Thursday, January 24, 2002

Mandolin #1: Tone Bars and f-Holes

This section would probably be more interesting if it was a Bluegrass prison song called "Steel Bars and A-Holes" but...

Tone Bars
With the top carved, next step was fitting the tone bars. As I understand it, the tone bars give strength to the to and may spread the vibrations from the strings (via the bridge) over the entire top. With a clear plastic template from the blueprint, I laid out the positions of the bass and treble tone bars. Notice that the bars and template are labeled to avoid getting switched around. I think there's a good explanation for the template having the labels backwards. For the time being, let's just say that they'd only be backwards if you were inside the mandolin in which case you would have more serious problems to deal with.

You can see in the second picture that because of the concavity of the top, the bottoms of the tone bars need to be shaped. To roughly transfer the curve from the top to the bar, I set the bar in place and slid a pencil along the curve of the top. With the tone bar in my cheapo bench clamp, I then chiseled down close the pencil mark and sanded the rest of the way. The MacRostie video shows him using his belt and/or spindle sander which would work significantly better. My problem was keeping the bottom of the tone bar square to the sides. Of course, after the final shaping, the bottom is not square to the sides because the top curves front to back as well as side to side but to get a consistent height at this point, I wanted it square. With a rough curve in the bars, I could now more accurately transfer the top curve using a pencil sanded flat on one side so that it drew a line on the bar as close to the top as possible. After more sanding to get close to this new line, it was time for lots of trial and error. Set the bar on the top, mark the high points. Sand them down. And repeat. As mentioned above, the bar also had to be fitted to the curvature across the top so more trial and error was needed.

With the tone bars fitted, they are set aside while the f-holes are cut.

Tone bar rough shaping Tone bar rough shaping 2 Tone bar rough shaping 3 

Tone bar placement Tone bar placement 2  

The next step was another one of those "you want me to saw into something I just spent a week on?" moments. It was also another aspect that I think really affects the look of a mandolin so I wanted to make sure I got them pretty close. At the same time, I had yet to learn that measurements thinner than those on the StewMac blueprint were probably not a problem so the concept of cutting into my top had me pretty anxious.

The MacRostie video shows starting the holes for the coping saw with a drill bit sized as close to the diameter of the top and bottom holes of the f shape as possible. According to the video, essential to drilling clean holes is using a backer block to avoid tear out. Of course, the backer block has to perfectly match the curvature of the inside of the top. Yeah right. I'm going to spend hours shaping the top of a 2 x 4 just to start the f-holes? Well, in hind-sight, my answer should probably have been yes. Then again most of the tear out I got was on the outside of the top. In any case, you can see my results in the pictures. I drilled holes for the the top f as close as possible and just used the coping saw through a starter hole on the lower ones. In both cases, I was able to use my plastic sanding sticks and various size dowels to smooth out the holes pretty well.

F-holes cut F-holes shaped 

With the f-holes cut and roughly shaped, the tone bars were glued in place and the glue was allowed to dry before final shaping. Meanwhile, thoroughly frazzled by my perception that my top was paper thin and now significantly weakened by the drill tear out fiasco, I followed a suggestion in the Siminoff book and shown in the Dude pix and reinforced the f-holes with gauze. This step also gave me a chance to glue and clamp a few chips that had broken away from the side of the f-holes during the drilling fiasco. The gauze seemed appropriate as at this point as I was not sure the patient was going to make it.

Tone bar fine shaping Gluing the tone bars

Tone bar final shaping Finished tone bars

The last step was the final shaping of the tone bars. Based on the blueprint, I chiseled the bars down close to their maximum middle height (different for bass and treble) and tapered the ends as flat to the top as possible. The goal is to have the bars thin to nothing as they approach their ends. The center of the bars was then narrowed into a scalloped ridge as seen in the Dude pix. The top bar in the picture to the left is close to being done. It makes sense that this almost triangular ridge would be the strongest part of the bar and would transfer energy out to the thin, flat ends and as a result more area of the top.

Index of Project


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