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Evaluation of CMM Fixture Effects on Diameter and Roundness

I haven’t posted anything related to engineering here for quite a while, so today I decided to write a new piece.

A. Main Technical Causes of Different Results

  1. Datum shift from seating differences:
    The curved fixture locates the part at different contact points than a flat surface, creating small rotations/translations that change the measured diameter.

  2. Form–datum interaction:
    The curved support follows surface irregularities and may hide actual tilt or coaxiality errors.

  3. Part deformation:
    Clamping or uneven support on the curved fixture can slightly deform the part and increase measured diameter/roundness.

  4. Lower repeatability:
    The curved fixture generally produces higher measurement noise than a stable machined flat.

  5. Different physical datum establishment:
    Even with identical evaluation settings, the coordinate frame differs because the part seats differently.

B. Short Validation Plan

Use same CMM, probe, environment, operator, and the original discrepant part.

  1. Repeatability Test

    • Measure the part 10× with Program #1 (curved fixture).
    • Measure 10× with Program #2 (flat fixture).
    • Compare mean, range, and standard deviation.

  2. Cross-Fixture Test
    Alternate measurements between Program #1 and Program #2 for 5 cycles to detect systematic offsets.

  3. Datum/Contact Mapping
    Record datum-establishing points for each fixture and compare the resulting datum shift.

  4. Seating & Deformation Check
    Compare “clamp,” “no-clamp,” and lightly supported conditions to detect tilt or deformation.

  5. Reference Artifact (optional)
    Measure a known master with both fixtures to isolate fixture bias.

Output: A one-page report summarizing the data and identifying which fixture is more stable and representative of the functional datum.

C. Practical Recommendations

  1. If customer requires Program #1:
    Follow Program #1, but document the variation and obtain written confirmation.

  2. If customer allows engineering judgment:
    Recommend Program #2 (flat fixture) for acceptance due to better repeatability and clearer datum definition.

  3. Long-term improvement:
    Redesign the curved fixture into a kinematic/datum-referenced support for consistent location.

D. Short Customer Email

Subject: Request for Direction — CMM Program #1 vs. Program #2

Dear [Customer],

We observed a measurable difference between your two CMM programs for the same diameter. Program #1 (curved fixture) reports a slightly oversize value, while Program #2 (flat fixture) reports the feature within tolerance. Both use identical evaluation parameters; the difference results from fixturing.

We propose a short validation to confirm whether this is a systematic fixturing bias. Pending results, our recommendation is to use the flat fixture for acceptance unless Program #1 is required for functional reasons.

Please advise whether you would like:

  1. Approval to perform the validation and follow our recommended fixturing, or
  2. Program #1 to be used as the official acceptance method.

Regards,
Fatmir Iliazi

E. Technical Attachment (Short Form)

  • Feature: Ø94.800 +0.000/–0.087 mm
  • Program #1: 94.8007 mm (slightly oversize)
  • Program #2: 94.786 mm (within spec)
  • Difference: 0.0147 mm (~28% of tolerance)
  • Cause: Different Z-support fixturing; no evaluation-parameter differences.

Recommendation: Confirm customer intent for Program #1 or approve use of Program #2 with supporting validation data.


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