The CMM Used in This Test
The CMM used here differs in several ways from a conventional coordinate measuring machine. Here are the key characteristics:
Air-bearing slides — standard on most high-accuracy CMMs — are sensitive to dust and temperature fluctuation and require a dedicated controlled environment. Linear guide machines trade some of that sensitivity for robustness, enabling direct installation on the production floor. This is a practical, line-oriented design.
Profile Tolerance — A Quick Review
The drawing callout in this test requires the specified surface to fall within a tolerance zone 0.2 mm wide.
Fig. 02 — Profile tolerance zone: TEF (blue) and tolerance band (green)
TEF (Theoretical Exact Feature) = the ideal design form. The tolerance zone extends 0.1 mm on each side. All substitute points must fall within this zone.
What is TEF?
TEF stands for Theoretical Exact Feature — called "tef" in geometric tolerancing practice. It is the theoretically perfect form defined by the design. The tolerance zone is centered on the TEF and extends equally on both sides.
Profile of a surface vs. profile of a line — a critical distinction:
Key Terminology
| Term | ISO 17450 name | Meaning |
|---|---|---|
| Double max deviation | Value of Double Maximum Deviation function | Maximum deviation from the TEF × 2. Used when a tolerance applies to both sides of the feature. In practice: the reported machine value = max deviation from TEF × 2. |
| Max deviation | Value of Maximum Deviation function | The largest single deviation from the TEF (× 1). Simply: how far off is the worst point? |
| Substitute point | — | A point obtained by measurement. In geometric tolerance practice, each CMM-acquired point is called a substitute point — representing the actual surface at that location. |
| TEF | Theoretical Exact Feature | The theoretically perfect form defined by the design. The reference from which all deviations are measured. |
Why "double" maximum deviation?
A profile can deviate both outward and inward from the TEF. To express the full width of possible deviation, the largest single-side deviation is multiplied by 2 — giving the total "band width" of deviation. This is why a 0.2 mm tolerance is evaluated against the double maximum deviation value.
Result 1 — Deliberately Wavy Sample
The first sample was 3D-printed with an intentionally exaggerated wavy profile. Three scanning passes were performed (a scanning-type measurement sweep), capturing a large number of substitute points.
Fig. 03 — Non-conforming sample: wavy profile exceeding the tolerance zone
Substitute points from all three scan passes exceed the ±0.1 mm tolerance zone. The wavy deviation is visually clear in the data.
All three scan passes showed substitute points deviating up to 0.3 mm from the TEF — clearly exceeding the ±0.1 mm tolerance zone. The wavy shape was confirmed visually in the measurement data.
→ Exceeds limit
Tolerance: 0.2 mm → Clearly exceeds
Result 2 — High-Precision Sample
The second sample was printed at the maximum accuracy the 3D printer could achieve. Measured on the same CMM, all substitute points fell within the tolerance zone.
Fig. 04 — Conforming sample: all substitute points within the tolerance zone
The actual profile stays very close to the TEF. All substitute points comfortably within the ±0.1 mm tolerance zone.
All substitute points fell comfortably within the ±0.1 mm zone. A maximum deviation of just 0.037 mm — from a printer costing less than ¥100,000 — confirms the sample's suitability as a verification specimen, and reveals something about the printer's own capability as a side benefit.
→ Ample margin
Tolerance: 0.2 mm → Passes comfortably
A useful side finding
A sub-¥100,000 3D printer achieving a maximum deviation of 0.037 mm is a noteworthy result in itself. The verification exercise produced not only a geometric tolerance test but also a practical capability benchmark for the printer — useful data for future sample production decisions.
Summary
Cantilever + linear guide design enables CMM measurement without a dedicated metrology room — opening the door to inline quality management on the production floor.
Surface profile fixes one 3D tolerance zone across all scans. Line profile allows the zone to shift per scan — a significant difference in strictness that must be intentional.
Profile deviation is expressed as max deviation × 2 (ISO 17450 double maximum deviation). A 0.2 mm tolerance means the double max deviation must be ≤ 0.2 mm.
Future instalments in this series will cover profile tolerance verification on different measurement machines, and move on to other geometric tolerance symbols. The aim throughout: knowledge that is genuinely usable on the manufacturing floor.