1. Standards-Based Measurement Requirement

Per ASME Y14.5-2018, the basic position and orientation of a threaded hole are defined relative to the functional pitch diameter axis of the internal thread. Inspection methods must therefore establish a datum axis that coincides with the pitch cylinder generated by the mating fastener. Any method that does not reference this functional geometry introduces systematic bias into positional and angular evaluations.

2. Why Minor Diameter Probing Is Insufficient

The minor diameter of an internal thread is not a controlled functional surface. Probing it on a CMM yields an axis that is influenced by:

• Circle form error
• Difficulty in picking up minor diameter
• Lead-in chamfers and surface irregularities

As a result, the axis derived from the minor diameter does not reliably represent the pitch diameter axis required for ASME Y14.5-2018 compliance and should not be used to evaluate either true position or orientation.

3. Solid Thread Form Gaging as the Correct Functional Reference

The Precision JO-PLUG® axis allows full engagement of the thread flanks at the pitch diameter, which is the true functional axis. When measured on a CMM, this provides:

• A stable and repeatable geometric axis
• Direct correlation to the mating fastener
• A valid reference for evaluating both basic position and orientation

4. Tru-Pos Locators and Flex Plugs: Functional but Limited by Gaging Length

Tru-Pos locators incorporate a solid thread form and therefore reference the pitch diameter correctly. However, they are typically manufactured with a very short gaging or shank length, (.375). This short engagement length limits their ability to establish a reliable axis over length, making them well-suited for determining true position at the entry plane only, but not for evaluating:

• Angularity
• Perpendicularity
• Orientation of the thread axis relative to datums

Similarly, flex plugs also have a short effective gaging length, (.375) and rely on elastic deformation to contact the thread form. While they can indicate approximate location, their compliance and limited engagement length result in:

• Reduced axis stability
• Increased sensitivity to probing force
• Lower repeatability for orientation measurement

Therefore, both Tru-Pos locators and flex plugs are best categorized as true position tools, not full GD&T verification tools for angularity, coaxiality and perpendicularity.

5. JO-PLUG® Solid Thread Gages: Full-Length, Rigid Axis Control

Precision JO-PLUG® tapped hole location gages are manufactured with:

• A solid, full-form thread
• A long, controlled gaging length coaxial to the pitch diameter
• A rigid cylindrical coaxial reference section for stable CMM probing

This geometry allows the CMM to establish a true pitch-diameter-based axis that is valid for both:
✔  Basic Position
✔  Basic Orientation (angularity, parallelism, perpendicularity)

6. Calibration, Traceability, and Accreditation

All JO-PLUG® pitch cylinders are calibrated in an ISO/IEC 17025:2017 Accredited facility. With every order, Spence Industries provides at no additional cost:

• A Certificate of Conformance
• Traceability to NIST
• Compliance with ISO/IEC 17025:2017 – A2LA Certification # 3692.01

• Compliance with ANSI/NCSL Z540-1-1994 and ANSI/ASME B1.2-1991 and B1.16M-1984.

This ensures:

• Documented measurement uncertainty
• Full traceability to national standards
• Defensible inspection results in regulated industries

7. Metrological Implications for CMM Inspection

For CMM measurement systems, axis definition quality directly affects:

• Basic Position results
• Basic Orientation evaluation
• Measurement repeatability and reproducibility (R&R)

Short stem length tools introduce tilt uncertainty. Precision JO-PLUG® threaded hole location gages eliminate this by providing a solid, calibrated pitch-diameter-derived axis coaxial to our industry leading .625 stem length that reduces systematic measurement error.

8. Conclusion

From a technical and standards-based standpoint, threaded hole inspection per ASME Y14.5-2018 requires referencing the functional pitch diameter axis over sufficient length to define both Basic Position and Orientation.

• Tru-Pos locators: Solid thread form, but short gaging length, best for true position only
• Flex plugs: Short gaging length, non-compliant engagement, best for position screening only
• Precision JO-PLUG® gages offer a solid thread form, longer gaging length (.375 vs .625) and ISO 17025 Accreditation, NIST traceability and ANSI/NCSL Z540-1-1994 and ANSI/ASME B1.2-1991 and B1.16M-1984 compliance

Precision JO-PLUG® gages are therefore not simply an alternative—they are the correct metrological solution for high-integrity CMM inspection of threaded holes.