Will somebody remind where a face coordinate is zero please. 
Is it at the outer or inner tip rail edge or in-between? 
   I've seeing a tip rail 0.8 mm wide and inclined over 10 degrees. 
Measurement differs on 0.8mm * sin(10 degrees) = 0.14 mm 
or 0.005" depending on what zero is assigned to.
   Experienced CNC person was skeptical how it is possible to measure 
the elevation of the outer edge accurately. The inner edge location 
moves during refacing but may be returned back to the 'design' 
position later, during the chamber work.
I am sure that there is some convention regarding that but 
could not find it in the archive.  
   Thanks in advance for comments.

PS. Is the word LAYMAN good as a refacer's nickname?  

When measuring facing lengths, the glass gage zero is the very tip of the
mouthpiece.  So, when you reshape the tip to match a reed, or thin a tip
rail, you are shifting the length zero. This usually helps the refacer
because it common to have a facing length that creeps beyond your target
while working a mouthpiece.

For tip openings, the zero plane is the reed table.  The tip opening is
most commonly measured up from the plane to the inside of the tip rail.  A
few makers seem to have specs to the very tip (Vandoren for one, I think). 
But most gages and wands "find" the inside edge of the tip rail.  

Yes, this is a moving target depending on how you approach your work.  I
try to measure the thickness of of the tip rail each time I measure the tip
opening.  For me, it is an XY coodinate that needs to lie upon the facing
curve target I have calculated.  


*******************
From Dictionary.com:

lay�man  n. 
A man who is not a cleric. 
A man who is a nonprofessional: His is just the layman's view of medicine. 

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Thanks Keith.

Mistakenly I used to plot tip opening value over zero coordinat.

As I understand the facing copying process now, if the tip rail width 
is unknown, it must be assumed before the work starts. Given data 
random errors must be analysed and corrected accordingly to refacer's 
sixth sense.

So the curve-making-helping fixture must be rather manual and have 
real time control of tip opening and facing length/profile .

Is a table refinishing procedure treaky? I've dragged the MP over a 
sand paper over 3/8" thick acrilic glass and got a convex shape with 
0.004" elevation in the middle. I did not try more rigid base and 
wondering if it is a big deal to have a table completed before MP is 
installed in a curve-making-helping-fixture.

Thanks to all
Dmitri Axakov
  

I think this is tricky.  The flatness of the working surface is not the
most important thing.  It becomes more important as you get to the final
flatness.

The way you hold and draw the piece over the sandpaper is most important. 
You need to fight the tendency of the heel of the table to dig into the
sandpaper as you draw the mouthpiece heel first.  

You can try drawing it the other way, tip first, but there will be similar
forces to balance.  If you do a combination of both, you have twice the
chance of getting a convex table.

I have on occassion resorted to sanding down the convex high spot with
small strips of sandpaper to make it a low spot.  This will create 2 high
spots on the table, one on either side of the original high spot.  These
two points can now rest on the sand paper as you bring them down to create
a flat table.  It is a lot more stable than working with one high spot.  I
mostly do this on SS Bergs.

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Sand paper digs better by clean granes, they get saturated on the way 
and higher cristalls may be lost. And any sandpaper is wavy. All of 
it must contribute to the table convexity as well. I know you know 
it. 
I am planing a trip into a glass shop.
I want thick flat frosted glass scrap piece to pour abrasive 
suspension on it. But it is known fact: copper tube and abrasive are 
used for a glass drilling. Softer material grabs sharp particles and 
digs a glass. I want to try it with a mild abrasive or maybe with a 
water without the abrasive. Would it be a good idea?
Dmitri



<kwbradbury@y...> wrote:
> I think this is tricky.  The flatness of the working surface is not 
the
> most important thing.  It becomes more important as you get to the 
final
> flatness...

For table flattening, and a lot of my facing work, I use Mylar
adheasive-backed sandpaper on 3/8" thick glass:

3M Micro Abrasive Film for Scary Sharpening - PSA , 15 and 40 mu

http://www.toolsforworkingwood.com/Merchant/merchant.mvc?Screen=PROD&Store_Code=toolshop&Product_Code=ST-MAF.XX&Category_Code=THS






		
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 Thanks !!