Rough Thickness Top & Back Plates Prior To Joining
With minimal sanding to take out some light planer marks the plates should finish out about .10″ or 1/10″ (2.6mm to 2.8mm), which is just about right for a steel string acoustic guitar. For classical guitars take the plates down by a few more tenths of a millimeter or thickened to about 2.4mm. Ukulele tops are usually about 2mm thick. For an easy method of doing this, visit articles on plate planing on this blog. There are a lot of opinions on thickness of tonewood, so do your research. Here are some general guidelines for finished plates and sides:
Take each half of the top or back plate and lay them out book-matched. Trace the Instrument shape on the wood with a white pencil on dark wood or a lead pencil on light wood. Choose the most flattering grain pattern and if there are any defects in the wood, try to work it out so they are in the waste portion of the plate.
If the grain is not perfectly parallel to the joint, now is the time to straighten it . Lay a straight edge parallel to the grain and mark the difference on the top plate (at the joint side of the plate). Mark with a pencil and carefully plane to joint(s) to your mark.
Now flip the plates over and use a hand plane, power hand plane, sand or some other method to smooth up the backside of the plates. Your goal is to eliminate all saw marks, planer scorch marks and other inconsistencies.
Once you are satisfied with the result, load a dual action electric sander with 80 grit paper and go over the entire surface until it is very smooth, level and consistent. Use long light strokes with the power sander and keep it vertical. Try not to work in any small areas to get out defects as these will cause thickness variations. This work can also be done with a sanding surface just as effectively, but you will really get a workout!
Now that the backside of the plates are smooth and level we can address the center joint of the plates, which is an extremely important joint and its critical that this be a perfect glue joint here.
First take a plate in each hand and hold them up against a strong light source so you can analyze the joint tightness. If you can see any light leakage, you will need to work on the quality of the plate joints.
Strive for the Perfect Joint
Here is the best way to get this joint razor-tight and you will need a couple of tools to do this.
First to be the most efficient you should make your first pass at this joint with a very, very sharp jack plane. These planes are 10 to 12″ in length and when sharpened properly you can easily take small amounts of wood off the joints.Start by laying the plane on its side. Alternatively if you have a stationary belt sander, that will work very good as well. You should have a fence setup on the belt sander though to keep the joints square.
When the joints are roughly straight you are ready for the next step. Stack the plates on top of one another and overlap the lower plate by approximately 1/2″ or thereabouts. While tightly applying pressure on the top plate to hold it in place, make long consistent strokes with the jack plane for the entire length of the joint. If the blade is set correctly, this should only take 2 or 3 passes. Reverse the plates and do the other joint. If the stationary belt sander is used, sand both plates at one time – in about 10 seconds!
Hold the joint up to the light again to check for light leaks. Repeat if necessary.
Here is the Secret For The Perfect Joint
This is a little trick that I devised. Purchase a good 2′ level with aluminum edges with milled faces. Place 180 to 220 grit adhesive backed sand paper on each face of the level and trim to fit the face. This type sandpaper is best purchased in rolls.
Next stack the plates as before and make smooth, consistent strokes on the plate edge. As you progress make the passes with lighter pressure. Repeat with the second half of the plate. The joint should now be perfect.
Now that the plates have been rough sanded on the bottom side, they are ready to insert into the plate joining jig for glue-up. If you would like to build one of these great devices, be sure to visit our Plate Joining Sales Page. We also sell Plate Joiner Plans where you can build your own form our easy to follow plans. If you need the exact hardware to make one of these tools visit our page on the Plate Joining Jig Hardware so you can just supply the wood and labor.
The Joining Operation
Completely open the three cams on the Guitar Joining Jig and run the fence to the completely open position. For easiest access to the plate joint, remove the center hold-down retainer. For the Ukulele Plate Joiner, open all 4 of the adjustable cams.
Note that in order to use the Plate Joining Jig you will need to square-up the plates to fit in the jig. Also, it works best when the plates are cut about 1/4″ oversize. This will help to obtain the best clamping pressure without the plates bowing out of shape.
Place a light bead of glue on the joint on both wood surfaces. If you are gluing a guitar top, you can use Titebond Glue. You can also use Titebond to glue any of the non-resinous hardwoods, which can be and of the Mahogany Family, Maple, Walnut or Sapale.
Exotic Species such as Bubinga, Ebony, Paduck, Zebrawood, Rosewoods etc., should be glued with slow-setting epoxy glue. Epoxy is best applied with a small stick of veneer or scrap stick and spread evenly over the entire surface of the joint until all surfaces of the joints are shiny with glue.
Place the plates centered over the non-stick HDPE sheet on the base of the jig. Gently rub the two halts of the plate back and forth to distribute the glue smoothly over the surface of the joint to begin adhesion.
Set the center retainer bar into place and spin the knobs down so the retainer comes in contact with the plates, but no pressure is placed on the plates by the retainer. This is just meant to hold the wood plates in alignment without restriction in sideways clamping pressure.
Move the fence into position to the edge of the plate and tighten the fence down tightly. Begin to engage the cams one at a time with gentle pressure. If the plate starts to slide out of position, carefully slide them back. Apply a little pressure on the center retainer bar.
Crank in more pressure on the cams and tighten the knobs one at a time. Again check the center joint and adjust as necessary. Crank in more retainer pressure.
Repeat as necessary until the plate begins to buckle very slightly (You will note that some of the stiffer woods will hardly buckle at all). Back off just enough pressure on the cams to relieve the buckling and completely tighten the center retainer. Let the plate dry thoroughly.
Once you remove the plate from the joiner, it is time to thickness plane the plate (if necessary). See our chapter on Thickness Planing Guitar Plates.