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.