Joining back plates, bracing, and tap tuning

Inside the Build, 1

Note: if you’re viewing this on a phone, you may need to turn your phone to landscape view in order to see the text/descriptions of what’s going on in these photos.

I join my top and back plates the same way. After I get them to the close final thickness of around 3mm, I’ll very carefully run them through my jointer which does a pretty good job of closing the gap between the two plates. I take them a bit further by using this little jig pictured here. It’s just a flat surface that can support the material, allow me to clamp it, and put the whole thing in my vice so I can work on it from a good, ergonomic, position. I glued 100 grit sandpaper to a very high quality level that I know is dead flat and straight and then take strokes in only one direction, doing both halves of the plates. I do this until there is zero gap when you place the joint together. Really, the joint should be tight enough that it doesn't require any clamps to make it close. Once there, it gets glued.

After the glue has dried for several hours, usually overnight, I work on inlaying the back strip. I always start with an undersized cutter and approach the final width. I have found that end mills are great for this. They come in every size imaginable and make ultra-clean cuts. I find the center, then one side of the thickness of the material that is to be inlaid. From there I can make the slot wider and wider by adding layers of tape to the edge of the router that rides against my straight edge. I just keep adding until it all fits nice and snug. No gaps. It’s better to inlay these backstrips instead of just making a sandwich during glue up. Much stronger joint with a lot more surface area to glue. Building guitars is most certainly not about cutting corners. Oftentimes, the longer, more difficult ways to do it, is the right way. 

Pretty self explanatory what's going on here. I just plane down the excess height of the backstrip until it is nearly flush with the surrounding surface. I scrape and sand until it is all perfectly smooth. This lie nielsen plane makes very quick work of removing the bulk of the material. 

It should look something like this when finished

Now is the time to install the reinforcement strip for the interior. I just use off cuts from the top from cutting it down to size. It is very important to make sure the grain is perpendicular to the grain of the back. This adds loads of strength to the back joint. Shape it down and make it look pretty. The inside appearance is just as important as the outside in my opinion. These things take forever to build, not sure why one would want to skimp on any part of the process. Pride, man. It matters.

All of my guitars have a domed back. It is stronger and allows you to brace it a bit more lightly giving you a lighter weight instrument overall. This photo here illustrates how I draw the witness lines for how I need to shape the brace. I just run a pencil along and it traces the contour onto the brace. Old carpenter tricks come in handy  for guitar building.

I remove the bulk with a handplane and approach the final shape by using this dome dish with sand paper glued to it as the sanding block to dial it all in. I sand until I get full contact. I just run the brace back and forth in its designated location. I mark out where the braces are located on the back and make sure there are witness lines in the same place on the dish.

I glue them down using go bars (bent pieces of wood) and use the sanding dish as the back side of a clamping caul. The guitar back will then take the desired shape. In this case, around an 18’ radius.

From there, it’s on to shaping. I keep them full height in the middle, and taper them down to the edge. I round them over. Again, to help reduce the weight as well as remove a bit of the stiffness of the spruce.

I scallop the ends to allow for some flexibility at the edges of the plate. While the back and sides don’t typically produce a ton of the sound and volume of the instrument, it CAN certainly change it. Think of guitars as speakers. The plates move in and out, just like a speaker, producing sound by changing air pressure at certain frequencies. If you allow more flexibility in the plates, you can allow them to vibrate more with the strings. Next time you play a guitar, hold the instrument away from your body when you play, and listen to the difference vs when you keep it against your belly. This will illustrate the difference between being active and not being active. I like to keep my backs a little bit on the active side. I just prefer the tone it makes. I do a bit of tapping at this point and usually just go for sustain and the widest variety of tones. I shave a little off the braces, which gives it more bass, but be careful! You don’t want to go too far and muddy the sound. Balance is more important than lots of bass. 

Here it is, all shaped, sanded, the ends cut to length, and the label glued on. That’s basically the whole process of making the back of a steel string guitar. Or at least the way I like to do it.