Monday, July 20, 2009

Gluing the Back

There was a lot to do to get ready to glue the back and top on. I placed the back on the sides and marked where the braces crossed the kerfing. I have to notch the kerfing at these intersections so that the back will fit tightly. I use my Dremel with a flat base and a 1/8" bit, and measured the depth so that the brace just barely clears.

Then I rout out notches for all the braces.

Now the back should fit flat on the sides, but it doesn't. There's a problem. The bracing pattern that is being used is for a regular OM without a cutaway, but the cutaway on this guitar dips down into where the top brace runs. The brace is rubbing against the kerfing. I don't want to notch the kerfing for the brace, because I need the gluing surface more than I need the support of the brace. In fact, I don't need the brace at all on the cutaway side because the side itself acts as a brace. This area is well supported. So I have to chisel out not quite half of the top brace, measuring constantly until it fits. Then I have to sand the brace again and the surface where the brace was removed.

And then I realize that if this was true for the back, it's probably going to be the case with the top, and it is. I have to carve away half of the transverse brace of the top, before it will fit as well.

That being done, I now have to sand the inside of the sides. I start with 100 grit paper and progress through the grits until 400. This includes the heel and end block. This is a lot of work for parts that will rarely if ever be seen and probably never touched, but it's worth it. It really makes the wood shine.

While I'm sanding I go ahead and clean up the inside of the top. There are a lot of pencil marks from when I laid out the bracing pattern, some of the pencil marks are now under glue squeeze-out, and there are weird angles where it's hard to get sandpaper in with any downward force. I use different sizes of sanding blocks, one as small as 1/2' square to really get into tight areas. I use a razor blade to scrape away the glue in tight places. It's a lot of work, and even with the air conditioner I'm sweating a lot. Eventually it gets clean and looks good.

But that was just with 150 grit paper. I have to do all surfaces again with 220, then 320, and finally 400 grit. I'm dirty, tired and hungry, and about 6 p.m. I call it a day. Pam and I have dinner on the deck then take the beagle for a walk.

Sunday was a long day and I was tired, so I just punted guitar building for the evening.

Monday night I get back to it. I cut some thin strips of spruce to glue to the sides with the grain running perpendicular to that of the sides. Some builders use these struts, and some don't. I didn't on the last guitar except on the tight curve of the cutaway, just to give this area a little more strength to guard against splitting.

I decided to take the time to do this at a couple of locations. If the guitar is ever banged, this will give a little protection against a split developing along the grain of the side.

I glue a real narrow strip in the middle of the horn of the cutaway, then two wider strips adjacent to it, following the curve, and now the horn is really reinforced.

I glue one on the opposite side of the upper bout, and one each opposite each other on the lower bout. Once again, these aren't necessary, and nothing is going to protect a guitar if it is banged real hard or dropped on something sharp, but it doesn't take much time and just might help.

Well, there's nothing left to do now except glue the back on. As alway I do a dry run using the go-bar deck, and it's a good thing, too. I find that in a couple of areas the kerfing wasn't sanded down flush with the sides. I hit these areas with a sanding block, and now everything fits fine.

The dry run also lets me know that I don't have enough fiberglass go-bars. I have 15 go-bars, and that's not enough. Not a problem though: I anticipated this, so on the way home I stop by Home Depot and buy a couple of packs of bamboo garden stakes. They're about 8 feet tall, and their rigidity varies from stake to stake, and along the length as well. But I cut a few, and I'm ready. If it seems like I'm flying by the seat of my pants here, these bamboo rods are what we used at Vermont Instruments and what I used for the last guitar, so it's cool. Unlike the bamboo, the fiberglass rods have consistent flexibility, they don't break, and they have rubber tips so I don't have to use wooden cauls to protect the guitar. But they're a little more than $2 each. I'll buy some more when I have a little fun money, but they aren't necessary.

OK, so I've done a dry run, got everything ready, so it's time. This is an exciting part. I spread glue on the kerfing as well as the heel and end blocks, line up the centerline, and start placing the go-bars.

This gets to dry overnight--or almost 24 hours since I won't get to it until after work. Then it's time to glue the top on, and we'll have a box!

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