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Building an arched/contoured top guitar in a mold with a "built-in" bottom seems to mystify
some folks ---- this is the secret, if you could call it that. Simply put a perimeter spacer
inside the mold.
The reality is that most users of the body shaped molds end up screwing
or clamping plywood to the mold at some point, so there is a bottom to work against.
This
spacer is made of ¼ inch plywood and it provides plenty on clearance for the arch of the
top. You could do the same thing with pieces of cardboard. The instrument in the
sequence has a 15’ radius back and 28’ radius top. Place the spacer in the mold, place
the guitar body in the mold top down. I always glue the top on first – one of the main
reasons is that I can tap tune after the top has been stiffened by the sides. On occasion I
have shaved the braces. Makes sense doesn’t it?
Line up the centerline of the back with the centerlines on the neck and tail blocks,
secure the back with a couple of rubber bands
Mark the location of the back braces
on the rim.
Using a straight edge mark the top of
the sides and kerfing.
To make sure the ends of the braces come through the rim and sit properly in the rim
notches you need to taper the end to about .20” from the perimeter to the inside of
the kerfing.
I found the best way to trim down ends
was to  grind the tapered ends with a
60 grit flapper wheel  --- naturally
chisels would also do the job.
Mark the depth of the notch with a square or compass to match the thickness of
the braces. THIS MUST BE LESS THAN THE WIDTH OF THE BINDING MINUS
THE BACK THICKNESS.
Use  a razor saw, sharp knife and a chisel to clean out the brace notches.
If the rim has been contoured correctly the back is now ready to glue in place.
It always intrigues me when I see photos of guitar tops and backs being glued in place.  
There are usually twenty or so huge clamps or thirty spool clamps. It is not real clear,
but the back in the last picture is being glued. Note how few rubber bands are used.
This back is seated all the way around and there is glue squeeze out all the way
around. This is a contoured back with a 15’ radius. I cannot repeat this enough  --- If it
takes bone crushing force to bring your parts together something is wrong. Building
stress into a musical instrument simply is not a good idea. I always glue the top on first
– this makes the assembly very stiff. To complete the contour of the back it does have
to be shaped over the waist to match the side taper. Again, if the back was braced
properly there will be very little force required to get the longitudinal bend.
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