Rotola: The Nut

So far, I've talked about the rotola's construction fairly idiosyncratically, discussing what I'm doing as I'm doing it, but, apart from sharing pix of the original design, I haven't contextualized what I've done within the instrument overall or within the principles of chordophones generally.  We're coming to a point where a little of that background might be useful.  

On most stringed instruments, the string runs between two anchor points that keep the string in tension and across two contact points that define the length of the resonating section of string.  Thus:

Note that the  contact points are by definition between the anchor points.  This is true of all stringed instruments in one way or another, with variations in how their different jobs get done.  

There can be some further specialization, however, within these jobs.  One of the anchor points will usually have a mechanism for adjusting the string tension and one of the contact points usually has the additional purpose of transferring the vibrations of the string to a resonating body.  We name these different points according to their roles:  

The anchor point responsible for adjusting the string tension may be called a tuner, tuning pin, tuning peg, etc.  The anchor point that holds down the other end of the string, usually immobile, is sometimes called an end pin (in some instruments, instead of a pin, there is a hole into which a thickened part of the string catches).  The contact point that transfers string vibrations to the resonating body is usually called the bridge and the contact point that holds the other end steady is the nut.  Most string instruments have some version of all of these, although sometimes a piece might do more than one job, as in the bowed psaltery, in which the end pin also serves as a nut.  The most important job of the nut is to provide a solid point against which the string can vibrate, i.e., by being a rigid surface, it inhibits the string's movement as minimally as possible.  (This property is necessary for the bridge, too, but I would say it's first job is to transfer sound to the body.)

Whether the bridge and the end pin are located at the same end of the instrument or not is usually dictated by the structure of the instrument and how it gets played.  In violin, viol, and guitar family instruments, for example, the tuners and the nut are usually near each other and the bridge and end pins/holes are together at the other end.  But it can be the other way around, which is the case for the rotola:

More precisely, per my initial design:

So, as you can tell from both the end of the previous post as well as the title of this one, I'll be talking here about the design and fabrication of the nut.  

Despite the octagonal cross-section seen in the above initial design, as discussed elsewhere, I decided as I began to build the instrument to aim for a cylindrical shape instead; thus, the nut will need to be circular as well.  However, the grain of the wood comprising the nut, in this case walnut, will need to run perpendicular to the strings it supports.  As no trees grow with the grain in a circle (growth rings don't count, as the fibers are still longitudinal), the nut will have to be assembled similarly to the pinblock -- what I came affectionately to call pizza slices, as I struggled to get yet again perfect 22.5° cuts.  Using a bandsaw, this is tricky:

The one on the left, you see the cuts are too shallow and on the right, too steep.  These are just two of my many attempts to get the jig set right using scrap wood.  In the end, I finally got one close enough:

You can still see some small gaps, artifacts of having to use the oscillating belt sander to get the angles just right, but I had wasted a good deal of wood already, and my tools and my skill with them were not going to let me get more precise than this anyway, so I glued it up:

Next step was to round it off, which entailed first circularizing the center hole and then using that as a pivot to do the same with the circumference.  

I spent quite a bit of time laying out and fashioning a jig that would allow me to center and hold the nut on the drill press; unfortunately, I neglected to capture that work, but here is a shot of the completed center hole with the nut correctly placed on the jig:

The slots in the frame allow the jig to be bolted to the drill press table; the alignment marks have complements on the table and those on the nut then line up with the jig.  Using this system, I was able to place the 3/4" blade bit precisely in the center of the inner octagon and make a perfect pivot hole.  It's amazing what good marking up can yield.  

Next was to make a jig for circularizing the outside of the octagon.  Using a scrap bit from a practice axle, I set up a way to hold the nut firmly in place on the table of the oscillating belt sander sufficiently precisely at the radius I wanted:

The long edge allowed me to keep the whole thing square, which was the key to controlling the radius (from the center pin to the sander face).  Thus:

I was very pleased with the end result:

In working with the walnut thus far (this was the first part of the rotola that used walnut), I began to realize that, while it is a relatively hard wood, it's not as dense as a nut really needs to be.  My original plan was that the strings would be supported directly by the nut, draping across its edge (as you can see in the third figure, the one labeling the various string supports), but I began to think that the walnut was just soft enough that it could have a dampening effect.  Given that the rotola's strings should be highly resonant in order to create the sound I aurelize, this was a risk I was unwilling to take.  

