Class 6: Progessing the body

 Last Wednesday's class (12/15) had me working on all parts of the body: sides, back and top.

Sides

The sides that I glued up with the end blocks last week were ready to prepare for gluing on the top and back.

The first step was to sand the back side on a circular sanding dish (seen to the right) with the same 15 foot radius curve used throughout the back.  This will prepare the edges of sides, and
especially the blocks at the neck and heel, to hold the same curve that the back is being set to.







Following the sanding, I started to glue in the kerfing that will run along the inside edges of sides along both the top and back.  The flexible kerfing provides a surface to which the top and back can be glued, extending the glue surface past the 3-4 millimeter thickness of the sides themselves.
This is a fairly simple operation, but one, as seen in these pictures, that make extensive
use of clamps.

End piece

Edward gave a demonstration during the class of fitting an tail piece in the body.  The shape of this piece is angular (long trapezoid) so that the piece of contrasting wood can be fit tight into the cutout made in the body.

This cutout is right over the heel block; the tail piece is decorative but also used ot hide a less than perfect fit of the sides.  The side joint at the neck end will be covered by the neck itself.  The notch is cut using a pull saw, then cleaned out
with a chisel.  I'll be using a piece of Koa to match the bindings, and will probably add black purfling strips for contrast.

Carving back braces

I worked on carving the braces on the back of my guitar.  Carving the braces reduces their mass while trying to preserve their strengths, hence the use of a curve.  The braces are tappered down to 5 mm at the edges where the ends of the brace will be fit into the sides.


Carving is done with a chisel, in successive strokes until the top of the curve is set about 60 mm from the edge of the top.  The trick to get a gentle cuve that preserves mass towards the center of the  back.  My braces tend to be carved a little on the steep side.

Top

I worked further in this class on gluing in the braces on the top.  In the picture, the transverse brace is being glued on.  This brace has a 1/4" hole drilled through the center to accomodate the end of the neck truss rod.  The hole is centered 4 mm above (below) the top, so that the outside edge of the hole is just inside the brace.

I also cut and glued the bridge plate from a scrap pice of walnut.  This is glued directly under the bridge to accommodate the ball ends of the steel strings which pull back up against the top.  The plate protects the soft spruce.  The piece is cut slightly off the direction of the grain so that the ends do not pull against a single fault line in the grain.

Back fitting

Edward provided a final demonstration of how to fit the back to the sides.  I'll be working on this when I return from our break.
The back is centered over the sides and the location of the braces is marked with a pencil on the outside of the sides.  It's important to remember the cuts for the braces run straight  across, not perpendicular to the outside curve of the side.










Next a back saw is used to make very shallow cuts for the edges of the notches.  Better to cut to shallow than too deep.

















The notches are then cut out with a small chisel.  Each joint can be tested by placing a small light inside the guitar and placing the top over the sides to see if any light comes through between the back and the side.










I'm hoping to come in once over our winter break to get everything else finished up on the sides and top so that I can begin fitting my back once class starts up again.

Class 5: Muti-tasking

Half of our class was missing this evening, which made it a little easier to spread out and get a number of tasks done.  Tonight I finished out the soundhole rosette, did the layout for the bracing on my top and back, cut the X-brace for the top, and glued up the back strip and braces on the back.

Finishing the Rosette

Completing the rosette at this point was down to just bringing the strips of paper and veneer added the week before down to the level of the sound board.  The strips were riding about 1/16th to 1/8th of an inch higher than the sound board, so using a sharp chisel, I worked them down to be closer to flush.  From there, sanding with a small block at 80 and then 120 grit brought the trim level.  A final sanding, prior to later final sanding for tap tuning and finishing, was down with some 220 and a block.  The complete rosette, I'm happy to report, no longer looks like an accident.  I'll try not to go down this course next time.

Gluing up the Sides
Here's a couple of shots,  from the bottom and the top, of the sides being glued up with the end blocks.  Spring clamps were added to the heel block to bring the sides up tight.  Cam clamps were used, three for each block, to bring the whole a assembly together.
The entire assembly is now sitting in the mold in the shop until next week.

Laying out the Top
The heart of the guitar, so to speak, is really the sound board.  This is the component that provides the core vibration and response that give the instrument the bulk of its character.  The top must be flexible, but strong enough to stand up to the longitudinal forces created by the pull and compression of the steel strings.  The layout of the bracing underneath the sound board, therefore, is perhaps the single defining design.

The design used by Edward on his steel string guitars is built around a classic 'X-brace' pattern developed by Martin (and possibly others) in the 1930s or 40s.  The 'X' is the two long braces running from the lower to the upper boughts, crossing and joined underneath the sound hole.  This pattern has been transferred from a template onto my guitar.  The transverse brace runs across the opposite side of the sound hole, and numerous finger braces spread out from the primary X.

After laying out this pattern, I did several test cuts of the X-brace joint before finally cutting this joint in two pre-cut
braces.  Top braces do not have a radius along their length, even though the top will be glued up on a support that will help to provide a curve with a 30' radius.  This curve is gentle enough it does not require the curvature to be set into the  braces before gluing.



Setting Back Braces
The back curvature, however, is more defined than the top.  We are using curvature with a radius of 15 feet.  In order to give some additional support to the back, a center strip of think spruce is glued down the middle of the back.  All of the supports for the back are glued on using a support that already has this curvature.  This is one of the many unique pieces available working in a shop that new luthiers must develop.  There is a caul with the same 15' curve that will be clamped over the back strip in order to provide support during the gluing.

First the back strip is put in place with some help
from masking tape. Masking tape is a great help to lutherie, but one of only a few tools that are cheap and readily available.   In addition to the back from and caul, we are using prebuilt templates and molds for the sides along with additional jigs (such as the router circle cutting jig that got me in to so much trouble) to help move the process ahead.

Another essential element of lutherie is the use of clamps.  Shown here and below are the standard wooden cam clamps that are used throughout the process of gluing braces and many other parts of the guitar.  They can easily run $15 to $20 each, and as you can see in the pictures below, we can easily use 12 or more at a time. 

After the back strip has dried, it needs spaces
set into it for the braces.  These braces, as mentioned, already have a curve set
to their bottom and have been given a preliminary shape on top as well.  Four are glued in to the back to give strength.  Setting clamps for gluing is its own art forms could all be set at the same time.






 Below I found space for 17 clamps on the back so that the braces could all be set at the same time.

 

Another subtle skill, as in all woodwork, is getting just the right amount of glue to ensure a firm joint, but not so much that things get messy and look sloppy.  An example of the latter is shown in the close up below.

Next week I will be fitting and gluing on the top braces as well as putting in kerfing strips along the sides to create a glue surface for the back and later the top.  There is a bunch of work ahead in carving are braces and tuning up the tops to complete the assembly of the guitar body.