In manufacturing metrology and gauging practice, is the following statement correct: ‘‘A plug gauge is used to check the diameter (external size) of shafts and studs’’—or is a different gauge type intended for such external measurements?

Introduction / Context:

Understanding which gauge applies to internal versus external features is a foundational topic in engineering metrology. A plug gauge, a ring gauge, and a snap gauge are often confused because all of them appear in incoming inspection, shop-floor go/no-go checks, and statistical process control. This question validates the correct mapping between gauge type and part feature—specifically whether a plug gauge can be used to assess the diameter of shafts and studs, which are external (outside) features.

Given Data / Assumptions:

• Shafts and studs present an external cylindrical surface whose size must be verified.
• Plug gauges are solid cylindrical gauges with GO/NO-GO ends sized for internal features such as holes and bores.
• Ring and snap gauges are purpose-designed for external diameters.

Concept / Approach:

By definition, a plug gauge is inserted into a hole to check whether the hole size lies within tolerance (GO end should enter; NO-GO end should not). For external diameters such as shafts, inspectors use ring gauges (GO/NO-GO rings that slip over the outside) or snap gauges/caliper gauges that straddle the part. Therefore, assigning a plug gauge to check shafts or studs is conceptually incorrect.

Step-by-Step Solution:

Verification / Alternative check:

Typical inspection work instructions list plug gauges under the section ‘‘hole inspection’’ and ring/snap gauges under ‘‘shaft inspection.’’ Coordinate measuring machines (CMMs) or micrometers may also verify sizes but are not GO/NO-GO gauges.

Why Other Options Are Wrong:

• Correct: contradicts the fundamental internal–external assignment.
• Correct only for threaded studs: external thread gauges are ring thread gauges, not plug gauges (which are used for internal threads).
• Correct if a taper plug is used: taper plugs still evaluate sockets/tapered holes internally.
• Depends on fit class: fit class affects gauge sizes, not the internal/external nature of the gauge.

Common Pitfalls:

• Assuming any cylindrical gauge can assess any cylindrical feature.
• Confusing ring thread gauges with plug thread gauges due to similar GO/NO-GO logic.

Final Answer:

Incorrect