Rotola: The Bridges and the Stand

 In my last post about the rotola focusing on the construction of the nut, I included an overview of basic string instrument construction; for review:  

In this post, I'll focus first on the other end of the resonating part of the string, the bridge, and then on something unique to the rotola, the stand on which the instrument rotates.  

The rotola has 16 strings and each string has its own bridge.  Here's a drawing of the bridges' front and side views:

They are not big, about 18mm high and maybe 6mm thick at the base and ~1.5mm at the top.  I decided to make them out of purpleheart, of which I have a large chunk.  I like the wood a lot, although I don't often use it, as it's really difficult to work with due to its extreme hardness and density; however, those same properties make it good for this application.  Additionally, while I have some hard maple, which is a more typical wood for string instrument bridges (particularly the violin and viol families), I wanted something that would contrast visually with the spruce of the soundboards.  

Fabricating these would be a challenge:  I needed to make at least 16 of them (spares would be nice, too) and they needed to be uniform and precise (note the curved base that fits exactly on the soundboard), plus, being small, they are hard to hang onto while working.  After much thought, I figured a way to make them from a long strip as a starting blank.  I could plane the strip at an angle along one face, then, supporting the planed side to keep the centerline correct, plane the same angle along the other face.  Next, I would mark out the width of the bases using a pair of dividers and then sand the correct radius between those marks.  Finally, I could saw out the rough triangles and clean up the sides using sandpaper on glass and file the notches for each bridge's string.  This more or less went to plan, although I mades some mistakes while working it out and had to start over.  

Here are some views of the process:

The purpleheart blank after it was cut, milled, and planed to size as precisely as I could make it.  

Two views of the jig I built to run the blank through the surface planer:  

For the second run (which I neglected to photograph), I had a shim with same angle as the cut, so the second side would be held at the same angle as the first going through the planer.  (For my first attempt at this, I used a hand plane and struggled to get the angles correct and consistent; the above success was due to having purchased a surface planer.) 

This is the setup I used to cut the radii into what would become the bases of the bridges.  The wheel is cut from 1 1/2" MDF (actually two 3/4" pieces glued together, leftovers from another project); I cut it initially on the bandsaw and then rounded it against the oscillating belt sander by rotating it on a fixed dowel inserted into the center hole.  I then stuck 120 grit sandpaper to the edge using spray adhesive.  I was able to raise the work surface by cutting some 1/8" plywood to fit around the disk and clamping it to the drill press table; this prevented leaving a ledge of unsanded wood where the bridge blank might slip under the spinning disk.  The setup worked fairly well, being only a bit off of parallel to the table.  

Here you can see the scallops made with the sanding disk.  

Next step was to cut the bridges out, which I did using this jig (left) and a pull saw.  The vertical piece of the jig had a magnet embedded in it to keep the saw at the correct angles (defining the side of the trapezoid and keeping the edge square to the centerline).  This also worked fairly well and you can see some of the cut bridges in the cup at center, but, unfortunately, I seem to have neglected to get any close-up pix of the finished bridges; you'll see later how they fit onto the instrument.  

Next, I went to work on the stand for the instrument.  As it is intended to be played by a single person, cranking with one hand and bowing with the other, it needs to be supported at the axle ends so that the pins and bridges have clearance from the stand, but not so high as to raise the business side -- the strings rotating across the top -- inconveniently high.  The simplest design would be some basic yokes at the ends and a plain platform spanning between them; however, I wanted the stand to be a part of the presentation and be attractive enough to compliment the instrument.  

I started out by hand milling down a large piece of fairly warped but beautifully figured walnut given to me by a friend.  This was before getting either a jointer or a surface planer and became an exercise in confronting my amateurism, ending with turning most of a 6/4 piece of wood into sawdust or shavings; you can see the resulting piece in the middle of the bench, below -- it's less than 5/8" thick.  

The two square pieces with holes drilled in the center will be the end supports.  These were resawn from 3/4" pre-milled stock and so was much less disastrous.  

I spent quite a bit of time experimenting with different designs before settling on these profiles.  

After routing roundovers on the stand's components, I rabbet-jointed them together with glue; I did not want any hardware or dowels showing.  Next, I cut two knees for each end to add strength to the joints.  

Here you can see the knees being glued in place.  

In the end, the joints were plenty strong enough and I am very pleased with the overall appearance of the stand.  

Notice that the left-hand end of the stand, for the nut-end of the rotola and over which the bow will rest, has a longer squared end than the right (below).  This is because, at the time, I was still considering anchoring the bow at the left end and making a hinge so that the bow -- and presumably some weight -- could rest on the string and not require a player to manipulate it; the hinge would connect to the stand somewhere along that straight edge.  I have since changed my mind on that bow design, but I still like the asymmetry.  

The right-hand end of the stand.  

Again, I neglected to photograph the finishing process, but the results will be seen in later posts when the instrument is fully assembled; it looks pretty good.  

Next post I will cover the fabrication of the crank and the bow -- and then it will be done!