STEP BY STEP - 2
With the exception of the soundhole and the bracing pattern, making a back
is very much like making a top - so the descriptions in this section will
largely focus on the unique aspects of making the back.
With the exception of the soundhole and the bracing pattern, making a back is very much like making a top - so the descriptions in this section will largely focus on the unique aspects of making the back.
Select wood for Back and sides
There is a lot of talk these days about selecting wood for the back and sides of a guitar. What follows are my thoughts on the subject. While I believe that the body wood has an impact on sound, it is relatively minor. First in importance is body shape and bracing of the top, followed by the top wood. Then you get to the back (and sides) wood. I prefer wood that is quarter sawn and straight grained, but I am seeing more and more guitars which have fairly slab cut wood (particularly in Brazilian rosewood where the supply is so limited). This is not the end of the world, although I still want sides to be quartered. As to what wood - I have a discussion of this HERE.
Join back halves.
Same as for a top.
Glue back halves together.
Same as for a top.
Locate and cut slot for center strip.
If the wood has a very attractive grain pattern I often will not use a center seam decorative strip. If I do install one I use a simple jig that holds the top firmly and has a straight edge to guide the router I use. I use a router bit the width of the channel I wish to cut and cut about 1/2 way through the wood. If I then sand equally from both sides the strip will be sufficiently thick so as not to disappear when sanding the body (believe me, it has happened).
Glue in center strip.
I have a clamping jig which clamps this piece while the glue is drying. Look here.
6. Sand back and sides to thickness.
Approx. .105 to .110 for back, depending
on the species, its weight and strength. and .090 for sides - final
sanding is done with 120 grit belt.
Layout back and trim to shape.
This is fairly self explanatory. As with the top, I use Plexiglas templates for each of my body shapes so that I can get the most effective placement on the back - showing the grain to best effect.
Glue in center cross grain strip.
I use the same jig I used for clamping in the back decorative strip. I use a steel bar to equalize the clamping pressure of the 4 veneer press screws.
Shape and sand center cross grain strip.
For most of my career I did this with a chisel and sandpaper tapering the edge of the center cross grain strip and then sanding it smooth. I am now doing this with a jig on my spindle shaper (a router table would do.) This is quicker and more uniform. I still have to sand the strip but I like the rounded edge and the uniformity. It took some doing to figure out how to round over the edge of the cross grain strip without tearing the grain. The secret is to hold it firmly down and very close to the edge with a strip of plywood, which also acts as a guide for the router bearing.
Cut slots in center cross grain strip for braces.
Again, for years I did this entirely by hand - and then I made a jig that does this with a router. It is more uniform in spacing, a tighter fit and the braces are always parallel.
11. Rough cut stock for back braces.
2 at 5/16 x 5/8 and 2 at 3/8 x 5/8
Shape bottom arch on braces on spindle shaper.
This is a simple jig - the arch is cut (and very carefully sanded) into the front edge of the jig. The brace is clamped in place with toggle clamps (I love these for jigs) and the assembly is run past the shaper blade which has a ball bearing rub collar the same diameter as the cutter.
Shape top of braces on spindle shaper .
The back two braces receive a rounded top and the front two braces a more triangular shape. Again, this is a simple jig. The cutter determines the shape of the cut and the jig guides the cutter in a straight line past the top of the brace. The ball bearing rub collar ensures that the cut is the right depth.
Sand all surfaces of braces
Fairly self explanatory. As the back is visible through the soundhole, it is essential that all sanding and glue clean up be as well done as possible.
Fit braces to notches in center c ross grain strip.
ross grain strip.
I try to pre sand the brace stock to the correct thickness before making the braces. I still leave them a bit large and then I lightly sand them so that they fit tightly in the notches in the cross grain center strip.
Glue in back braces.
The problem with this is that the braces are curved and so must be glued in some sort of curved jig that will maintain this curve. Look HERE and see the jig I use. Fairly simple and I have been using these for many years.
Again, fairly self explanatory. Do a nice job here.
Trim and sand brace ends
I generally do this as I am fitting the back to the ribs. I notch the braces into the linings on the ribs.
19. Final inspection and clean up of back.
Do a nice job. It is one of the first things a customer will look at to evaluate the workmanship of the maker. At this point I use a wood burning device to place my logo on the center cross grain strip.
Select wood to match back.
Most suppliers match backs and ribs so this is easy.
Thickness sand wood to .090" - .095".
This too is done on a thickness (wide belt) sander or on my drum sander. I sand to a 120 grit paper. Some of the rosewoods which are very heavy (such as cocobolo) I will sand thinner - perhaps to .080".
3. Joint top edge of side.
I like to have this edge straight before I bend the side.
I do this before I bend the side. I do this on my spindle
shaper. Many makers profile the side after it is bent, but I
find it is easier to cut this shape when the rib is flat and straight. The
trade off is that I must be very careful to bend the side in exactly the
right place so that the profile is in the right place. Since I use a
variety of bending jigs (see below), I find that this is relatively
Soak side in water .
I used to do a lot of soaking. Now I do less. Generally 10 minutes in cold water is sufficient. For very curly wood such as koa an even shorter time (less than a minute) is sufficient. Too much soaking will weaken curly wood and make it more likely to break or crack on the curl. The first time a piece of very curly (and very expensive) koa breaks you will take this lesson to heart.
Heat bending jig.
I use two types of bending jigs. Two of them are old heated aluminum jigs made by Arthur Overholzer. They are no longer available but are simply wonderful. I plug them in for about 15 minutes and they heat to the desired temp. (About hot enough to make a drop of water dance on it). These must be hot before beginning to bend the wood.
The second type is made of wood and covered with metal sheeting. The heating element is a silicone rubber heating blanket (I believe that Jim Olson was the first to use these). These heat up very rapidly and so I start clamping the wood to the jigs before I turn on the heating blanket.
Once the jig is hot I slowly start bending the wood around the shape of the jig, clamping where necessary. The two ends and the waist are a minimum for clamping although I sometimes clamp a strip of wood across the rib at other places to help minimize cupping which is a chronic problem. I also use a thin metal strip on the top of the wood and pull this quite tight with toggle clamps. This helps to even out the heat distribution and to minimize cupping.
This website and all of its
content, text and images are copyright ©1997-2011 by Charles A. Hoffman.
All rights reserved.
This website and all of its content, text and images are copyright ©1997-2011 by Charles A. Hoffman. All rights reserved.
2219 East Franklin Ave.
Minneapolis, MN. 55404