Acoustic Guitar Construction

I think Ken's description (John?) is still available. Way I heard it, Martin sanded the upper rim dead flat and then tilted the body to the angle desired, and ran the sanding disk over the upper bout, taking wood off the rim from the upper edge of the soundhole to the neck end of the body. Sanding disk didn't change angle, the body did. Ken's Mega Mold lends itself to this kind of shaping, too, and I've used mine for that, too. Ken was big on trig and measuring, I think not much TLAR engineering came out of his shop.

Number 9 Undone: ADVICE REQUESTED 12/25/23

Unfortunately #9, along with #6, was at a local guitar store without humidification for sale last winter, and the back cracked badly. Yeah, you'd think an Art School boy would know enough to inquire about humidification but he assumed that all stringed instrument shops shared that necessity with art galleries.

This was the first of two long scale Norman Blake 12 fret variations I've built, along with Plain Jane, #11. They both sound loud and well voiced, except #9's not talking at the moment.

I'm hoping to get some input from those of you with repair expertise on how to proceed. If possible I'd like to reuse the original red oak 3-pc back. I also would agree with most any woodworker that red oak isn't a preferred wood for acoustic guitar bodies, but it started out as practice side bending for my friend Otto.

I made a pass at Harbor Freight on the last grocery run and for $20 bought a heat gun, 700 and 1,000 degree heat ranges. The lower one works just fine for disassembly.

I'd also had to previously remove a bridge from #6 way back when, and had modified a thin, flexible putty knife (in photo) to be a little thinner yet at the business end and took that bridge off with a brooder light but the heat gun's waaaay better.

Bindings came off easily. Working around the back on the linings with the putty knife also worked fine. Where the braces for the back were let into the linings I had a small straight screwdriver I could tap in under while applying the heat gun and the brace would pop loose from the lining.

The split was such that I couldn't compress the joint together with the cross-braces retaining grip on some parts, so I separated the treble side from the braces.

My plan is to reglue any braces loose on the bass part of the back first. I've already glued the split #4 brace to the bass side.

Then to glue the split back and reattach the braces on the treble portion. How to do that all at once with any assurance of bonding the original break well? I've got no idea whatsoever at this point.

I'm hoping some of you can find the flaws in my projected plan, and/or give me some good ideas for clamping the original back split to produce aesthetically useful results.

All input welcome...

Holimoli Will,
I would consider this a basket case. If it was mine, I would make another back.
It you take it all apart and true the crack, than it will not fit the rims anymore.
If glueing it blunt together, you'll end up with a not so appealing back.

But you're the wabi-sabi man here. Go find an idea to be different!
Herman

With heating on a cold day Michigan winters are hard. I would take the braces completely off then glue the back parts together. If that looked good I would re-brace it. You may want to add a purfling strip inside the bindings so that you can reroute the channel and deal with any issues with the channel. Also you may find a need to put the body in a mold or find away to get the rims aligned like it was.

12/28/23

Well, those are pretty bleak assessments from two guys whose expertise I highly respect. Not what I wanted to hear but kind of what I expected.

There are some more of you out there yet, feel free to chime in too. I'll tinker with it a while yet.

Nor do I really understand how repairing broken ceramic tea bowls with gold lacquer that highlights the breaks, what the Japanese call kintsugi, doesn't displace the components, either.

Herman, I think the real wabi-sabi solution would be that if the repair still showed a crack to just put masking tape on it. Or, if you want kintsugi, I could enhance the tape with the green stripe, hah! Photo.

If this was my headache, I'd do as John suggested, in effect backing up until the repair amounted to making a new back reusing the original (glued and cleated) back with fresh bracing joints. I'm assuming the neck is off and the body can go back into the mold and there's enough breakout spreaders inside to hold shape till the back gets glued on.

New back? I'd not do that, for me it would be too great a PITA to try and match wood. Already did that (cleats, please).

Please continue the story!

12/30/23
Attempted fix for #9
Broken edges reglued. Along with the break itself, I reglued the braces from the large piece only as far as the backing strips in the narrow piece for where the maple strip divides the back (3-pc back). The braces are of course radiused and I wanted to just pressure the break, not stress the rest of the assembly. Then, glued the four of these unsecured brace ends.

Did this face down on a strip of wax paper. If you zoom in you can find a thin, straight-line portion of the excess glue where it squeezed out. Also, in the right light, probably not visible in the photos, there are a couple thin ponds of glue along the repair also.

A fingernail can also detect a minute misalignment. I’m hoping a single-edge razor blade converted to a scraper and maybe a little judicious sanding will bring it level with a minimum of stain and finish removal, that can be recoated.

All this begs the question you, collectively, raised however, about being able to reglue it to the linings at the proper placement. I did save and label the bindings for side removed from but they were thicker than my usual .100” at closer to .130” due to improper setup on my part when routing the rabbets. They’ve also straightened out seriously and are not reusable. John’s idea of a purfling addition would probably let me make up misalignment and easier-bent bindings.

And, I need to remove and reglue the saddle. It looks like it was starting to pull loose, but wasn’t evidently properly glued to begin with.
Salvage, if successful is still less effort than starting over.

01/04/24
Glued the back on last night. I tried putting the sides/soundboard assembly in the mold it was built in as well as the other allegedly identical OM mold, but, as John foretold, it would fit in neither. So I just eyeballed the alignment of the back to the sides to try to make the non-alignment as even as I could and clamped it down. I will need to re-rout the binding rabbets. My original binding was .030” too wide to begin with. I'll also need to, as was suggested, add purfling.

Then I nicked the back at the mortise for the neck tenon. I had a thin piece of pine glued in above where the rabbet depth is but there wasn't any below it, so spent another afternoon making a maple patch.

01/07/24
Binding rabbet is now close to .200” wide, not desirable, but necessary due to misalignment. This itself, may eat my lunch.

Glued up potential binding from 3/32 cherry and a scrap of teak big enough. I tried to bend some teak on an early guitar and found it not conducive to bending, but I had the material so will try it. Make two then bend the waist on my hot pipe and the bouts on my crude version of a Fox bender. Stay tuned....