The Springboard (Pt. 6, Sanding and Bridging)

This is the sixth of a number of posts pertaining to the progress of my 21M.299 (The Social Lives of Instruments) final project, The Springboard.

After a busy week of psets and other final projects, I spent today working on the instrument. The goal for today was to figure out the “bridge” idea I mentioned in the previous post, and to cleanup the instrument and make it presentable.

The resulting “bridge” on the cleaned up instrument.

In order to make the bridge and also cleanup the instrument, I was going to need some more supplies. 

Getting Supplies

A found piece of wood that, unfortunately, did not make the cut. Yes I added the face.

Relying on found supplies can only get you so far. I found another piece of wood that I attempted to use for the bridge, however it kept splitting and wasn’t very easy to cut, so it was off to the store.

At the store I picked up a small rectangle of project plywood, sand paper of various grits (80 through 220), a sanding block, wood stain and polyurethane, and a couple of brushes (for the poly, I plan on using rags for the stain). The wood stain is “Red Chestnut”. Should be classy.

Cutting the Bridge

So why am I bothering with this part? Well currently, the springs are too long and essentially useless when played in their open position. There is not enough tension for the lower frequency that they vibrate at, resulting in a sound that lacks any useful harmonics. Adding this “bridge” will allow the same springs to be used while shortening their effective length.

Though I referred to this part as a “bridge” in my previous post and continue to do so here, the part I’m making is more like the nut and retainer bar found on guitars. For the sake of consistency I will continue referring to the complete assembly as the “bridge”, the lower portion lifting the springs as the “nut”, and the upper portion pressing down the springs as the “retainer”.

Initially I was going to use a simple rectangular piece of wood to lift the springs up. I realized with a quick experiment that this wouldn’t work- the departure angle of the springs after the square was very shallow, meaning that the portion of springs above the nut still vibrated when the lower portion was played.

To counteract this, another simple rectangle of wood, the retainer, was used to press the springs down above the nut. While this worked, it looked pretty ugly and I knew I could do better.

While playing my bass guitar, I was looking at the nut and its notches for the strings, and realized I could do the same thing for the springs. With my drill in hand, I tried making half holes at the top of the nut- this didn’t work so well. I ended up having to make new pieces, this time drilling out the holes before cutting. This process took forever, as the drill really, really sucked.

The nut and retainer for the bridge after a rough sanding.

Sanding Down

With all the pieces of the instrument made, I decided it was time to make the instrument a bit more presentable. After disassembling the instrument into its components, I hand sanded the wood to see what I had to work with.

It turns out when wood sits around for a long enough time without any coating, the outer most layer oxidizes. Having not worked with wood since some time in early high school, I wasn’t very familiar with what kind this was- I suspected pine, but the coloring was throwing me off.

A quick pass with the 80 grit paper made quick work of the oxidation layer. The wood was much lighter underneath and confirmed my suspicion that it was pine. The image to the left shows the drastic gradient after a single pass near the top. The lower portion remains un-sanded.

After sanding the entire board and other components of the instrument, I began rounding all the corners. For the most part this was just an aesthetic choice, however on the main board this was done so that the sharp edge wouldn’t damage the bow (ironic given that I’m playing metal springs with the bow, but I digress).

The below pictures illustrate the drastic difference before and after sanding.

The spring holder before sanding.
The spring holder after sanding.


The dry-fitted instrument in all its glory.

Now that all the pieces were sanded, it was time to dry fit them. This process was relatively painless. The nut was positioned to provide three octaves of range on the smallest spring, while the retainer was placed close enough to provide enough tension that the springs did not buzz against the nut.

New holes were drilled for the nut and retaining bar, while existing components reattached without problem. I was worried that the nut and retaining bar would split while drilling the holes to attach them, however this did not happen.

The instrument feels considerably more substantial with the addition of the bridge and the cleanup- it feels more like an actual instrument rather than some cobbled together junk.

Things That Didn’t Work

When cutting out the bridge, I also cut out a piece of wood to press the center of piezo pickup against the instrument in order to create better tone. This ended up not working- the tone became quieter and more anemic. I’m not sure if it was too much pressure, not enough pressure, or pressure in the wrong spot. More internet research is necessary.

A cursory glance suggests mounting the piezo pickup using epoxy on the center of the transducer. Based off my experience of applying pressure to the center of the pickup in order to get a more full tone, I believe this would work.  I am currently using hot glue around the outer edge of the transducer. I need to be cautious though, as epoxy is a very permanent adhesive. This problem and hopefully solution will have its own post at a future time.

Next Steps

My plan for tomorrow is to stain the instrument. This will involve disassembling the entire instrument, more sanding, and staining all the components. A clear coat of polyurethane will then be added in order to preserve the stain. While this isn’t really necessary, I think it would be a really nice thing to do for the instrument. After that I will figure out how to best mount the piezo pickup.

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