Rotola: Compromises and Progress

My last post about the rotola focused on the production of the cross-wise bent strips of spruce that would ultimately comprise the cylindrical soundboard.  Of the 14 months since that post, 10 were spent in frustrating, off-again-on-again experiments trying to get the right radius in the strips without cracking them.  After gaining some confidence with scrap pine, the Sitka spruce that I had been husbanding for the "final" steaming ended up with more strips cracked than not.  Swallowing my pride and scratching my head, but with undimmed determination, I brainstormed with a luthier at the timber supply company I like to go to about new approaches.  We decided to try thinner stock (the original stock was 4mm; the new is 3mm), which helped with the cracking, but now my molds were too tightly radiused (the thinner stock rebounded less after drying).  My near obsession with getting the radius perfect became a running gag in my family and, eventually, I found that even identically milled and carefully steamed and molded strips varied too much from one to the next to produce a consistent radius using steam bending alone.  

In the end, I threw up my hands and used a heat gun to get each strip to something close to right radius.  As most of them were too tight, flattening them with heat and a little water worked to a degree -- although I snapped a few from too much confidence and too little heat and moisture.  When all was said and done, I could only get a few of them to exactly the right radius, so the cylinder I was ultimately able to make was far from perfect.  As I have said, though, this is a prototype and, as such, I don't expect it to be perfect (even if I want it to be).  

But I'm getting ahead of myself.  Let me pick up at the point where I decided to go with the strips I had.  Next step was to create an axle.  Just as a reminder:

(This original CAD workup was octagonal in cross-section; the current version is cylindrical, but the location and function of the axle is the same.)

Unfortunately, I did not photograph any of my initial work on this part (spaced it), but I will describe it.  The axle needs to extend far enough from the ends of the cylinder to allow for a support at each and a crank handle at one.  Obviously, it needs to be round (cylindrical) at the points of support, but it does not have to be so within the instrument.  In fact, I realized that the shoulders of a square cross section could help relieve some of the compression stress on the soundboard by allowing the pinblock and endblock to rest against them (if this is not clear, I do have pix of this aspect below).  Further, as the original mahogany dowel I had purchased to serve as the axle was not sufficiently straight, I decided I needed to make my own from scratch.  

After thinking about what type of wood would best suit as the axle, I settled on ash for its strength and straightness of grain and purchased a bit from a nearby timber supply company.  I started with some oak offcuts to practice my cutting and planing on and was able to produce some satisfactorily square cross sectioned blanks, then made the final from the ash.  Next was to cylinderize the ends.  Again, practicing on the oak blanks, I used a quarter round bit on the router table for this; once I had my setup right, I ran the ash through.  This process went surprisingly well, which was a great relief in the context of my struggles with the soundboard.  This was my end result:

Here is a shot of the major parts laid out together:

Note the endblock blank on the left still needs to be cut to a circle and both it and the pinblock need to have center holes cut to the correct diameter.  The soundboard strips have been taped together to get a rough sense of how they will join, but have not been properly fit.  

A close up of the endblock:

...and the pinblock:

Note the pencil marks aligned with the inside edge of each block; those represent the shoulders to be cut against which each block will eventually rest.  

Next step was to round off the endblock blank and to cut the center holes in it and the pinblock.  Again, I didn't get pix of cutting the endblock to a circle, but I used the same jig that I used for the pinblock (see the top of the last post).  That done, cutting the center holes had to be extremely precise, lest I build in a wobble.*  The process was very fiddly and I ended up getting absorbed in it and forgetting to photograph it, as well (I will, however, show in an upcoming post the very similar process of cutting the center hole for the nut).  Here are the results:

Endblock: 

...and pinblock: 

I was very happy with these results; despite my post hoc approach, the enlarged holes were concentric with the blocks' outer circumference. 

Next was to cut the shoulders of the axle.  Here you can see the slope left by the router cutter and the nicks I cut with the ryoba saw to mark the shoulders:  

I then chiseled in the shoulders roughly:

And:

Followed by more detailed chiseling:

I was then able to use the endblock and pinblock to finalize the shape for a nice, sufficiently tight fit:

Honestly, I'm pretty proud of the pinblock side.  

Next, I glued up these three parts to create the dumbbell structure to which the soundboard will be attached:

In order to keep these updates in bit-sized (or at least single-serving) pieces, I'll put the subsequent steps -- fitting, shaping, and gluing the soundboard, drilling the pin holes, and constructing and shaping the nut -- in the next post.  

* If I make more of these in the future, I will create the center holes first, then round of the circumference to guarantee concentricity, but the nature of this experiment meant that I ended up building the pinblock, endblock, and nut more or less from the outside in.