Soundboard Preparation

This post assumes you have purchased the tonewood for the soundboard and it arrives rough-cut (as is typical of most tonewood), and now it is up you the luthier to thin the top, and join it to prepare for additional operations.
Tools and Supplies:

24″ Metal Level
Roll of Self-stick 220 Grit Garnet Sandpaper
Guitar Plate Jointing Jig
Titebond Glue
RO Type Power Hand Sander

Upon arrival the top tonewood should have a chance to acclimate to the shop atmosphere for several days or several weeks – longer is better, so if possible order the wood for the instrument(s) well ahead of time. The main reason for this acclimation process is to allow for the wood to make adjustments to shop humidity. It is likely that the wood has been stored in any outdoor covered storage area or an unheated storage building. The atmosphere of your shop will be considerably different. Check our our article on Shop Humidity.

The wood usually arrives in a rough sawn state and is rarely sanded. Most luthier supply houses will finish sand or finish plane the top and back plates for you at an additional cost. This is something you should consider as this is a very difficult operation to complete properly without the proper tools. If you cannot get this service or you already have the tonewood purchased, see the article on Proper Thickness Sanding Guitar Plates for instruction to complete this operation without the aid of a shop planer or other stationary thicknessing device.

Straighten the Grain at the Joint:

In order the keep the soundboard grain as parallel as possible to the global centerline of the guitar, the wood grain at the center joint usually needs straightening. This is completed by using a metal straightedge and drawing a line close to the edge to be joined that follows the grain exactly. This usually results in a small triangular piece of the wood to be removed on each half of the tonewood. To remove this scrap of wood, place one plate on top of the other and cut to the line with a low angle block plane on its edge.

  • Adjust The Edge of the Tonewood By Drawing a Parallel Line to the GrainAdjust The Edge of the Tonewood By Drawing a Parallel Line to the Grain
  • Sand The Edge of the Plate With a Level Faced With Adhesive SandpaperSand The Edge of the Plate With a Level Faced With Adhesive Sandpaper
  • Bookmatched Plates Ready For JoiningBookmatched Plates Ready For Joining
Preparation of the Center Joint:

Lay out the top halves of the plate so they are bookmatched. If there is any runout, make sure it is at the outside edges of the plate. If the center joint is crooked (and normally it is), it will need to be planed and sanded straight.

To properly plane the edge of the wood, set one plate half on top of the other and slightly overhang the top plate. Place the Jointer Plane on its’ side and make long light strokes the full length of the joint until it is straight. Repeat this with the other plate. Note that if you have a joiner or a stationary belt sander, this can be accomplished with either of these tools quickly and efficiently.

Adhere the 220 adhesive backed sandpaper to one edge of the 24″ Metal Level and make sure it is tightly set. In an operation similar to the planing, overlap the top plate and make long, slow stroke with the Level, after making some progress, lighten your strokes until you barely touch the joint. This process should take about 50 strokes or so to do it correctly Repeat this process with the second plate edge joint.

Time to Check Your Work:

Hold the plates together at the joint, just as they would be when they are glued together and hold both halves up to a bright light. You should not see any light coming through the joint. If you do, repeat the jointing process with the Level and use lighter strokes. Check again, and you should be good to go.

Get out your Plate Jointing Jig. Place a piece of waxed paper in the center beneath the glue joint. Position the plates on the jig. Adjust the fence so it snugs up against the plate very lightly and tighten it down with the knobs at either end.

Do a dry run of the glue-up. Place the joint hold-down fixture over the joint on the top and very lightly snug it up. Now tighten up the cams about half way and check that everything is tight. Tighten the Joint hold down just a bit more and adjust the cams all the way. If everything check out you are good to glue.

Remove the plates from the Plate Jointing Jig. Apply a small amount of glue on each edge of the joint. Do this with your index finger and gently run it along each joint. Make sure ALL surfaces are covered and there is no dry wood. This should just be a thin layer, not a liberal layer as you will be dealing with too much squeeze-out.

Now rub the joint together in a back and forth motion. This will distribute the glue evenly along the joint and assure you there will be no gaps in the joint itself.

Place in the Guitar Plate Joining Jig and tighten just as you did with the dry run. This time remember to place a strip of waxed paper over the top of the joint as well.

Potential Problems:

Too much downward pressure on the top joint hold-down will prevent adequate joint clamping due to the friction. Use only minimum downward pressure almost like just setting a weight on the plate.

Too much cam action on the side cams. The way you can usually see this is that the plate buckles between the cam and the joint hold-down. If you feel you need this much pressure, use heavy weights to hold down the buckle.

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