13.2 Datum-Axis Simulation and Rotating Parts
Key Takeaways
- Runout is evaluated about the datum axis established by the specified datum feature simulator, so the inspection setup must reproduce the drawing-defined datum system.
- When two coaxial journals are identified as Y14.5-2009 multiple datum features establishing datum A, both are simulated together to derive one datum axis.
- Between-centers, V-block, collet, chuck, and arbor setups are not interchangeable by name alone; the chosen method must realize the applicable simulator and constrain the specified degrees of freedom.
- Datum-feature form participates in datum establishment under the standard; fixture, spindle, arbor, seating, and measurement errors are additional metrology errors that must not be misreported as part runout.
- A credible result requires calibrated equipment, correct seating, a setup matching the datum reference frame, and sufficient sampling of the controlled surface.
13.2 Datum-Axis Simulation and Rotating Parts
Quick Answer: The datum axis is not whichever axis the inspector finds convenient. It is established from the datum feature simulator specified by the drawing. When two journals are identified as Y14.5-2009 multiple datum features establishing datum A, both must participate in establishing that single axis.
Drawing Definition Before Fixture Choice
The datum feature is the imperfect feature on the part; the datum is the theoretically exact axis derived through the applicable simulator. Inspection equipment attempts to realize that simulator. The setup must therefore follow the datum reference frame and boundary conditions on the drawing before any indicator reading is interpreted.
For one cylindrical bore datum A, an appropriate precision arbor or expanding simulator may establish A. For two separated, nominally coaxial journals designated together as multiple datum features establishing A, both journals are contacted together so their combined simulation establishes one datum axis. This is not an A|B precedence sequence and is not an ordered A|B datum-precedence sequence.
Common Physical Methods
| Method | Typical use | Error to manage |
|---|---|---|
| Precision arbor or expanding mandrel | Internal cylindrical datum | Clearance, arbor form, seating, spindle runout |
| Collet or precision chuck | External cylindrical datum | Gripping distortion, jaw/collet error, seating |
| Two supports or purpose-built simulator | Two journals jointly establishing one axis | Support geometry, simultaneous contact, journal sampling |
| Between centers | Center features specified for the setup | Center condition, alignment, spindle error |
| CMM mathematical alignment | Accessible measured datum features | Sampling density, fitting rule, alignment implementation |
A generic V-block or chuck is not automatically the correct datum simulator. Its validity depends on the drawing's datum definition and whether the setup reproduces the required contacts and degrees of freedom.
Part Geometry Versus Measurement Error
Form and size of the specified datum feature legitimately influence the datum established under Y14.5; that is part of the requirement, not automatically a false reject. Separately, fixture runout, worn supports, arbor clearance, dirt, poor seating, indicator error, or an incorrect fitting algorithm can bias the measured result. Those are metrology errors and should be found with calibrated masters, repeat setups, and uncertainty checks.
Diagnostic Sequence
- Confirm the datum feature identification, order, and modifiers on the drawing.
- Confirm the fixture or mathematical alignment realizes that datum system.
- Clean and reseat the part, then repeat the measurement.
- Check fixture and spindle error with a calibrated master.
- Sample enough of both the datum features and controlled surface for the stated runout control.
The Senior-level distinction is crucial: do not subtract datum-feature form from a result merely because it is inconvenient, and do not accept fixture error as though the drawing allowed it.
A middle shaft diameter has 0.05 circular runout to datum A, where two separated bearing journals are identified as the multiple datum features establishing A. What must the inspection setup do?
Two coaxial bearing journals are identified as multiple datum features that jointly establish datum A. Which setup principle is correct?
Which practice best distinguishes fixture error from the part geometry included in a runout requirement?