Why “Nominal from Part” Is a Critical Mistake in CMM Programming

In 3D metrology, one of the most common — and most dangerous — shortcuts is creating nominal geometry directly from a real part.
This technique, often called “nominal from part,” may seem simple and fast, but it can completely compromise measurement accuracy, repeatability, and compliance with ISO GPS and GD&T requirements.

In this article, you’ll learn:

what “nominal from part” means
why it is risky
how it affects measurement and production
what ISO/ASME standards require
what you should do instead

What Does “Nominal from Part” Mean?

“Nominal from part” means creating planes, lines, circles, or datums by clicking directly on the surfaces of a real component, instead of using:

the CAD model
the technical drawing
GD&T definitions
ISO 1101 / ISO 5459 / ISO 8015 rules

It may look like a time-saving shortcut, but it introduces significant measurement errors.

1️⃣ Your Program Becomes Valid for Only One Part

If you create nominal geometry from a real part, your CMM program becomes dependent on the shape and manufacturing errors of that specific component.

The next parts will have:

different inclinations
different curvature
different machining deviations
different deformations

➡ Result: inconsistent and non-repeatable measurements.

2️⃣ Manufacturing Errors Become Your “Design Intent”

This is the most dangerous outcome.

If the surface of the part is:

tilted
deformed
warped
shifted

…all these defects become the “reference geometry” in your CMM program.

You no longer measure parts against the drawing —
you measure them against the defects of the part used during programming.

This contradicts ISO GPS and ASME GD&T at a fundamental level.

3️⃣ Circles Become Ellipses and Geometry Is Corrupted

If the nominal plane or surface is incorrect:

reconstruction vectors are wrong
circles are measured as ellipses
axes shift
position tolerances become unreliable
you get false NOK or false OK results

This directly affects the quality and credibility of the measurement report.

4️⃣ Collisions and Path Errors Increase

Incorrect nominals lead to:

wrong approach vectors
probe collisions
unstable path generation
missing points
unpredictable movements

A robust CMM program needs stable, standard-based nominal geometry.

5️⃣ Total Loss of Traceability — Major Risk in Audits

Audits such as:

IATF 16949
PPAP
VDA 6.5
customer audits

require proof of:

what was measured
what datum system was used
which GD&T definitions apply
alignment with ISO 8015 and ISO 1101

With nominal-from-part, you cannot prove any of these.
It is considered a major non-conformity.

What You MUST Do Instead

✔ Always use the CAD model or the drawing

This is the only correct source of nominal geometry.

✔ Follow ISO 5459 for datum definition

Datums must not be created from real parts.

✔ Use proper alignment strategies

RPS alignment
ISO-based datum systems
Best-fit (when allowed)
Constrained alignments for flexible parts (ISO 10579)

✔ If CAD is missing — request it

Never program a CMM based only on real parts.

Conclusion

A real part is for measurement, not for defining geometry.
Nominal-from-part is a risky practice that compromises:

repeatability
accuracy
program stability
traceability
ISO/ASME compliance

If you want professional, consistent, and audit-proof measurement:
Nominals must come from the drawing. Always.