[step-manufacturing] Minutes of Conference Call #11

[Martin Hardwick]

Attendees
Martin Hardwick, STEP Tools, USA
Mikael Hedlind, KTH, Sweden
David Loffredo, STEP Tools, USA
Fred Proctor, NIST, USA
Bengt Olsson, Sandvik, Sweden
Fiona Zhao, NIST, USA/New Zealand
Leon Xu, Boeing, USA
Erik Jacobson, GE Energy, USA*
*Ronnie Fesperman, NIST, USA
Stephen Newman, University of Bath, UK
Michael Mariani, IQL, USA
Andreas Archenti, KTH, Sweden
Mike Fair, Siemens, USA
Thomas Charlton, ASME, USA

We reviewed the minutes of the last conference call. The stacked 
Homogeneous Transformations Matrices are stacked by error type, not 
axis. Each matrix estimates a different type of error with respect to 
positioning, straightness or some other criteria. The matrices are 
stacked in the sense that they are applied in series to the nominal 
position and location of the tool to determine a predicted position and 
location. Each nominal position has its own matrix stack with 
interpolation used to fill in for positions that have not been measured.

We discussed methods to represent this data. There is an XML format in 
the B5.59 standards but the standard is a draft and the vendors have 
been making extensions.

We discussed how the data might be used by STEP-NC. There are several 
scenarios. In the one discussed a STEP-NC program is run on a machining 
simulator and the error prediction data is used to convert the nominal 
tool path points into actual points and a simulator (for volume removal 
or volume addition) predicts how material will actually be added to, or 
removed from, the workpiece on this machine. The final workpiece 
computed by the simulator is then compared to the STEP-NC workpiece and 
the GD&T information on the STEP-NC workpiece is validated by measuring 
the differences between its dimensions and those of the simulated workpiece.

We discussed how much machining error is due to errors in the machining 
program and how much is caused by other factors such as bending forces 
and thermal effects. The estimate is that on a good machine the 
machining program contributes about 40% of the errors and on a less good 
machine it contributes more.

We discussed methods for correcting or eliminating the errors. The first 
method being discussed is to develop a very accurate machining simulator 
that will predict the result of machining a part on a machine and so 
tell the user when a program is not suited to a particular machine. A 
second method is to monitor the machining very correctly, taking 
periodic measurements for critical tolerances and correcting the 
machining as necessary using closed loop programming. This was 
demonstrated at the Boeing Renton meeting in October. A third method 
which is a hybrid of the first two is to make experimental cuts during 
the machining, measure those cuts and use the results to correct the 
main machining program.

A recording of the call is on the ftp site at the address below. The 
next call will be at the usual times on Friday May 7.
ftp://www.steptools.com/private/Undecided/stepmanuf_telecon_20110422.wmv

Action Items
1. Try again to get copies of the ASME B5.59 draft standards.

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