Top and Back Bracing:
A new construction year has begun in the shop meaning that all the soundboards and backs that I will need between now and next January should be built by the beginning of February. That's only four guitars this year...I think I'm getting old.
Winter is the best time to glue up bracing... while the ambient humidity is low_about 35-45% in the shop. In Virginia, winter is the only time when one can count on several low-humidity days in a row. So, in January, Woody glued up bracing on four guitar tops and backs.
The braces, of course, are glued on after the soundboard and back stock have been thicknessed and center-joined. Then the rosettes have to be made and inlayed into the tops. All of this actually took about three weeks during which there were about 16 days of outdoor ambient humidity of less than 45%. The humidity in the shop stayed below 40%.
Woody doesn't have any type of mechanical humidity control in the shop (maybe we should). When I see from the weather report that we are in for 3-4 days of dry conditions, I am ready. These tops and backs were center joined last winter so, when the weather is cooperating, I am prepared to go into the shop early and stay late... and sleep well.
But first, rosettes and soundholes.
Gluing up back braces is much more straight forward than top braces if you have a radius disk fixture or radius cauls to hold the back in an arch shape when the braces are glued down. Woody uses a 15-foot radius for the arch of the back.
The photos shows two fixed-caul workboards that Woody uses exclusively for this purpose. One has four ribs (cauls) and the other six. A third, not shown, has five ribs. It just depends on the type and size of the guitar being built. Today it's a 6-brace dreadnought.
Every brace gets clamped at the same time to this type of workboard/ fixture to form the shape of the back into the proper radius.
One final note about bracing several backs and tops at the same time. It is likely that these backs will not be built into a guitar body for several months. Therefore, to maintain the contour of the back while hanging free in the shop, I have cut out an arched caul in the radius of the back (~15 feet). With one clamp and this caul the back will stay in shape until it's time to glue it to the sides of a new guitar.
As is often the case in writing up these journals, with so many words of explanation it might seem as if the process is more work than it actually is. Woody made up, inlayed and surfaced in place four rosettes in about 3 hours, albeit spread out over several days.
ROUTING the SOUNDHOLE
Routing for the soundhole can be done before, after... or at the same time as the rosette. Woody usually routs both the rosette channel and the soundhole at the same time. That way, one can ensure that both circles are on the same center.
Briefly explained, the router base has two 3/16" holes drilled in it at exactly the distance from center needed to cut a 4" diameter soundhole with a 1/2" bit. There is also a 3/16" hole in the router base a distance from center that allows a 1/2" bit to cut a 5-1/2" outside diameter rosette channel. Woody rosettes are 1/2" wide and set 1/4" from the edge of the soundhole.
The top is clamped to a workboard. A 3/16" hole is drilled in the top and the soundboard at the center of the soundhole layout. A 3/16" dowel is set in this hole. Then the router base slips over the dowel which will allow the router to rotate 360 degrees. The cutter is set to a depth of half the thickness of the top and the rosette channel is cut by rotating the router around the dowel.
The same is done for cutting the soundhole. In the photo below, the soundhole was being cut after inlay and sanding the rosette. Before cutting out what will be the soundhole, however, the disk must be secured (with brads in the photo) so that it will not fly out when cut loose from the surrounding soundboard. This is very important.
The rosette that Woody uses is actually 13-15 strips of wood_ bent and wrapped around a cylinder to cool and stay in the approximate final shape until ready to be glued up.
CROSS BRACING: The cross bracing is the most critical bracing in terms of structural integrity of the top. With steel strings and no cross bracing the pull of the strings on the bridge and top would create a torque that the upper bout and soundhole could not withstand. Cutting the lap joint that allows the cross bracing to act as a single unit is essential. Cutting this lap joint is not as difficult as one might think. (The 99 degrees is my own. I've seen it 98 and I've seen it 100. Go figure.)
TONE BRACES: The two braces in the lower bout between the cross braces are referred to as tone braces. The lower bout most significantly impacts the tone and volume of the guitar. The upper bout is primarily structural and somewhat inert relative to the lower bout in tems of tone production.
