Good questions.  I was wondering when they would be asked.

> I'm looking at the facing curve spreadsheet that you have under 
> the Files section (FacingCurve.xls), and I have a few questions:
> 
> 1) R - what does this represent? (The Radius of the curve?)

Yes.

> 2) M - not sure what it stands for, but it's obviously the facing 
> length

I'm not sure why I called it M.  Max facing length I guess.  I was already
using F for Feeler.  This is the where the curve meets the flat table.  I
think a few manufactures may call this the facing length (Vandoren for
one).  But most call the facing length the length to the the .0015 feeler.

> 3) Spec (specification? What is this, and what does it represent?)

You will occasionally find measurements on the web or elsewhere.  There
were a few on alt.music.sax post by Jon Van Wie for a popular Meyer and
Link facing he used.  So in the spreadsheet, this is a target spec for
fitting the curve through to see how close you can get with a pure radial
curve.  If you can not get a curve to fit well through all your points, you
need to decide if the disagreement is a defect in the facing or a feature
you wish to duplicate.

> 4) Diff - difference? between what?

This is just an aid to fitting the curve.  Minimize the Diff between the
calculated curve and the target "spec" for the best fit.

> 5) L Inc - I think I have this one: Length Increment, correct?

Again, just an aid to illustrate the spacing of your chosen set of feeler
gages.  

You can customize this spreadsheet to your liking.  The one I use is much
more elaborate, but would be confusing to present and support.

> 
> Questions on Usage:
> "Adjust M until L at Tip = 0".
> I'm not sure what M is? Is this spreadsheet to be used for plotting a 
> facing that I've measured from a mouthpiece, or is it for determining 
> the feeler gauge points for a facing curve that I want to apply to a 
> mouthpiece?
> 
> In other words, let's say I want a facing length of 50 (on the glass 
> gauge, which is 25 mm, correct?), with a tip opening of .110.
> 
> How do I use the spreadsheet? I adjusted M until L at Tip was zero 
> (Length was 58.2 - seems rather too long)
> 
> Then I adjusted R to minimize the Error Sum - 7.33 gives an Error Sum 
> of 0 (though I really don't know what this is).
> 
> But now my L at Tip is -6.1. Unless I adjust M again, but then the 
> Error Sum is off again.

You are on the right track.  I did not specifically say repeat steps 1 and
2 until both goals are achieved.  So after you change R you should work on
M again, then R, then M, etc.  Stop when your changes become very small. 
There are powerful and useful built-in solver features in Excel, but you
need to walk before you can run.

Also, the goal of finding M that gives you L=0 at the tip gives your target
tip opening at zero on the glass gage.  This means your tip opening as
measured at the very tip of the mouthpiece, outside the tip rail.  If you
would like to work towards a target tip opening as measured inside the tip
rail, this is a little more complicated.  Now what you are saying is you do
not want your target tip opening to occur at L=0 on the glass gage.  You
want it to occur at a small positive number away from zero.  This number is
the width of your target tip rail as expressed in glass gage units.  So if
you would like a tip rail thickness of .030", this is equal to ~.76 mm or
~1.5 on the glass gage.  So now you need to adjust M until L=1.5 at F=Tip
Opening.  (Tilt?)  

Except for F=0 and F=Tip Opening, F is equal to your set of feeler gages. 
For refacing work, you can just ignore the F=0 row.  Its just there to
illustrate that M=L at F=0.  At F=Tip Opening, you use a wand, caliper or
whatever tool you have that works for you to measure the tip opening.  
  
> 
> Obviously, I don't know what I'm doing. Are there instructions posted 
> somewhere? If this has already been covered in an old post, please 
> forgive me, and point me in the right direction.
> 

I posted some notes previously, but the above is more detailed on usage.

I recently did another derivation and came up with a slightly simpler
formula (no trig functions), but the one in the original spreadsheet works fine.

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