Rotola: An Instrument Takes Shape

In my last post, I focused on finalizing the basic frame of the instrument:  two blocks at each end of an axle.  With that complete, and having given up on further refinement of the soundboard slats' radiusing, it was now time to attach the latter to the former.  This post will cover those steps.  

Before I could glue the pieces of the soundboard to the frame, a few things needed to happen to them.  First, they needed to be fit, i.e., cut to final width (or more properly, degrees of arc) and jointed along their long edges so as ultimately to create as unified a cylinder as possible.  Once that was done, the soundholes needed to be cut.  

Jointing was done in several steps.  First, one edge was selected as most straight; this was then made as perfectly straight as possible by either sanding or planing or both.  Given the thinness of the soundboard the wood was brought to the cutter, rather than the usual other way around:  for sanding, I used sandpaper attached to glass and, for planing, my #7 Stanley jointing plane was placed upside down in a vise and the piece of soundboard was moved along it.  Care was taken that the edge being jointed was perpendicular to the tangent of the cross sectional arc (or coplanar with the radius) so as to join as tightly as possible with the adjacent slat.  Hopefully, the below diagram clarifies this statement:

That done, a wheel marking gage was used to mark a parallel edge just a bit wider than the final width, e.g., given there were eight slats, each slat would be 22.5 degrees of the cylinder's circumference, so the edge would be marked at 23 or 23.5 degrees; translated into millimeters at the outer radius of the pinblock, this was something like 45mm on the wheel gage -- I don't recall exactly.  The excess would be planed down, then, if necessary, sanded to fit precisely with the adjacent slat with the same method as the first side.  Here I'm test fitting two pieces of the soundboard, taped to the frame:

Next, the eight soundholes needed to be cut.  This would normally be a tricky operation, made more difficult by the curve of the soundboard sections.  A typical way of doing this would be to drill holes into the center of each strip and then connect them by cutting with a coping saw or knife, risking splitting the thin and delicate spruce, requiring much clean up, and making it difficult to get them all identical.  Alternatively, one could make plunge cuts with a router or router table; this would require yet another jig -- probably a fairly elaborate one -- and, again, the curve of the soundboard would make it difficult to keep the soundboard stable during the operation.  

After much deliberation, I decided instead to route out half-holes on each side, which would then create soundholes between slats.  There were several advantages to this:  it would be a very simple operation on the router table and require only stops on the fence, rather than a jig, it would be simple to keep the soundholes neat, straight, and centered, since they would register to the precisely jointed edges rather than a curved face, and it would reduce the surface of the edges being glued (and thus save time during a tricky glue-up).  

I spent a fair bit of time testing with scraps and getting the table and fence set up and then proceeded cutting sixteen half-soundholes, two into each of the eight slats.  Rough off the router table, they looked like this:

As you can see, the routing left some rough edges, especially at the ends of each hole, but these were easily cleaned up with sandpaper.  Too, there were a couple of holes I screwed up, one in which the router bit climbed out of the intended hole and bit a little too far into the spruce and one where the bit chipped out the edge of a hole.  I managed to repair these passably well by cutting tiny bits of spruce, matching the grain as best I could, gluing them carefully into place and the filing and sanding them to the correct outlines of the soundhole.  

In the end, I was pleased with the overall results and, if I go with this soundboard and soundhole design again, will probably use this method to cut the soundholes.  It was a bit fiddly, but I think less so and much easier to make consistent than the two methods I described above.  

So, now I was finally ready to begin gluing up the body of the instrument.  I had numbered the strips to keep the jointed edges together:

Six of the eight are shown here.  Number one is off camera, either taped or already glued to the frame; number eight is yet to be finalized, as I decided that would be easiest to do once the rest are on.  (You may notice in the first pic above that the pinblock sections are numbered as well; they correspond with the slats.)

Gluing arc-cross-sectioned strips of wood to cylindrical hunks of wood using square-faced clamps was not easy.  I tried several configurations, one of which was this: 

Note how the two clamps on the right cross each other, each taking a basically diametrical orientation, which is the only way a flat clamp can get any purchase on a cylindrical surface.  The one on the left is serving to bolster the rope holding the soundboard down, as I could not get enough radial pressure with the rope alone; that experiment was not repeated as subsequent strips were glued.  In the end, I solved these problems by making custom, cork-lined clamping cauls (excuse the accidental alliteration); although I don't have pix of these glue-ups, the cauls show up some of the next steps.  Here is the instrument with more slats attached:

You can see the clamping cauls being used as supports there.  Next is a view of I believe the same stage (six of the slats glued) showing the soundholes:

The end result was not what I had hoped:  most of the slats had radii tighter than the cylinder they made up, which meant that, rather than being perfectly round, the tube of the instrument was slightly lumpy.  So, instead of having a cross section like this:

It's more like this:

This effect was further exaggerated by the need to scrape and/or sand some of the glue joints.  It is, however, largely an esthetic concern at this point.  Unquestionably, the unroundness will affect the sound of the instrument, but my goal here is to see if it will simply hold together and produce something like the sound I have in mind; presuming it does, I can experiment with the effects of structure on timbre on later iterations.  Presuming I do so, I have some ideas as to how to increase the chance of ending up with a nice, smooth, round cylinder, if that seems like the shape to pursue.  

One other outcome of this stage was learning something about the acoustics of a stopped, cylindrical soundboard.  The tap tones for the individual slats were decent; they were clear and reasonably sustained.  As I added each strip to the frame and glued them into an ever larger resonating surface, they retained this quality and I was increasingly excited about how the whole would sound.  However, after gluing in the final piece, the soundboard deadened dramatically; it is now much stiffer than I expected.  It has some resonance, but nothing like I had hoped.  In retrospect, this makes sense, but I'll share my speculations on that elsewhere.  Ultimately, this design -- a capped, tubular resonating body (as opposed to an open or stopped-at-one-end chamber, like a marimba) -- may not work.  I expect to experiment further with it later on.  

With the body of the instrument assembled, it was time to drill the holes for the tuning pins.  My original design (along with being octagonal and having both the soundholes and the bow at the other end) had only eight strings.  As a reminder:

Back when I was assembling the pinblock, I realized I had grossly overestimated the space I needed for the pins.  My ultimate vision for this instrument is of something much larger, with, say, 30 or 40 strings; being able to put more strings on this prototype would allow for a better proof-of-concept, so I decided to set it up for 16 strings rather than the original eight.  

After doing the layout for drilling the holes (which can be seen in one of the pix below), I needed to create a drilling jig.  Not only do the pinholes need to be perfectly radial (i.e., run from the surface of the cylinder exactly toward its center), but they also need to be tilted a bit along the longitudinal axis to counter the pull of the string; I decided, more or less arbitrarily, on five degrees.  (You can see this tilt in the pic above of the original design.)  To accomplish this, I repurposed the clamping cauls and set up a jig on the drill press thus:

The carriage bolts secure the jig solidly to the drill press table and the extension helps stabilize it.  In the next pic, it's easier to see the five degree rake.  

Setting up and doing the actual drilling was quite nerve wracking:  if I screwed this up, I'd have to scrap what I'd built so far and start over and, having spent more than two years on this project already, that would be a bad thing.  So, following the maker's adage "measure twice, cut once" -- or in this case, measure many times -- I forged ahead with a trial hole, which, much to my relief, came out exactly as planned.  Testing it with a pin: 

You can see the layout marks for the pinholes here; "x" marks the spot!  The first of sixteen holes being successful, I plunged on (sic) and completed them.  

The next step is to create, assemble, and attach the nut:

The nut also has had some significant redesign, but I will cover that in the next post.