The top of an acoustic instrument is solely the most important element for the production of quality tone and volume. Sure other element contribute to sound and volume, but nothing compares to the top plate. The string vibrations are transferred directly to the saddle and bridge and then spread across the entire top. The more we vibration we can attain from the top plate the more focus can be placed on tuning that vibration toward the tone we desire and the volume that is balanced and appropriate for the instrument.
About the best example of this theory is the classical guitar. The strings of the classical are rather low tension, when compared to a steel string guitar. Yet, a well made classical guitar with a superior top and bracing can have just as much, if not more volume that any 6 string acoustic guitar. This is accomplished by the usage of the best grades of woods and construction techniques, and the most important of these is the top plate and how it is braces and connected to the balance of the instrument.
So, it is crucial that not only you pick out the best wood you can afford for the top plate and bracing, but use the proven techniques to properly install correct bracing and shape the braces with care and quality. Then, be sure not to compromise all the hard work you have in your top plate with a poor fit or even a force-fit on the sides, kerning and blocking.
Tools and Materials:
Japanese Pull Saws
Fine Dovetail Saw
Sandpaper 180, 120 Grit
Large Rubber Bands
The Top Installation Process:
First of all, make sure your Top Plate is ready to go. The braces should be glued on the top, shaped and sanded. The bridge plate should be attached to the top and sanded. The rosette should be in installed as well as the rosette reinforcement strip installed beneath the rosette.
Put the sides in the inside form, top side facing up. Put in the Side Spreader Jacks, and make sure the sides are tightly following the form. Place the Top Plate on the sides, and line up the center of the top to the global center-line of the instrument perfectly. Put several large rubber bands over the top to hold it firmly in place so top braces are resting on the sides of the guitar. Double-check your alignment. Put another rubber band at the waist. These rubber bands should be tightly fit to allow only minor adjustments of the top location.
Visually Inspect Your Work and Mark All Braces:
Do a visual inspection around the guitar to make sure everything is exactly lined up – this is especially true of the center of the top aligned to the center of the sides. Now take a sharp pencil and trace the perimeter of the sides onto the scallop portion of the braces – just on the top of the braces. Be sure to be accurate with this and place this mark directly next to the wood side.
Next place a sharp pencil line along the side of each brace, onto the kerfing, where the brace crosses the side. Again place this line as close to the brace side as possible and don’t let the top plate slip or slide.
Trim Braces and Cut Channels Into The Kerfing:
Carefully remove the top by loosening the rubber bands. First locate the marks on the braces. With a straight end mark a second place of each marked brace to represent the thickness of the sides. Measure this value if you need to. When all braces are marked, carefully cut the braces to length using a fine Japanese Pull Saw. Be cautious as you approach the top and stop just short of cutting into the top plate. Usually the blade of the saw can be tilted in the kerf slightly and the end of the brace will pop off and leave just slight residue which can easily be cleaned up with a sharp chisel.
The channels that are cut into the kerfing are the next task to tackle. I prefer to use several tools to preform this operation. First I cut at each brace line at about a 45 degree angle down through the kerfing. Stop short of cutting into the side wood. Next, I set the depth of a Dremel router, which is mounted on a router base to the brace end thickness and carefully cut most of the wood out of the brace channel. Final cleanup is completed with a sharp chisel.
Repeat this for all top braces that extend to the instrument sides.
Dry Fit The Top:
Now is the time to see how accurate your were. Place the top on the sides. It should fall right into place with very little pressure. Your channels should not be really tight but they should not be sloppy either. It should feel like you are placing a plastic lid on a can and the top plate will snap into place. That is a good feeling!
If you run into too much resistance. Inspect your work to find the problem areas and adjust as necessary. The top should lay perfectly flat on the sides with no gap. Troubleshoot as necessary. Possibly some braces may be a bit too high and need to be cut down slightly.
Now trace the side profile onto the bottom of the top plate. Remove the top and carefully band saw the outline to within 1/16″ of the line – closer if you are careful. Sand the rest of the way to the line or just a bit beyond the line. You don’t what a big overhang on the top as this can split the top when it is glued on.