I played with several ideas for solutions, including remaking the nut with ebony or maple on its edges or even fashioning a new one out of those materials.  However, in the end, I took inspiration from the bowed psaltery, which has a slim bar of brass on its bridge.  Brass is hard enough to support the steel strings rigidly, malleable enough to work with in a woodshop, and looks good, too.  

Having procured some 1/8" brass rod from my local Big Box Home Store, the next step was to mount it onto the nut's edge, most simply by cutting a groove into it.  As with much of this project -- indeed lutherie in general -- this would require making a jig.  Fortunately, I needed only modify the one I'd made to circularize the nut, standing it on its edge and mounting it to the router table.  After some trial-and-error, I came up with this:

A v-bit in the router could be raised (via the black and yellow handled adjustment shaft on the left) into the edge of the nut, then the nut could be rotated on its axle, creating a fairly straight and even groove.  Both the axle and the router bit are seen better from the below angle (seen from the other side of the fence): 

I was pretty happy with the result; it's not perfect, but more than good enough for a proof-of-concept:

Next, I wanted to taper the outside edge, giving a smoother transition for the strings on the end pin side (outside) of the nut.  After more trial-and-error, I came up with a setup that would allow some consistency:  

The rounded nut (here a practice scrap) rests on the bench, with a bench dog through its center hole (serendipitously the same size as the axle), then a guide keeps the plane at the same distance -- and thus the same angle -- relative to the nut; counting strokes and rotating the nut the same number of degrees as I worked my way around helped me rough in a taper:

These large facets (seen below in the nut itself) were smoothed by making smaller facets in between:

These were then blended and rounded over with sandpaper.  

Next step was bending the brass rod.  I expected that, in order to get a good fit, the rod should be wrapped around a cylinder that was somewhat smaller in radius than that nut itself, to account for springback.  I made a form, which turned out to be too large, then a smaller one -- 

-- which was still too large.  Rather than continue to make circles out of wood, I decided to look for ready-made cylinders, like cans of finish:

-- which turned out to be just right.  

I got the brass rod to a rough fit, cut it, and then taped it to the nut to see a) how well it would sit in the groove on the edge of the nut and b) how much pressure from the strings would be required to get it to sit straight and evenly in the groove.  

You can see some of the wobbliness, especially in the lower of these pix.  Masking tape was capable of holding the brass down fairly strongly, but I'm unsure if the strings (even considering that there are 16 of them) will generate enough power to keep it firmly in its slot.  That said, I decided those were problems to resolve another time and I needed to keep going to finish the nut.  

The last fabrication operation with the nut would be drilling the holes for the end pins.  This would require some accurate marking out:

Here, you can see I started by marking out the radii at 1/16th circumference (or 22.5°) arcs.  I'm using the seams between pizza slices as references, which, in turn, I will match up with the seams in the soundboard slats; this will then allow the end pins to line up perfectly with the tuning pins, which were also drilled in with an analogous method at the other end of the instrument.  Then, returning the nut to its jig, I used a compass to draw circles at 15mm and 20mm from center.  These intersections I then marked to indicate alternating drilling points, like so:

The places with the hash marks (or crosses) are to be drilled.  I drilled a few test holes in some scrap to see which of the candidate pins would work best (I had several ideas:  brads, screws, etc.) and settled on brass-plated twist brads (no pix yet; to be revealed later).  To keep the alignment consistent, I returned to the edge circularizing jig (the one I used on the oscillating belt sander and the router table), now mounting it onto the drill press and tipping the drill press bed 5° to give the pins a tilt away from the direction of the pull of the strings.  After drilling the holes, I ended with this: 

And that is as far as I'm taking this blog post.  I have since cleaned up the markup pencil and begun pre-finish sanding on the nut.  Still remaining for the nut is to be glued to the body, which I'm holding off on only because I have this nagging feeling that I'll regret it if I'm too hasty, and for the brass rod to get a final fitting (i.e., hand-bending to get as nearly perfectly circular as possible) and possibly to be glued (epoxy? cyanoacrylate?) in place.  

Next post I plan to be about the bridges, which I have just begun to experiment with.  After that, there will be very little left of the complex, inventive parts of this build:  I'll need to make a support for the instrument (basically a couple of Ys to hold the axle), glue up whatever isn't yet glued and apply a finish (very likely shellac), put the strings on and tune it, and make a bow (the design of which is still unclear).