Understanding Guidelines for System Performance

By Dirk Steiner / April 26, 2021
The gold standard for 3D measurement equipment is the DIN EN ISO 10360 Geometrical product specification (GPS). It is designed to help compare different equipment and create a clear understanding of when a system is within the specification and when it is not. There two series of standards under the VDI/VDE umbrella that are applicable for CT:
 
  • VDI/VDE 2617 Accuracy of coordinate measuring machines
  • VDI/VDE 2630 Computed tomography in dimensional measurement
 
These crossover standards offer the guideline for the application of DIN EN ISO 10360 for coordinate measurement machines with CT-sensors. VDI/VDE 2630 part 1.3 provides three guidelines:
 
  1. Fundamentals: Definitions of characteristics (errors of probing, length measurement, size, and form) and limits
  2. Acceptance test: How to determine a CT system meets the limits of the characteristics
  3. Monitoring: Long-term compliance with the characteristics
 
The standard recommends the use of spheres or spherical caps made of a “suitable material”.  A common material is ruby since high-quality ruby spheres are commercially available and are thermally stable. The spheres by themselves allow for determining probing error form and size. The results are compared to the manufacturer’s limits and a compliance report can be created.
 
A common way to decide if the process is suitable for the application is to compare the tolerance and uncertainty. Here is a list of common standards used in the industry:
 
  • VDI/VDE 2630 Computed tomography in dimensional measurement, Part 2.1
  • Germany: VDA 5
  • USA: Measurement Systems Analysis (MSA, guide: ASTM E2782)
  • Gage repeatability and reproducibility (GR&R, standard practice: ASTM E177)
  • Company specific standards (mostly based on MSA: e.g. GM EMS, Ford EU 1880)
 Fig.1: Ruby sphere specimen

Part 2.1 of the VDI/VDE 2630 was developed for users of industrial CT systems. Part 1.3 was written so buyer and seller can agree when and how a CT system is performing to meet the specified characteristics. The specified MPEs (maximum permissible error) have nothing to do with a specific application. That’s where part 2.1 comes in play. It serves two purposes:
 
  1. To determine the uncertainty of measurement of specific features
  2. To give the user help deciding if the process is suitable to be used for the required tolerance of that feature
 
It is based on GUM and traceable. The core to determine is this formula for the expanded uncertainty of measurement.


 
Fig.2: Equation for the Uncertainty of Measurement
 
U: Expanded uncertainty of measurement
k: coverage factor (2 for 95%)
ucal: uncertainty of the calibration measurement
up: standard uncertainty of the process
uw: uncertainty of the workpiece
b: systematic deviation
 
To solve the equation several measurements must be run. The standard suggests having one sample calibrated with a method that is more accurate, normally done in a certified lab with a highly accurate CMM. The certified lab will provide the uncertainty of the measurement (ucal).