Stay in Tune vol# 26
By Luthier Patrick Podpadec
I know that I left off in the in the last issue talking about replacing or reglueing a bridge that has mysteriously loosened itself from the top of the guitar. Sometimes this is due to the improper grain orientation of the bridge that causes it to warp and lift itself or sometimes the bridge was not properly glued so that the constant string pressure will lift itself from the top.
To recap the procedure from last week, I left off at the part where you must be extremely careful with the amount of heat that you apply to the bridge so that you do not damage the surrounding finish . If you are planning to reuse the existing bridge you must also take in the consideration of scorching the top of the bridge so that you don't damage it as well. It is not an issue if you are going to replace it anyway. Many times bridges will have “cracked” or “split”when they lifted themselves from the body. At this point the obvious solution would be to replace it. I will explain my methods of replacement in a future issue, Today we will concentrate on reglueing the original bridge because we think that it is still salvageable.
After we have successfully removed the bridge without too much damage to the top. If there is you must take the proper measures to repair that damage first before you can continue your repair. Assuming that there is no significant damage you would proceed to remove any remaining glue from the top of the guitar and the bottom of the bridge. I usually use a sharp ¾ “ chisel and use as a scraper to remove the dried glue from the top of the guitar. I check to make sure not to go outside of the footprint of the bridge. I use a small straight edge, such as a 6” machinist ruler to see that I have removed all of the glue so that there is a small gap between the ruler and the top. I also scrape the bottom of the bridge and check it to see that it is straight and true from side to side and front to back.
After I'm confident that the glue is removed and the fit is good all around I get ready to glue the bridge. The first thing you must do is to make a clamping caul that will fit on the underside of the bridge which is where the bridge “pad” is. This is a small, thin patch of wood made of maple, or rosewood and even plywood that serves to stabilize the area under the bridge. It gives the area some stiffness and protects the underside of the top from the balled ends of the strings from digging into the soft spruce top. The idea is to use a caul that will be of a substantial thickness that will “flatten out” the top a bit when you clamp the bridge into place. You must make sure that it does not get hung up on a brace anywhere underneath, you most certainly do not want to crush or crack any braces while clamping the bridge. I usually make these cauls from small pieces of 3/4” plywood scraps. Although I have made quite few different size ones, I always prefer to make a new one that fits the job at hand perfectly, rather than use an already made one that doesn't quite fit. It doesn't take that much time if you have a bandsaw available. There are many different clamping methods that can be used. There are a variety of clamps that are readily available from supply houses such as Stewart Macdonald or Luthier Merchantile. A clamp with a 5” throat will be sufficient for most guitars I own a few 5” and 6” and even 7” clamps that I use to clamp my bridges. I use as many as I can fit in the soundhole with out damaging it. I use a mirror that I place inside the guitar so that I can see where my clamps are lining up underneath. Make sure that you do not get a bad fit of the foot of the clamp, so that it is sitting firmly in the proper area . I have also had to make “specialty cauls” that fit on a brace. This is only done when there is no other options and done with extreme caution. I recommend that you try your clamping procedure several times in a “dry” run, so that you get very familiar on how the hole thing is to proceed.
Patrick from Wood-n-Strings