The angle of the tone braces relative to the grain lines also affects the tone that the top will produce. More perpendicular to the grain enhances the treble end. More parallel to the grain enhances the bass. Dreadnoughts don't need any help with bass.
CUTTING THE LAP JOINT: Cutting the cross bracing lap joint is straight forward and relatively simple if one is careful. First carve the bottom of each cross brace. That is, carve the edge that will be glued to the top. Woody carves the lower bouts of the cross brace to a 30-foot arc leaving the upper bout ends flat and straight down to the lap joint. Other luthiers will cut these braces differently...but only slightly. Putting an arc in the lower bout will assist in keeping the top from splitting as it goes through shrinking and swelling due to changes in ambient relative humidity. Keeping the upper bout flat will assist with assembly of the box, joining of the neck, and set up of the guitar fretboard.
Once the arc of the cross braces has been carved, line up the two cross braces side by side so that they both have the glue side down. Make sure that the 'upper' bout and 'lower' bout edges are also side by side. Now, turn one of the cross braces over so that one brace has the glue edge up, and the other brace has the glue edge down. The arrows in the picture point to the edge of the brace that will receive the glue. This is very important.
Back clamped to the Caul
When the glued-up rosette comes out of the fixture it is about a 1/4" thick and rough on the top and bottom. The rough rosette is placed in a simple, home-shop fixture with two circles routed.
The first circle is routed to a depth of .350" and is used to flatten what will be the bottom side of the rosette. Here, Woody's old thickness sander does the trick. Then the rosette is flipped over and placed in the second routed circle which is only .120" deep. This takes the rosette down to approximately 1/8" thickness which will allow it to be inlayed and sanded flush to the guitar top...again on the thickness sander.
After cooling and drying (overnight) the bent stock is wrapped around a circular fixture, also home-shop built out of 4" PVC pipe. While gluing up the layers, spring clamps are all that is needed. Once all are glued and laid up (to a width of .500") PVC cauls are used with 2" C-clamps to keep the 15-piece assembly round as the glue dries (overnight).
The number of rosettes that one can make at the same time is limited by how many circular fixtures one has and how many C-clamps.
Woody's theme for 2019 is New Beginnings. So far, that has meant 'out with the dremel' and 'in with the CNC' for inlays. We talked a lot about that in November and December.
Pictured below is Woody's 35-yr old Fairbanks guitar peghead. The inlay was done with a dremel tool. The MOP was cut the old-fashioned way, with a jeweler's saw and bench pin. The finished inlay came out OK but not as tight as I would have liked. See December 2018 for details.
Enough about inlay for now.
The rest of the braces are glued in according your bracing plan. Many of the various bracing schemes are available on line. Many are free, some are not. The one thing that they all probably have in common is that brace ends are scalloped and tucked in under the cross brace or lining, or both. Pictured below is an example of what I mean.
TOP AND BACK BRACING
Now we are ready to brace the top and back. First, the cross braces. These are the longest braces on the guitar. Woody uses standard C.F. Martin style cross bracing on all of his dreadnoughts (so far). It looks like this.
Set the bandsaw table to 9 degrees as shown in the photo below. Tape the braces together making sure that the edges to be cut are flush. Both braces are cut at the same time. I have a 9" bandsaw so I have to keep the 'upper' end of the braces on the throat side of the blade. The upper part of the brace is about 7-1/2".
Cut between the lines which should be the width of your cross braces and only half way through the brace. Use the bandsaw blade to carefully hog out all of the wood between the lines. WORK SCANT and you should still see all of your lines when you're finished cutting. Being too tight is not a problem. You can always shave a bit more off.
Woody uses a guide template to place the braces at the 99 degree angle on the top. Once the X-braces have been glued down, the final step is to glue on a reinforcement patch over the top of the lap joint (pictured). This strengthens the joint significantly.
OK. That's it for this month. In February Woody should be bending some sides and gluing up some boxes. I gotta stop working so hard.
The stock for these circular bends is wood binding stock from LMI or StewMac (maple and rosewood, .250" x .080"). Also used are maple and rosewood strips (.250" x .020"). These thin ones don't need heat.
Also pictured is what happens when Woody gets over anxious and tries to bend the rosewood too quickly on the pipe.