are. Most are probably usually plugged in. This has no plug. I'm trying to build a guitar that is responsive, and has good tone and volume with no amplifier. It is a third shorter and 25% narrower than my other archtop. It felt really good.

Then I cut the soundhole/fake humbucker out!

I should have put a soundhole patch around it. Didn't think about it.

So here I am.

The patch should help. I think I'll make the new tone bar go up higher. It does need stiffening around the sides. Something that bends easier, and thicker would be good. Maybe laminated sides. Yes, that would work. Then I'd need to make some sort of form.

With a sound hole patch, the weakest part is the ribs. I didn't know that Padauk was not a great choice for sides. If you have them fairly thick, and have supports they should be fine. I'll put some bracing on them.

I didn't mean to make a guitar that is too weak. I didn't want to make it too strong either. How do you find out?

Taking the tone bar out makes it fairly floppy. It isn't that bad with the bar in, so it works.

It's like a prototype race car.

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Why be normal?

Well Ken you are a braver and more adventurous man than I am I would be terrified to put this much effort into something with no basis on something substantially similar suggesting success is possible. Many here want the utility of a playable instrument and or may have some hopes of going to market with their creations at some point. Some want to give a gift that is appreciated and not a problem in time.

When I worked at GE for us Job One was not making money it was shielding the corporation, the largest in human history when I was there with 375,000 employees from unwanted liability. Martin knows how to make a guitar that sounds better than the production models of today and yesterday. But they also know some fool will keep it in the trunk of a hot car in the summer in Nashville and then make a warranty claim when the HHG under the bridge decides to get liquid again.

I'm all about prototypes and doing what is hopefully high performance stuff like that race car you mentioned and I did build 15 prototypes before going to market. My business partner built close to 200 prototypes for one manufacture before they went to market so I get the prototype thing.

But for me I limited my innovation intentionally to only 2 - 3 things that I changed with each build in an attempt to isolate my changes so that I was not fooling myself with what I believed the results were providing. No offense intended I am describing what I did and what worked very well for me.

I also created a build sheet with the serial number of the new build and the goals and changes bulleted to keep me on track. You might call me a documenting mofo.

I would describe my 15 prototypes that more than half of them are here to this day and playable instruments as a minimum for what I needed to do to get my in some cases 2.9 lb guitars to market. I went light too but I crept up on it only changing a very few things with each build so I could see more clearly what impact the changes had.

So I would go about it and did go about it differently. I also understand that this is what you may enjoy more than I do and did, risk.... so I get it and this is not criticism but it is as all my posts are intended to maybe, maybe be sage advice for the lurkers who are considering hanging out a shingle someday.

Someone else used a method similar to my approach to guitar building, NASA. They knew human lives were on the line so they took small, measured steps always based on tech and science that had panned out in the past. Even still all risk can't be eliminated.

Anyway to me as you asked, how do you find out, well I found out and still find out with small, measured steps based on previous known successes.

I may not be having as much fun as you are though

Thanks Hesh. I'm just on a different wavelength, I don't do normal. You can buy normal in a store. The sound coming out of this thing is different. It sounds like an archtop. It sounds sort of electric, because it keeps ringing. You'd probably have to mute strings a lot to play intricate pieces on more than one string at a time. Or just let it ring!

The top rises a lot in this space. It is curved somewhat concave on the outside, that is hard to show. I decided to use .02" veneer in layers; like laying on fiberglass cloth. It is way stiffer now, Even without the tone bar, the tap tones on the top in this area, are where they were before I planed the tone bar off. I checked them, and wrote them down beforehand, just as a reference.

The scale length is about the same on this one as my 16" archtop, 635mm or something like that. But the little one looks tiny. Thinking about weight; surprisingly, the little one, with a massive upper block, and the heavier tuners is 3 pounds 4 ounces! The big one is 3 pounds 10 ounces. I think it is the neck and the block that carry all the weight. It's basically a massive neck, with a soundboard attached.

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Why be normal?

Hey Ken,

I wouldn't pay much attention to the ney-sayers about what you are doing. Does the name "Padma" ring a bell. If not do a search here, he could be your long lose uncle

So looking at the structure you have (assuming you are using steel strings), it looks as though you do not have enough structure after cutting that rectangle pickup/hole.

If you look at the pic I marked up you can see the stress points in red. When you cut the rectangle out just in front of your neck, you are allowing the neck to rotate.

That is what caused the big crack to start right where the block (on the left in the pic) stops. The crack that is from the corner of the rectangle is only natural since you are twisting that section of the wood. FYI, Square holes cause stress points at the corners.

What may help is if you could put in an UTB bar across the bottom of your neck block and tie it into your sides.


I have a Epiphone Casino which is a double cutaway hollow body electric. That has 2 bars that run parallel to the guitar similar to your tone-bars. But I am pretty sure they are quite tall. Not sure if they are called tone bars.

Possibly see if you can find out more about how they are using those bars. But I am confident they are there for structure.

Anyway,
Sorry you developed that crack and now have to figure something out, but that is price you have to pay to venture into the unknown...


Cheers
Bob

Ken,
A while back I bought a 1950 Gibson ES125 that needed repair. I remembered that I took pictures of the inside of it. Just happen that my phone just fit in the pickup opening.

And it just so happen I saved them.

Now this is a bigger guitar and can't be compared to yours (Like my Casino I mentioned above), But they are similar in some respects.

I only had pics facing back but you can see they go just up to the lining. But the span the entire length of the body.

And you can see they are fairly beefy.

