Class 7: Back from Break

Class met again last week for the first time in four weeks following a two week winter break and a snow day.

Over the Break

I went in once over the break to work on catching up; I glued most of the bracing on my top and installed an end wedge from a piece of Koa that should match the bindings I'll be using.  There are two purfling strips surrounding the wedge; the wedge shape is used to try and get a tight fit with the sides.  By cutting in this shape, the wedge can be pushed into the sides until it fits.  I ended
up with a slight difference in the angle of my wedge and the angle of the slot in the sides, so the fit wasn't as tight as it could have been, but  cosmetically it should be fine.  This week I cut the excess wood and purflings flush to the sides with a back saw.

Fitting the Back

Another step tonight was notching out the sides and kerfing so that the braces on the back would fit snuggly within the sides.  This increases the instruments strength (in case it's dropped).  The notches are made by laying the back onto the sides while they are in the mold and marking the sides of the braces.  The notches are then cut out by making a shallow incision in the side for each notch, then using a small chisel to cut out the notch.
The trick, which is hard for me, is to not overcut the notch with the saw.  It's better to start with a shallow notch and to work it out and deeper with a small file.  The fit with the back can  checked by putting a small light inside the mold and then placing the back on top.  The seam between the back and sides can then be checked for light, indicating a brace that may need to be made deeper.

Glue Up

The sides are back were glued up by running a generous seam of yellow glue all the way around the kerfing.  Leaving the sides in the mold, the back is placed onto the sides and secured with a generous serving of clamps (that's about 2 dozen wooden cam clamps at about $18/clamp in this picture).  Clamping pressure is exerted down the sides with the bottom of the clamp pressing on the side, not on the mold.




Here's the results.  Some clean up of the squeezed out glue is still needed.  Also, the back still extends out past the sides - but
it's a good milestone.

Progressing the Top

When I came in over the break I glued on most of the top bracing.  At the start of class I still had two long braces to glue on, extending out from the X-brace.  Once these dried I began the process of planning and curving the braces.  These are the first steps in tuning the top - probably the single most determinate process in giving life to the guitar.  The goal is to remove wood from the braces in order to lighten the top and allow for greater sustain of the sound.  The top is the vibrating membrane that gives the guitar depth and volume of sound.


At the same time, there needs the braces need to remain strong enough that the top can resist the longitudinal forces applied at the bridge by the strings.  This is a compressive force that tries to bring the neck and body together.  Removing wood from the braces, therefore, must be done with an eye towards retaining strenght.  The initial step is to plane down the X-brack and curve the ends of the other braces.  From there we'll go towards scalloping the braces - adding horizontal curves to complement the vertical curves.  We'll also finish the tuning next week by sanding the top.

Unfortunately, I've set my top up to need some extra sanding.  While working on curving and planning the braces, I left wood fragments between the top and my work surface, thereby adding a bunch of small imprints and one good divit.  The little ones will hopefully sand out, but the larger divit is probably there to stay.  All in all, lutherie remains a challenge.  I'm not yet at the point where I've anywhere near full command of the steps
that I'm executing - probably be a number of years (and a few more instruments) to get towards any basic command of the tools and skills.

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.