Effects of thread wrapping, Series 2:

Original bore shape



In the first article in this series, we met the flute we are going to be working with, and saw the distorted shape of its bore as found.  In this article, we look for evidence of what it looked like originally.  This will be your opportunity to exercise some imagination and indulge in some detective work.

What should the bore look like?

That question is not immediately answerable, but we can put some limits on the possibilities.  Firstly, let's remind ourselves what the top tenon looked like in our original survey:

Our flute is the orange trace in the graph above.  Just looking at the trace, we might think that flattening out the trace over the region X=0 to X=26mm is all that's called for.  Keep that in mind as we look at the bigger picture.

Now let's look at the bore of the whole flute, as we saw in the Introduction.  Except this time, I've added a few guesses as to what the original bore might have looked like.  It's our job now to decide which of these have the most merit, or to suggest other possible original scenarios.

Scenario 1 - This is the original bore, silly. 

In scenario 1, we deny that any compression has taken place.  We regard the bulge around X=50mm as a product of deliberate chambering, remembering Rockstro's quote:

Much was, however, done by actual experiment to improve the defective notes of the older instruments by "chambering", or slightly enlarging, the bore at certain places.  Some of the flutes made by the old firm of Rudall & Rose were marvels of ingenuity in this respect.

Except of course we have no reason to believe Potter employed chambering.  We also don't really know what Rockstro meant by chambering.  Did "slightly enlarging" mean that the bore at the particular point actually became larger than somewhere upstream, as it has been interpreted in recent times, or simply became a bit larger than the prevailing taper might have suggested.  I don't believe we know.

Scenario 2 - Just flatten the bent tenons

In this scenario, we look at the effect of flattening the squashed tenons, as we discussed further up.  The thin purple, brown and dark green lines on the three tenons above show what would happen if we did that.  What do you think?  Leaves some pretty big dips in the bore, and some pretty large discontinuities between the bores of adjacent sections.

Scenario 3 - Two slopes in the Left Hand section (orange)

This scenario, shown in orange, accepts the idea that the bore was conceived as having two slopes, meeting at the high point at about X=140, and conforming to the current bore as far as 160mm.  It accepts the notion that the maker used more than one reamer, or a complex reamer to make the two-slope bore, and had reasons to introduce that additional complexity.  The top tenon will need to be expanded considerably from its current condition to conform to the upper slope.  The lower tenon needs to be expanded even more dramatically, in order for the second slope to merge with the generality of the RH bore. 

You'll notice I haven't chosen to aim it at the start of the RH bore, because of the compression that bore seems to have suffered there. Instead, I've aimed to pick up the unaffected bore taper from around 235 to 290mm.

Scenario 4 - Join the flat bits (red)

In this scenario, in thin red, we seek out those parts of the total bore that seem to have a chance of lining up.  We allow that some parts of the LH section might have bulged outwards, perhaps in reaction to the constriction at the ends.

Given how well the red line fits some parts of the LH, and most of the RH and foot, it's tempting perhaps to think that it was the original cut, with a spot of chambering applied to create an enlargement between around X=100 to 160mm.  But we don't know if Potter (or anyone else other than allegedly Rudall & Rose) indulged in chambering. 

Scenario 5 - Join the head and foot (aqua)

If we were feeling particularly bold, we might argue that the flute originally had a single taper, joining the end of the headjoint bore with the foot RH section.  I've illustrated that on the graph with a thin line in aqua.  Note that I've taken it from the original location of the top end of the LH section, before the first 13mm was cut off in the late 19th century. 

The severe crack in the barrel of the Potter reminds us that the wood has shrunk around the tuning slides by about 1.5%.  So, presumably, it has shrunk approximately equally everywhere else.  The head bore cannot shrink however, as it is lined in metal.  If it did, we might expect it to drop to about 18.6mm.  So I've started the aqua line at 18.6mm to allow for the same rate of shrinkage that appears to have occurred to the barrel.

Scenario 6 - Ignore the shrinkage

This most ambitions scenario is the same as Scenario 5, excepting we choose to reject the notion of allowing for the barrel wood shrinking.   We draw a line (in violet) that joins the head bore to the foot bore.  On the plus side, it now takes in the widest section of the original LH bore.  But it skims over the head of the RH bore, even more than Scenario 5 did.

This Scenario accepts the notion that the thread band not only causes constriction under the tenon, but as far along as 140mm from the top, and as far back as 170mm from the bottom of the LH section.  Leaving only 30mm of original diameter bore!

Scenario 7 - Other, please specify

So, now it's your turn.  Hold a piece of paper up to the screen, below or above the blue trace, whichever obscures the least.  Or print out the graph and use a pencil and rule.  Look for a more likely scenario than any I have presented so far.  And let me know which scenario you support!

Don't get too excited?

Now, we don't know that Potter et al had the capacity to make a conical bore as straight as our scenarios demand.  Dr. Robert Bigio argues that reamers even into the mid 19th century were hand forged by blacksmiths, and therefore restricted to simple tapers, and limited in precision.  Helmholtz, writing back in 1843, seems more confident:

"Thus the carpenter employs only the rimer [reamer], which is a fluted tool like the generality of his bits, as shown in Fig 518 so as to act like a paring tool."  [His image reveals that the rimer has a "(" cross section, sharpened along the edge.]  "Flutes and clarionets [period spelling] are first perforated [bored] with the nose-bit [shell-augur], and then broached with taper holes by means of tools of this kind, which are very carefully graduated as to their dimensions."

Unfortunately, that's all he tells us, so we are left to interpret "very carefully graduated" according to our prejudices.  Given that Helmholtz is a man demanding of considerable precision in his ornamental turning engines, perhaps we should take him pretty seriously.

Even if Potter could make a long straight tapered bore, we know that wood moves with time, especially boxwood as this flute is made of, so we could be seeing a lot of other errors that have crept in over time.  Against all that, some parts of the bore do line up very well, which seems more than coincidence.  Anyway, let's keep in mind that maybe we're just trying to reading too much into it.  But that's not an excuse to throw our collective hands up in the air, shrug our collective shoulders and exclaim collectively that "it's all too hard!".  If we are going to cajole Mr Potter's Patent flute back into shape, we're going to have to know what that shape was, or at least know when we have got there.

Welcome to my world!

This is probably the first time you've been confronted with the grim reality of what lurks inside a period flute, and asked to make sense of it.  But don't think we're the first to venture there.  Flute makers have to do this every time they draw a new model from a period instrument, although perhaps not always in such detail.  Welcome to my world!

And flute makers not always coming to the same conclusions.  We see out there the full gamut of possible reactions, from the "single taper" adherents to the "heavily chambered" enthusiasts and everything in between.  We can't all be right!

Conclusion, so far...

We've attempted to fit a number of scenarios to the reality presented by the Potter flute.  All of them help us see just how distorted the bore has become due to the compression by the thread band over the two centuries since the flute was made.  But which of them is the most likely, or is there another more plausible scenario I've missed?  It's over to you.  Get in touch!

  Created 21 April 2011