Saturday, July 18, 2009

Heel Block and Kerfing

The heel block arrived from LMI, so it's back to the body. It comes as a 1 1/4" x 3" x 5" block, and it needs to be 1 7/8" x 2 3/4" x 3 1/4", but a couple of whacks on the table saw takes care of that.

If this weren't a cutaway I'd be ready to glue it on, but nooooo! Everybody wants a cutaway these days!

Just kidding.

But this is one of the areas that a cutaway complicates. Rather than just needing one good gluing surface, I need two good gluing surface, and one has to be curved. First I trace the curve of the cutaway onto the end of the heel block.

This curve must go all the way down the length of the block, which complicates things. The band saw can be used to hog off a lot of the material, but it's a little dicey to cut things that thick when I'll be holding it by the short dimension. It gets my fingers close to the blade on a thick, curved cut, and I like all ten of my fingers.

So I use the curved end of my belt sander. It's really impossible to put even pressure on the block as I hold it against the sander, so it doesn't remove material evenly down the length of the block. Eventually I decide that the best way to do it is with a chisel. There's a lot of chiseling and checking the fit, and there is no way it's going to be perfect, but it doesn't have to be perfect, I just need the there to be enough of a gluing surface for the cutaway side to have good adhesion. It actually takes about two hours, but finally I'm ready.

As always I do a dry run to make sure I know the best way for all the clamps to be positioned, and then I glue it to the body.

That's a shadow on the other end of the block which makes it look like the block isn't tight against the side. Here's a view from the other direction and you can see that there's a tight fit on both surfaces.

After some time with the radius dishes to get the sides, end and heel blocks down to dimension, it's time to glue the kerfing. The sides are less than 1/8" thick, which isn't a sufficient gluing surface to hold the back and top, so kerfing is used to increase the gluing surface. Also called linings, these are narrow strips of wood that are glued on the edges of the sides. Originally the linings were solid strips that we bent to match the shape of the sides, but at some point little slots or kerfs were cut into the linings to give them flexibility, and eventually the strips were called kerfing.

Most kerfing is glued on the opposite side of the kerfs, with the kerfs facing toward the inside of the guitar. However, a few years back someone experimented with gluing the linings with the kerf-side being glued to the side and discovered that this made the sides more rigid. This is probably because the glue gets in between the kerfs and then dries hard, making the whole strip more rigid. Anyway, since I was taught using reversed kerfing, it's what I continue to use.

It's a simple process. Just spread glue on the kerfing, position it on the side so the end sits just above the edge of the side, and clamp. I use little spring clamps. Unfortunately I only have enough to do one half of a side at a time. It takes about 50 clamps to do all the kerfing on the front or back, and I have about 75. Whatever, it's just time. I start with the back. Here's one side:

I let the glue dry for about an hour, then do the other:

Flip it over and do the same for the top. On tight curves like the cutaway the kerfing breaks, but that's not an issue. I don't need the lining to be tight to itself, I need it to be tight to the side and provide a good surface for the top and back. Here's what the kerfing looks like unclamped:

It's Saturday morning and I just have one more strip of kerfing to glue on. The reason the kerfing is left proud of the edge is so that I can sand the back and top radii on them, and that will be next in the process. After that there's a lot of sanding and cleaning up the inside of the sides so that they are smooth, all pencil markings are gone, and there is no glue drips, because once the back and top are on, there's no going back inside the box.

And we are very close to having a box. Maybe even later today.

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