Luthier Tips du Jour - Steel String Back Braces

Uploaded by OBrienGuitars on 02.03.2009

When it comes to back bracing, there are basically two schools of thought.
Reflexive and responsive.
Reflexive meaning that the back is stiff or rigid so that the sound waves are reflected off of it,
and responsive meaning that the back is looser allowing it to work together with other variables,
like the top in producing the overall sound of the instrument.
On my guitars I choose to work with the responsive back idea.
I start by cutting my bracing stock according to my plans.
Here I am using a typical Martinesque OM style bracing pattern with some variations.
Next I need to radius the braces and there are a couple of ways to do this.
One way is to use a jig called a bracemaker jig.
This well designed little jig allows me to radius both top and back braces to my desired radius.
In my case here I am using the 15 foot radius for the back braces.
I place a back brace into the jig, push it down in to the radius and secure it by tightening the knob.
You can also place another brace on the other side to help apply even clamping pressure to the brace.
I then run the brace through my joiner until the edge of the brace is flat.
You could also use a table saw or even a sander.
When releasing the clamping pressure the brace springs back into a perfect 15 foot radius.
It doesn't get much easier than that
This jig puts a single radius on the edge of the brace.
However, the back of the guitar has a dome shape radius.
In other words it is arched along the length and across the back.
I place the braces in a 15 foot radius dish with sandpaper
in the approximate position they will be on the back.
A few strokes in the dish sands the domed radius into the brace.
Another way to radius the back braces is to do it by hand.
I place each brace in my radius dish and trace the radius on it with a pencil.
With the brace clamped in a vise I then use a block plane to pre-radius the brace.
This will make the process go much quicker and also save you some work in the sanding dish later.
You also don't have to breathe all the saw dust if you use a plane to pre radius the braces.
Now I place each brace in the dish at the approximate position that they will be on the back of my guitar.
A few strokes with each brace in the sanding dish will put the exact radius into each brace.
When sanding be very careful to not round over the edges on the bottom of the brace.
You want the edge to be crisp so you get a nice glue joint when gluing it to the back.
I also number each brace so that I know where each brace goes
and that it is facing the right direction on the back.
My braces are cut to length so that they are hanging over the perimeter of my guitar
by about 1/4 inch or about 6.5mm.
I like to pre scallop the edges so I don't have a much work to do later.
I come in from the edges about 60mm or about 2 and 3/8ths inches.
There are two types of scallops you can put on the end of a brace.
One is to have a drastic radius or scallop like I am showing here.
In my opinion this is a weak brace due to the large amount of end grain exposed on the end of the brace.
The other type of scallop is almost a parabolic shape.
This gradual contour makes for a much stronger brace and is less vulnerable to movement
due to humidity changes.
With the back thicknessed and cut to shape I can now lay out the braces.
You will notice there is no center reinforcement strip on this back.
You can glue the braces on the back before the center strip or vice versa.
There are pros and cons to both methods and I will show each way.
I will be gluing the braces in my 15 foot radius dish using the go bar method.
I have found that if I place a thin flexible material like a router pad in the dish
and under the back I get a better glue joint.
I apply the glue sparingly to the brace and spread it with my high tech glue spreading device.
Very carefully place it in position and use one of the fiberglass go bars to apply downward pressure.
Be careful as the brace will want to skate out of position as you apply the pressure.
Continue putting on the fiberglass rods until the brace is completely clamped.
Usually about 6 rods per brace is sufficient.
A piece of scrap wood with an edge on it works well for cleaning up excess glue along the brace.
Go ahead and glue all braces at once.
Now that all the back braces are glued to the back I can now glue on the center reinforcing strip.
One of the advantages of gluing on the braces first
is that you don’t have to notch out the centerstrip to attach the braces
and run the risk of having gaps where the strip meets each brace.
The disadvantage is that you have to keep the pieces in a straight line
and it is harder to sand or plane the centerstrip than if you glue it on first.
Make sure you use a clamping caul to apply even pressure on the thin strip.
If you decide to glue on the center strip first
you can just clamp a straight edge next to it to help keep it aligned.
Then use a long clamping caul as you glue it on.
After applying pressure with the fiberglass rods
then you can immediately remove the straightedge and clean up the excess glue.
After adequate glue time you can shape the centerstrip by planing it or sanding it
to the thickness and shape you prefer.
However, now you have to notch the centerstrip at the locations the braces pass through it.
This is easily done with a small saw or chisel.
Make sure that you keep the opening tight. It is better to cut on the small side.
I also use a razor blade to remove any excess glue where the centerstrip was.
Go ahead and mark and notch the locations for the other braces.
You can then glue on the braces in the manner I have already shown.
Congratulations. Your back is now ready to voice and be fit on the rims.