Probe System Determines the Valid Evaluation Method
The correct center/midpoint calculation on a coordinate measuring machine (CMM) depends first on whether a scanning or a non-scanning (single-point) probe system is used. The GX (maximum inscribed / Pferch) and GN (minimum circumscribed / Hüll) methods require filtering and outlier elimination, which is only reliable with a large number of measured points — i.e., a scanning probe. With single-point probing, GX/GN cannot be applied reliably because there are too few points for meaningful filtering.
Single-Point Probing: Use Gauss Elements (GG)
When measuring with individual points, always use Gauss (least-squares) elements, designated GG. Gauss evaluation is stable and largely insensitive to outliers caused by dirt or similar surface contamination. Because single-point strategies lack the point density needed for outlier elimination, the robustness of GG makes it the only dependable choice in that configuration.
Envelope Requirement (E) on the Drawing
If the drawing carries the (E) envelope-condition symbol, it prescribes envelope-based evaluation for size: use GX (inscribed/Pferch element) for internal features and GN (circumscribed/Hüll element) for external features when determining a diameter. For position evaluation, GX/GN should be used wherever possible — but again only with a scanning probe system, since the filtering requirement still applies.
Measure Bores as Cylinders, Not Circles
GX/GN evaluation only delivers its full value when bores are measured as cylinders rather than as individual circles. Inscribed or circumscribed circle elements are a poor compromise; scanning the bore as a cylinder provides the point count and geometric coverage that make GX/GN results meaningful for both diameter and position.
FAQ
Which CMM evaluation method should I use with single-point probing?
Use Gauss (GG) elements. With single points there is no reliable filtering or outlier elimination, and GG is stable against outliers from dirt or contamination.
What does the (E) symbol on a drawing require for CMM evaluation?
It prescribes the envelope condition: GX (inscribed) elements for internal features and GN (circumscribed) elements for external features when determining diameter.
When can GX/GN methods be used reliably on a CMM?
Only with a scanning probe system, because GX/GN need many points for filtering and outlier elimination. Bores should be measured as cylinders, not circles.