Hope these help,
Cheers,
Bob

Thanks for pointing out the stress points Bob. I think the corners, where the crack started are the worst. Padma? Oh, I see. Thanks Bob. I don't see any naysaying, they are just pointing out that it is flimsy. Yes. It turned out flimsier than I intended!

Yesterday I fit a bar, and glued it in. Chalk fit without moving it back and forth. Tap it, and maybe move it a bit, .5mm, 1/64" I glued it in with some 192 high clarity. I can't get my usual 3 or 4 bass bar clamps on it. But I used to just do rub joints there. That's what I used here. Put glue on both parts, rub them till they stick, and let it be. An hour later I saw one end was up a little. A little more glue, hold down with a piece of wood, and shoot it with the heat gun. It is fine this morning.

So I cut the top profile on it from the start. I use a formula with exponents, to get the shape. It counts the thickness of the top in. I think the top is about 2.5-3mm. I used 3mm. The bar is 15mm high. Then to find what it is at 3/4, 1/2, 1/4 and the end, divide 18 by 1.1, then that by 1.1 squared; that by 1.1 cubed. That by 1.1 to the 4th for the end. For some reason I used a number to make the ends go to nothing on the first reiteration. It would work if it had f holes maybe.

Gluing it on, I could see that I need to do the same on the other side. It should solve the problem.

I was one step ahead of you Bob.

I think I will add an extension on the one I just glued on to tie it to the block holding the "humbucker frame" to the block. I don't think it is real loose there, but it won't hurt. I will plane off the other, and make another the same length as the I just added. I will place the lower part where it is now, and have the upper go right along the side of the cutout. I have lots of ideas on how to make this really work now.

First I have to find some stock to make the bar with. That was the last long one I had cut out. I have some red spruce bracing, but this one is more like a test puppy, so I'll save that for something real.

I will have to glue on the few tentalones? that broke off, and glue on some side supports between the tentalones to stiffen up the side some. My idea for the next one would be to use ribs of easy to bend wood, and cut notches into the blocks to they would blend in flush when glued on. Make a solid block for the tip of the cutaway arm, and let the sides into that the same way. Flatten everything, and make it smooth. Then cover with .02 veneer.

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Why be normal?

Hey Ken,
I am by no means a structural engineer, but I have a pretty good understanding of where weak points can form. The Gib CF100 of mine came to me with the neck pulled up and off to the right. These guitars were built with a small UTB that didn't go all the way across the upper bout. This allowed the neck to pull up and deform the top. It is a similar situation that you have. Not enough support for the neck not to pull up and twist but instead of just warping a thin top yours had no choice but to crack. Gibson CF100 was the only acoustic they made with a cut-away. Probably for a good reason was the structural issues it had.

As far as the "nay-sayers" I didn't read the comments from him since I have blocked his posts. I was just going off of your comment



I like your outlook on things... I have a tendency to over-think things and usually play on the safe side. But I admire your adventurousness and ability to think outside of the box.

I would like to hear what this sounds like, I am guessing with minimal bracing it probably sounds larger than it is.


Cheers,
Bob

I did the other bar yesterday afternoon. Was kinda sloppy with the glue. The bar wanted to go further to the outside, so that's where it went. When you do a rub type joint, or even if you hold it for a few minutes, you don't want to go in and start wiping up extra glue. Dry glue is so hard to get off.

I forgot to cut the height down, so I'll have to do that first. Then I need to angle the sides. Just the sheer massiveness of the straight side makes me cringe. I was thinking about adding a piece on the top of the first bar connecting it to the block through the piece gluing on the fake humbucker. I'll see how it seems after I tone those bars down some.

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Why be normal?

Yesterday afternoon, I trimmed the bars down, cleaned it up some, and added a dozen little stick to stiffen the sides.

I like the way the bars look now. I can't flex the top, but from the top to the bottom both sides rise in pitch from the top to the bottom. That seems like it would be a good thing. There are tap tones all over on it.

The sticks really worked! Before adding them it was very flimsy, because of the tentalones. Usually you have them on a belly, and that makes more sense; freeing up the belly right to the edge. Now the body, with the back off feels like it has a regular lining on it. It has some flex, but you can feel the stiffness.

I think I'll just glue the back on, and order the black binding. I'm never using that stuff again. That's going to be the worst part. I like wood.

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Why be normal?

Yesterday I read a book in the afternoon. I put the clamps on later. Ready to glue up today.

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Why be normal?

I glued it up yesterday. It was fairly easy. Take off a few clamps. Use a palate knife to put glue on the joint, and put the clamps back on. But. It has the little tentalones. They weren't the problem. The problem is just getting a space for the knife! For some reason the back must be really flat, and the top and bottom of the sides push in some. It is VERY hard to get the knife in! When you get it, you have to poke a fingernail in to try to keep it open. It just doesn't want to lift up at all! Ahh, for the ease of violins and cellos, with an overhang.
Ken Parker does it.
Anyway.
An hour or so later I took off the clamps, and tapped around the edge. It all sounds snug. I'm sure that I have a lot of glue drips to clean up today.

Yeah, you can see that it was cracked. It isn't cracking there anymore. The next trick will be getting that dumb thin black plastic binding on. Did I mention that I'm not using that again?

Thanks for making me see that it needed more bracing. I don't know what I was thinking. Moving the braces up close to the top will really help. I was shocked at how much the little toothpick like strips on the sides stiffened it up.

It will be fun to hear it again. Will it gain some higher overtones? Will it still vibrate like crazy?

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Why be normal?