Friday, February 29, 2008

BRACING THE BACK

CUTTING THE BACK PLATE
The back plate was first marked using the template, first one half then the other. In order to draw a saw line, a second marking was done with the pencil stuck inside a 1/4" wide brass washer, rolled around up against the template.

The back was sawed out on my bandsaw using a 1/4" 6TPI raker blade. The marking was done on the 'true bookmatched' side that eventually will be on the outside, in order to eliminated potential tearout from the saw on that face.

GLUING THE BACK CENTER REINFORCEMENT GRAFT
In Cumpiano's book he does this by lining up the back graft against a metal ruler and so I went and did the same and it worked very well. Though I think I was a little frivolous when I added the glue as the ruler ended up being heavily soiled in all of the squeeze out. But, after a couple of minutes, once the glue had set, I was able to remove the ruler and clean it up without the back graft moving and all was fine and dandy. I waited about half an hour to clean up the glue on the back itself.

For the gluing I made a 20" long caul out of some scrap walnut that I stuck to the back graft with some double sided sticky tape before gluing. The caul was about 11/16" wide, just a hair narrower than the graft and about 1/2" thick.

The graft was glued in the go-bar deck, using a 15' radius disc and a number of go-bar rods - 13 I believe. But, also of importance, the go-bars were stuck in there slight off center along the graft gently pushing the graft towards the metal ruler and thereby preventing it from slipping out of place.

CUTTING THE GRAFT PROFILE INTO A GENTLE CURVE
I have seen many different tools being used for this depending on the preferred profile and look - sanding blocks, chisels, gouges and planes.

I started off by using a block plane to cut the initial facets, but I kept having difficulties controlling it as the plane wanted to rub up against the back plate itself as the facet started to form. Soon I switched to my little bullnose plane instead. As it is a rabbet plane, the blade sits flush to the sides and I felt it gave a lot better control for faceting the edges, where the block plane kept rubbing up against the back plate. One caveat with the bullnose plane though, it had to be kept off the back plate all together in order not to mare the back plate.

Like when I did the heel carving, I started of planing a primary facet of both sides of the graft and finished the curve off with to secondary facets on either side of the primary one. This produced a gentle curved over the graft consisting of seven facets in total.

I think using facets as a great way to monitor the progress and evenness of the contour or curve you are trying to get. Using a light positioned at a low level produces a clear visual reference of the consistency and accuracy of the facets.

The graft was finished off by sanding with 320 grit paper, smoothening out the edges of the facets until one single smooth curve was present.

CUTTING NOTCHES IN THE GRAFT FOR BACK BRACES
The Torres plan I scaled down for this guitar calls for a certain positioning of the back braces. However, when I laid them out on the back I noticed that the spacing was smaller between the middle and lower brace than the spacing between the middle and the upper brace. I couldn't help but think that it looked a little odd and out of balance.

Being the vain person I am, I took the liberty to move the middle brace closer to the upper bout so these two braces were at an equal distance from the center of the soundhole. I don't know if this is going to be detrimental to anything, but, from a little bit of research I found that the spacing between the back braces to be seemingly rather random; Some keep the spacing even while others moves the middle brace closer to the upper bout, yet others again keep it closer to the lower bout. In Cumpiano's book he suggest using four braces instead of three.

I started by marking the position of the braces on the graft with a pencil. Then each brace was held in position on the marks and squared off with a engineers square held against the graft. Once the braces were in place, a mark was scored with a scalpel on either side of that brace.

With a small razor saw, cuts were done on the inside of the marks for each brace and the waste was pared down with a 1/8" paring chisel. As when the Transverse braces were trimmed to length on the soundboard, I had to be careful not the mare the back plate with the saw. As the cut progressed I gradually moved the sawing to the tip of the saw, sawing with small strokes only taking care not to keep the heel of the saw off the back, stopping just before the cut was done. The rest was taken down with a 3/4" chisel while clearing out the waste.

Finally a last fitting was done where small adjustments were performed to widen the notches here and there for a snug fit.

