Tools and Materials:
Go Board Gluing Tool
Fine-Toothed Japanese Pull Saw
Woodworkers Marking Knife
Wood Chisel Set
Narrow Sanding Stick
The bracing needs to be selected for the top and acclimated to the shop.
Preparation of the Top Bracing:
If you purchased Instrument Plan, from Georgia Luthier Supply refer to the bracing profiles for the top plate. Rough-cut all of your bracing to length and width. Finish sand the sides and the gluing surface of the braces in preparation for the gluing process. The thing that is of most concern with the soundboard bracing is the X-Brace intersection joint.
As I mentioned in another article, the X-brace is the main structural element for the typical acoustical top. It does the job of supporting the tension placed on the top by the strings. Its’ second job is to distribute vibrations from the strings over the entire surface of the top. The main area of concern with the X-Brace in the joint where the two braces intersect. Typically, and this applies more toward production guitars than hand-made guitars, one brace butts into the side of the other brace. The joint is then wrapped with a open-mesh cloth fabric that is soaked with glue.
While this method is used a lot, I am not a proponent for this method of construction. I feel this is a weakness that is in an area of greatest stress and this joint simply is not adequate. I feel a much better solution is to construct this joint as a lap joint of sorts. What I mean by this is one brace is notched out for 1/2 its’ depth from the bottom to the middle of the brace. The other brace is notched out from the top to the middle to receive the other brace.
Advantages Of The Half-Lapped X-Brace:
- Both braces retain structural integrity – neither is compromised – both are equal.
- There is about two times the gluing surface for you to spread glue, giving the joint a lot more strength.
- The vibration from the top can travel along an unbroken path because the one brace is no longer cut in half.
Possible Concerns With The Half-Lap X-Brace Joint:
This is a difficult joint to execute properly. It takes very careful layout and even more careful execution to get this joint perfect. The reason that it has to be perfect is:
1. A tight joint is better structurally.
2. A tight joint will have less glue required and thus better have tone production.
3. A tight joint looks more professional than a sloppy joint. Show-off your skills.
4. Less glue in a tight joint will improve tone and volume characteristic of the top plate
Construction of the Half-Lap Joint:
- Carefully mark the intersection of the braces on each brace. Be sure to accurately mark the depth of each cut.
- Take a fine tooth dovetail saw or my preference a fine toothed Japanese pull saw, and cut two parallel cuts from the bottom of the first brace to just shy of the depth cut.
- Take a sharp 1/4″ chisel and place it just above the depth cut line, being careful that it is parallel with the depth cut mark.
- Chip out the cut.
- Clean up the cut with the chisel and finish with a narrow sand stick and/or file.
- Check the joint by sliding in the other brace. It should be snug but you should not force it. Sand a bit to get it just right. If you sand too much and the joint becomes loose, you need to start over. If you can wiggle it – it is too loose.
- Now cut the second brace in a similar manner only make the cuts opposite – that is from the top of the brace down to the half-way mark.
- Once you chip out the cut, start checking the two braces by sliding the joint together. It should be snug and not forced.
Now take the Soundboard and bracing over to the Go Board Tool and carefully glue all of the bracing onto the top. See the article on Glue Cleanup Tips to aid you in cleaning glue squeeze-out.
Next, refer to the article on Shaping Tone Braces to final shape and finish your soundboard bracing.
GLS Tip #1:
To get all of the soundboard braces consistently flat on the gluing surface, assemble them together as one group, clamp them together and sand the surface of all the braces at one time with a flat sanding block. It is far easier to keep the surfaces of the gluing surface perpendicular to the sides this way and you are also forming all of the braces exactly the same.
An addition method you can use: Make a sanding slab from a piece of 8″ x 11″ x 1/2″ marble flooring. Glue sandpaper to the surface of the marble with feathering disc cement and make sure the sandpaper is evenly adhered to the slab. Now sand the gluing surface of the braces on this slab for a perfectly flat brace bottom.
Another method is to cut the braces out of a single piece of wood. Finish sand the gluing surface side. Next cut the bracing to width on a band saw or table saw and finish sand the sides.