Archive for April, 2008

Back Prep

The first photo below shows the attachment of the back strip. The back strip is a very thin piece of wood (maybe 1/16″) and maybe 5/8″ wide with the grain running laterally (rather than longitudinally which would be much easier to rip) which is used to reinforce the glue joint on the back, historically one of the weakest joints on the guitar. The second photo below shows my logo branded into the back strip. I use a brand I bought from Rockler which is low-tech by today’s standards (I can’t justify a laser, at least not yet) but works really well. It makes me nervous every time I do it, however, like bending sides.

gluing on the back strip branding

The first photo below shows your’s truly cutting out braces from rough stock.  I use either Sitka spruce or mahogany for the back braces and usually use Sitka for dreadnoughts to keep with tradition.  I use 1/4″ bracing exclusively now rather than a more common (at least among factories) 3/8″ wide.  Martin has taken to offering 1/4″ bracing on some of their guitars, I think several of their “vintage” models.  1/4″ is a little harder to work with but it’s used for a good reason.  By narrowing the brace 33% and increasing its height  by a few percent, as any engineer can tell you one ends up with the same rigidity with significantly less mass.  Mass is the enemy of sound (on the soundboard and back, that is, not necessarily elsewhere–sustain is often improved by mass in other areas).

cutting back braces shaping for the back radius Go-Bar Deck

The second photo above is me shaping the backs of the braces to the back’s 12 foot radius.  I use a template to trace the radius on the brace then use the belt sander to remove the stock to the line with a final sanding on the sanding dish.  This gives the back tremendous strength for its weight.

I use a razor and a chisel to cut slots in the back strip to accomodate the braces.  I just think it’s easier than cutting relief into the brace itself and, as any of you following this blog so far might have sensed I try to take the easy way out whenever I can.  I’ve used the bandsaw to pre-cut the “ramps” on the braces but will use a chisel to do their final shaping.  I try to use power tools to remove as much stock as possible during production.  While purists will scoff at this I do it for several reasons:

  • It reduces tedium.  I have a short attention span and removing stock is boring.
  • It improves quality.  If I’m bored I’m more likely to make a mistake.
  • It improves consistency.  Using machines and jigs makes following specifications much easier.
  • It improves safety.  Boredom equals complacency which increases the likelihood of me doing something stupid and getting injured.
  • It improves health.  Much of stock removal, especially when shaping necks, can be physically hard on the builder.  Carpal tunnel syndrome and repetitive stress injuries are greatly reduced by the judicious use of power tools. 

Years ago as a carpenter I developed the philosophy of using the right tool for the right job and I try to do the same in my shop. 

I want to make the point here that I don’t believe that my approach to building is the only one or even the best, while I think it is the best for me.  We are in what many have called the “Golden Era of Lutherie” and there are many builders out there building guitars with different approaches and different philosophies of building and each has validity.  While the dimensions and basic design of a guitar are pretty similar from one builder to the next, the final products are amazingly different in their smell, sound, playability and overall feel.  So much of the personality of the builder insinuates itself into the product.  There’s more than one way to skin a cat.

 I use a Go-Bar deck to glue the braces on. The Go-Bar deck is a really handy tool for the small shop.  It’s fast, easy to set up and allows for the distribution of pressure over a wide area. 

All that’s left for the back now is to shape the braces using a tiny block plane, finish the scallops with a really sharp chisel and sand.

Kerfing

The sides are now ready to have the kerfing glued on in prep for receiving the back and top.  You can see in the first photo that the inside of the sides is a little darker than the outside; that’s the result of the treatment with CA.
Sides Ryan A4 Kerfing 3D

The second photo shows Kevin Ryan’s A4 kerfing.  Kevin uses a laser cutting machine to make very thin-kerfed cuts of the wood but cuts both sides which allows the kerfing to move in three dimensions like one of those old watch bands from the 60s and 70s from Speidel.  Very, very clever.  It solves a lot of problems.  You no longer have to break the kerfing in several places to allow it to curve along the radius of the back, a problem especially noticeable around the cutaway.  The installation is faster, too, and ends up with very little or no squeeze out (fourth photo.)

easy clean up Chalking

After both the top and back are applied I whip out my daughter’s chalk again and mark up the edges, then sand the kerfing flush with the rest of the sides using the radius sanding dishes again. This step takes only a couple of minutes.

Next steps: putting the neck together and inlaying the rosette in the top.

Filling Bug Holes

Like much old Brazilian this set has some flaws.  It has some small cracks on the backs near the bottom, a couple of weak spots on the sides and half-a-dozen worm holes on each side, two of which are fairly large.  The cracks on the back I’ll probably fix after the backs have been attached to the sides.
holes in sides
I’m not going to go out of my way to hide the holes, just fix them. I read somewhere recently that the existence of wormholes is a sign that the wood is genuine Brazilian; it also fits in with my philosophy that the flaws give the wood character and add to the overall beauty.

I’ve used a scraper to collect a little bit of sawdust from the sides (that’s what’s in the little plastic cup). You don’t need much. I’ve also cleaned some of the stains near the bug holes with naptha. Naptha is pretty effective at removing mineral deposits from rosewood as well (that’s the white stuff that won’t sand off, often found in trees that were grown for shade in plantations–not sure why).

back up the hole with tape Fill with sawdust glue

First I use a tiny piece of tape to back up the hole on the inside, opposite the side I’ll repair.  This keeps glue from running all over the place.  I drop a little mound of sawdust on top of the hole and then rub the sawdust back and forth pushing it into the hole, then put a tiny drop of super glue (cyanoacrylate or CA is the glue of choice here, thin viscosity). After it dries I add another drop of glue, sometimes using medium viscosity for this. I don’t use accelerator for this operation since the reaction can trap gas bubbles which turn white. Then, using a tiny sanding block (I use a tiny piece of pine or mahogany with a piece of 150 or 220 grade paper glued to it), sand flush. On the inside, I peel off the tape and then use a scraper to make the repair flush.

For the two larger holes I’ve taken a piece of scrap from the end cuts and cut tiny plugs.  The principle is the same:  push the plug in following the grain orientation, tape the back, fill hole with CA, dry, fill again, dry, sand flush.  If you see the guitar in Maryville you’ll be able to see these holes as I’ve made little effort to really “hide” them, just fix them.  But they look “right” if you will.

The last step today is probably going to seem a bit unorthodox.  I got the idea from C F Martin & Co.  I read somewhere that they, like everyone else, can always use more Brazilian and will, depending on the grade, immerse the wood in a bath of cyanoacrylate before they use it.  (That’s super glue.)  Using CA as a way to fill and harden the fibers of wood is an old repair guy’s trick and works extremely well.  I didn’t take any photos but the process is simple.  Using a respirator and working in my spray booth, I use a disposable brush to apply glue on the wood.  After drying I use a scraper and sandpaper to remove any excess.  During the process you can see that some areas absorb more glue than others: these are the weak spots.  Besides improving the strength of the wood it will also improve the tone, making the wood more rigid.  It also darkens the wood considerably.  If you plan on using an absorptive stain you should apply it before you begin this process since the CA effectively seals the wood.

 Next:  Kerfing, with Kevin Ryan’s really cool new material.




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