We are blanking pieces out of niobium sheet using an automated XY
punch. We are encountering difficulty with registration, because the
sheets are stretching during the punching phase. As a result, the
first row of parts we punch are very accurate, but then the subsequent
rows are further and further off the mark.

Why does the sheet stretch? 
Is there a recommended punch pattern to eliminate, reduce, or
standardize stretching of the master sheet while blanking? This
relates to a computerized XY punch, and we want to make sure that we
can design coordinates so that it will punch it where we want it to
be.
Is there anything else you can recommend to eliminate, reduce, or
standardize strethcing of the master sheet while blanking?
If any, what is the relationship between the size and/or perimeter of
the blank to the stretch/deformation of the master sheet?
Our master sheet is 013 or 024 14" by 4" niobium.

http://www.rembar.com/niobium.htm

"High speed tooling with a proper lubricant will allow machining of niobium."

"However, note that tool wear is high and the cost of machining is
high in comparison to conventional metals. Tools will wear fast and
high rake angles should be maintained."

"Niobium may be machined using standard techniques. However, due to
the tendency of the material to gall, special attention needs to be
given to tool angles and lubrication.
Niobium also has a tendency to stick to tooling during metal forming
operations. To avoid this, specific lubricant and die material
combinations are required in high pressure forming operations."

(Also there is a table on this site with tooling specifications)

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http://www.espimetals.com/tech/machiningniobium.pdf

"Although carbide tools can be used, the tendency to gall is more
pronounced with them than with high-speed steel."

"The minimum surface speed of 80 feet per minute is important.  Slower
speeds will cause the metal to tear, particularly annealed stock."

"Beryllium copper, aluminum bronzes, and steel may be used for tools. 
The techniques can me the same as used for stamping steel.  The tools
should be polished to reduce the tendency to gall as much as possible.
 Light oils or similar lubricants should also be used, again to reduce
the chances of galling."

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http://www.cabot-corp.com/cws/businesses.nsf/CWSID/cwsBUS01232001113830AM345?OpenDocument&bc=Products+%26+Markets/Niobium/Application+Info&bcn=23/4294967167/3045&entry=product

(Similar info to espimetals.com)

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Hope some of the above links help.  The topic kind of interested me,
although I would have to say that more detail on your current
punching/stamping method would be needed to more directly deduct your
problem.

It seems that the over all key factors in punching this material is
tool speed (80 feet per minute minimum) and lubrication.  Also keep in
mind the quicker degradation of the tools being used, and how they
might also adversely affect the process.

From the lack of data I was able to find in my search I believe your
question too highly industry specific and also considered proprietary
information or know-how by the machine shops or manufacturers who
utilize niobium punching.

Best of luck.  Let us know if any of this resolves the problem.

-Twyl (My free $0.02)

The problem may also be that the metal heats up as more metal is
machined, causing expansion and misregistration.

     -welte-ga

Well, if you are punching the metal, there is no tool speed issue that
I know of.  Sounds like the sheets are too thin to maintain their
form.  You might try doubling or tripling up the sheets to add some
stiffness.  Alternately, if you are getting good punches in only the
first foot (or whatever length) of the sheet, you could simply cut the
sheets so they're only one foot long.

Good luck!