GLUING THE BACK BRACES

With the notches done, one last mark was made before I was ready for gluing. With a plastic drafting square a mark was made with white pencil at the template line. This was done to have a reference mark for keeping the braces square to the graft when they got glued on.

The braces were glued to the back in the go-bar deck using a 15' radius hollow form. I used 5 rods for the upper and middle braces, whereas the lower brace was glued on using 7 rods.

A note: The braces had previously been carved when I made them a while back. In retrospect I don't know if I would do that again. I think it made them more difficult to clamp in the go-bar deck. The tips of the rods had to be positioned more carefully in order not to slip off and using a caul was not really possible either. I can see why some people prefer carving them before gluing, as it may avoid problems like damaging the plate should the chisel accidentally slip and that sort of things. But, I think I prefer using cauls and I don't really mind carving the braces once they are on. Maybe another clamping method would be more suitable for the precarving approach.

SCALLOPING THE BRACE ENDS
I made a little template out of Lexan for this and used it both the mark the braces and as a reference while carving and it work quite well. I had drawn some reference lines parallel to the base that I used keep the template held aligned to the top of the brace and thus not following the radiused bottom.

The scalloped was carved with a 1/2"chisel with the belly down. On the top and middle brace they started 2" inside of the template and 2½" on the bottom brace. All of the scallops tapered down to a 1/4" at the ends.

I was quite surprised to see how much the grain in the mahogany was interlocked. Often one half of the brace, along the length, would cut fine with the grain while the other half would tear severely cutting against the it. It was quite difficult to control, but with a freshly sharpened chisel and thin slivers I was able to keep the tearout to a minimum.

Each scalloped was finished off with 320 grit sandpaper rolled onto a piece of 1" dowel.

THE LABEL
Since this guitar is a 3/4 classical size it lends itself to be played by a child, and I though what more appropriate than to have one of my sons do a drawing for it. Albert, my 4 year old, came up with a drawing of a person, me I think, playing a guitar that I thought was quite delightful (as parents do).

Kay put it all together in Adobe Illustrator and voila! the label was done. I printed it on some Strathmore Series 400 Drawing Paper, an acid free paper that is slightly cream colored and had a little bit of tooth to it.

Our Inkjet printer did the printing and that left something to be desired. The problem was that the inks bled hugely as they are water based and the glue itself contains water and when the paper absorbs it, the moisture makes it expand and buckle up. One way to remedy this is to add water to both sides and, Haha, the colors bleed.

A fellow builder had suggested using fixative to lock the inks before gluing/wetting the label. I did some test with this and found that it mostly solved the problem, but I had to literally soak the label in fixative to prevent the bleeding, and even that didn't control it 100%. Some bleeding still occured but I found that dabbing the label with tissue paper restrained it.

The label was glued in using normal yellow wood glue thinned with water. The glue was applied to the paper with my finger and once the whole surface was covered, I gently wetted the printed face with water too. This kept the label fairly flat. I held it in position for a few second and then dabbed the excess moisture off.

Despite all the care I took, the inks still ran a little and left the final appearance a little less crisp than I would have liked. But, no big deal. I wonder how it would have turned out had it been printed on a color laser printer instead. Next time........


MARKING THE BACK WITH THE TEMPLATE

CUTTING THE BACK

GLUING THE BACK GRAFT

GLUE CLEAN UP

CONTOURING BACK GRAFT IN FACETS

SANDING FACETS SMOOTH

MARKING BACK BRACE PLACEMENT

SQUARING OFF BACK BRACES

MARKING BACK GRAFT FOR BRACE NOTCHES

CUTTING BACK GRAFT NOTCHES

PARING WAIST

ADDING SQUARE REFERENCE MARKS FOR BRACES

GLUING BACK BRACES

MARKING BRACE END SCALLOPS

SCALLOPING BACK BRACES

LABELS

ADDING THE LABEL

THE LABEL ON THE BACK

THE FINISHED BACK

THE FINISHED BACK

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