The next step in building of the guitar was to do some work on the back. Braces are needed across the back and down the center seam to arch and reinforce the back. As I mentioned in the previous post, the back is radiused from neck to tail and also from side to side so, some creative clamping solutions were in order.
Building the Go-Bar Deck
There are a few alternative methods that can be used to clamp the raduiused braces to the back. Though, most involve the use of a lot of deep throat cam-action clamps. I do not have any of these in the workshop and it would be a fair amount of work to make them so, I opted for an alternate route.
A very old method often used by luthiers to perform this clamping task is to use flexible sticks of wood or other material to brace the objects to be clamped between between two fixed parallel plates. The fixture to do this is called a Go-Bar Deck and usually makes use of flexible wood or fiberglass rods to clamp parts of the guitar during a glue-up.
I made a very simple version of the Go-Bar Deck with some 24″ x 24″ particle board pieces and some threaded rod. As you can see in the picture (click for larger view) the threaded rod is used to stand off one particle board plate from the other. I did also glue on some reinforcing strips to the bottom of the lower plate as well. The threaded rod is screwed into T-nuts in the lower plate and secured with a washer and nut. In the upper plate, there is a washer and nut on both sides of the plate to lock it into position on a plane equidistant from the lower plate.
Preparing and Clamping the Back Braces and Center Reinforcing Strips
Because Rosewood is a tropical wood it can sometimes have residual oils on it, the first step I took was to layout for the braces and then clean the areas for the braces with some Naptha. This step may not have been necessary but, because the radius on the braces will force the back into a fairly significant arch, I wanted to be sure that there was a good glue bond. However, even with the Go-Bar Deck for clamping, the braces could not be clamped to a flat back so, an alternative was necessary.
Many luthiers will use a radiused dish for the purpose of backing the back plate to maintain the needed arches. Two of these dishes would be required (i.e. the back a top have different radii) and I do not have them in the workshop. They can be made by dishing out MDF with a router and a jig but, I was not too interested in the copious amounts of dust this would create. My alternate approach was fairly simple. I created some strips of hardboard to place under the edges of the back plate so that when the braces were clamped to the back the appropriate arches would be formed into it. In the picture, you can see the back with the four main braces clamped in the Go-Bar Deck. I used 5/16″ wood dowel rod with rubber screw protectors on the ends as the go-bars. You can also see the strips of hardboard under the edges of the back to maintain the necessary arches in the back.
Because this was my first attempt at this, I decided to clamp just the four main braces as a first attempt. It’s amazing how much even clamping force can be applied using this method. Because the wooden dowel rods were under a fair amount of pressure, I made sure to wear eye protection during this operation – I was concerned that one of the go-bars could shatter while I was working on this. In the picture you can see what the back looked like after the glue up of the four main braces was complete. In the second picture I attempted to show the arches that were forced into the back during this operation.
After the clamping of the four main braces was completed, I still needed to add the center reinforcing strips to the back. The process for this was the same as for the braces. Though, I did add some spring clamps for the two end strips because they tended to slide around a bit when the go-bars were applied to them. I think that my concern regarding the wooden go-bars was a valid one because when I returned to un-clamp the center strips in a few hours, I found two of the wooden go-bars broken and lying on the shop floor! Luckily, this must have happened after the glue had already set, so there was no issue. However, I think I will attempt to find some fiberglass alternatives for the clamping of braces to the top.
Shaping the Back Braces
The back braces were rough shaped prior to gluing them to the back plate. However, the idea behind bracing a guitar is to achieve a strong, stiff back without adding excess weight. The shape of the braces also affects the sound of the guitar and how it will project when played. These braces extend all the way to the ends of the back and will intersect the sides and kerfed linings. So, they needed to be trimmed down to about .100″ at their ends. I did this with a sharp chisel and a caliper, checking the thickness as I went. After that I did a quick sanding of the braces just to clean things up.
Next up will be the bracing of the top and assembling the back and top to the sides. That effort will also call into play some unique clamping requirements. I’ve already started gathering materials to accomplish that task. Until then, please feel free to contact me via e-mail at firstname.lastname@example.org or leave comments or questions here using the comments link at the end of the posts.