Tools and Materials Required:
Fitting the Back Plate:
This article applies equally to the Acoustic Guitar as it does to Classical Guitars and Ukulele’s. Fitting the back can be a very fussy job. You have to be very precise so everything will fit together like a puzzle. It is of utmost importance all joints are aligned, braces and have been glued on the back, the back center reinforcement strip has been installed and the entire top and sides of the instrument have been finished sanded with 220 grit sandpaper. I like to fit my backs so they just “snap into place”.
One final step to attach the luthier label to the inside of the guitar back. This is the easiest time to do this, while there is still easy access to the back.
All linings shall be glued in place and ready to receive the back plate. The sides shall be shaped and contoured following the side template that was made form the building plans.
Slip the sides into the Inside Form and check that the top is dead flat on the bottom surface of the form. Make one final inspection to the back and check that all surfaces are silky smooth.
Lay the back on the sides. The braces should extend over the sides slightly or be slightly long. Take a couple of 1/4″ rubber bands and secure the back to the sides with a couple of loops around the Inside Form and tie off the rubber bands. Place another rubber band over the waist of the instrument body. The back should be loose enough to make minor position adjustments, buy not so tight as it can’t be moved.
Line Up the Back Plate:
There is a global center-line for the acoustic instrument. This center-line is the dead-on center for all parts of the instrument used for
alignment and it should be clearly marked on the Inside Form. The back plate should be aligned on this center-line and it also should be lined up perpendicular to the this global center-line. This will place the back plate in perfect alignment with the top plate (if you aligned it properly), the head block and the tail block.
Mark the Brace Locations & Cut Channels Into Kerfing:
Take a sharp pencil and mark all of the brace locations. Mark on either side of the brace and strike a sharp mark over the top of the lining and the side. At this time it is also import that the ends of the back reinforcement strip are trimmed to the correct length. You can to this careful measurements when the back location has been finalized.
Once marked, take the Fine-Toothed Japanese Pull Saw and carefully cut down through the kerfing without cutting into the back material. Now take a 1/2″ Woodworkers Chisel and chip out the scrap, clean the remains of the strip and the glue holding in place and make this area smooth and ready for gluing. A Dremel tool with a router base attached can also be used for hogging most of the material from the brace channel. Be sure to stop routing short of cutting into the side wood. Also take care in cutting near the edges of the kerfing as this is a very fragile area.
Upon arriving at a good brace fit into the kerfing, carefully finish sand the sides and kerfing with a crown sanding jig. Also account for the back arch over both the head block and the tail block. It is essential that the quality of joint at these two block be perfect to assure a structurally sound instrument. One thing to avoid is trimming the block down too far, in other words not consistent with the back arch. If this is not detailed and the blocks are too short you could have a “hump” in the back where the back meets these blocks, so take your time when fitting these blocks.
One Final Fitting:
Fit the back and adjust as required. Trim the back reinforcement strip as required to fit tightly against the foot and the tail block. Fit the back one more time. This time loosely strap it down the the 1″ Elastic Tape (use 1/2″ elastic tape for ukuleles),and check that it is lying flat against the linings and sides, without any gaps.
For the Classical Guitar, also check that the classical guitar foot is tight against the back and that the head and tail blocks of the acoustic guitar are a tight joint too. Make sure there is not a hump in the back where the plate lies on the head and tail block. If there is, trim down with blocks with a Sanding Surface.
Remove the back and you are now ready to glue the back to the sides.