Comparators, Gauges and Inspection Planning

Introduction

Metrology provides the language by which manufactured dimensions are specified, measured and controlled. In production engineering, however, it is often unnecessary to determine the absolute size of every component with a universal measuring instrument. Frequently, the practical question is whether a feature is within its permitted limits. Comparators and gauges answer this question rapidly and economically.

A comparator does not usually measure absolute dimension directly. It compares an unknown dimension with a reference standard and indicates the deviation. A gauge, in contrast, is generally a fixed or adjustable device used to determine whether a dimension lies within specified limits. Inspection planning connects these instruments with process capability, sampling, measurement uncertainty, cost and product risk.

The central engineering challenge is to select an inspection method that is sufficiently accurate and reliable without imposing unnecessary time and cost. This chapter examines mechanical, electrical, pneumatic and optical comparators; the design and use of limit gauges according to Taylor’s principle; gauge tolerance and wear allowance; and the development of economical inspection plans.

Learning Objectives

After studying this chapter, the learner should be able to:

1. Explain the operating principles, construction and applications of major comparator types.
2. Distinguish comparison, gauging and direct measurement.
3. Design GO and NO-GO gauges using Taylor’s principle.
4. Allocate gauge-maker tolerance and wear allowance correctly.
5. Develop inspection plans using risk, process capability and economic considerations.
6. Identify sources of measurement error and establish suitable control practices.

1. Measurement, Comparison and Gauging

A direct measuring instrument, such as a micrometer, produces a numerical estimate of a dimension. A comparator magnifies the difference between a workpiece dimension and a reference dimension. If the reference is set to zero, the comparator indication is the deviation from that reference.

If the measured dimension is D and the reference dimension is D₀, the deviation is:

Here, ΔD and the dimensions are expressed in the same unit. A positive value indicates that the workpiece is larger than the reference; a negative value indicates that it is smaller. The equation assumes that the comparator has been correctly zeroed and that systematic errors have been compensated or are negligible.

A comparator normally has three functional elements:

• a sensing element that contacts or detects the workpiece;
• an amplification system that magnifies a small displacement or physical change; and
• an indicating element that displays the deviation.

Comparison is especially useful when:

• many similar parts must be inspected;
• the tolerance is small;
• a master component or setting standard is available;
• rapid accept/reject decisions are required; or
• the operator must avoid reading a fine scale repeatedly.

A comparator is not automatically more accurate than the standard used to set it. Its performance depends on resolution, repeatability, sensitivity, hysteresis, alignment, temperature and the stability of the reference.

2. Characteristics of an Effective Comparator

Important performance terms are as follows.

Range is the total dimensional variation that can be indicated. Comparators generally have a smaller range than universal instruments.

Sensitivity is the change in indication divided by the corresponding change in input dimension. If an input displacement of 0.01 mm produces an indication of 1.0 mm, the displacement magnification is 100:1. Sensitivity is a ratio and has no unit when both quantities use the same unit.

Resolution is the smallest change that can be distinguished on the indication. Resolution is not the same as accuracy.

Accuracy describes closeness to the true or accepted reference value.

Repeatability is the closeness of repeated results under the same conditions. Reproducibility concerns agreement when conditions such as operators, instruments or laboratories change.

Linearity indicates how closely the output follows a proportional relationship over the operating range. Hysteresis is the difference in indication for the same input value when the input is approached from opposite directions.

Drift is a gradual change in indication with time, temperature or component ageing. A comparator may have fine resolution but poor accuracy if it is not calibrated or if its mechanism is unstable.

3. Mechanical Comparators

Mechanical comparators convert a small contact displacement into a magnified pointer or scale movement. Common mechanisms include levers, gears, racks, pinions, springs and flexure elements.

3.1 Dial indicator

A dial indicator uses a contact spindle, rack-and-pinion or similar mechanism, and a pointer moving over a graduated dial. A small linear movement of the spindle is converted into angular pointer movement. Dial indicators are widely used for checking run-out, alignment, parallelism, flatness, concentricity and comparative dimensions.

The instrument should be mounted rigidly, and the contact direction should be as close as possible to the direction of the dimension being inspected. If the contact is inclined, the effective displacement is reduced. For an ideal geometric arrangement, if the actual displacement is d and the contact angle from the measurement direction is θ, the indicated component is:

where dᵢ is the measured component. This relation assumes rigid contact and neglects stylus geometry and mechanical errors. At θ = 0°, the cosine error is zero; as the angle increases, the indicated displacement becomes smaller.

3.2 Lever comparator

A lever comparator uses a pivoted contact lever. The ratio of the distances from the pivot to the contact point and sensing point provides mechanical amplification. It is useful where access is restricted or where very small variations must be detected. The contact force is usually low, which makes the instrument suitable for delicate surfaces.

3.3 Sigma-type and reed-type comparators

Mechanical comparators may incorporate crossed strips, reed mechanisms or compound levers. Flexure mechanisms avoid backlash because they operate by elastic deformation rather than sliding contact. They can provide high sensitivity, although their range is limited and overload may permanently damage the flexures.

3.4 Advantages and limitations

Mechanical comparators are relatively inexpensive, portable and easy to use. They do not require electrical power and are suitable for workshop applications. Their limitations include friction, backlash, wear, limited range, sensitivity to mounting and possible parallax when the scale is read manually. Regular zero checks and controlled contact force are essential.

4. Electrical and Electronic Comparators

Electrical comparators transform a mechanical displacement into an electrical signal. A typical system contains a probe, transducer, signal-conditioning circuit and display or data-acquisition unit.

4.1 Inductive comparator

An inductive probe commonly uses a movable ferromagnetic core and coils. A change in core position changes the inductance or differential output voltage. The device is highly sensitive, has low mechanical friction and can be connected to a digital display, statistical process-control system or automatic sorting machine.

4.2 Linear variable differential transformer

A linear variable differential transformer (LVDT) has a primary winding and two secondary windings arranged symmetrically around a movable magnetic core. When the core is centered, the differential secondary output is approximately zero. Displacement in either direction changes the magnitude and phase of the output, allowing both size and direction of deviation to be determined.

An LVDT requires suitable excitation and signal conditioning. It offers excellent repeatability and a robust, nearly frictionless sensing mechanism, but it is sensitive to electrical noise, magnetic conditions and incorrect installation.

4.3 Capacitive and resistance-based comparators

Capacitive probes detect displacement through a change in capacitance, often caused by variation in gap. They are useful for small displacements and non-contact measurement, but humidity, surface condition and stray capacitance can affect the result. Resistance-based systems use strain gauges or variable-resistance elements. They are compact and suitable for electronic data collection, though temperature compensation may be necessary.

4.4 Applications

Electronic comparators are used in automated gauging, in-process inspection, coordinate measurement systems, precision assembly and closed-loop manufacturing. Their digital output supports trend analysis and statistical process control. Nevertheless, a digital display does not remove uncertainty; probe calibration, fixturing, software validation and environmental control remain necessary.

5. Pneumatic Comparators

A pneumatic comparator uses air flow or air pressure to compare a workpiece feature with a calibrated restriction. A typical system includes a regulated air supply, filter, pressure regulator, master setting device, measuring head and indicator.

When the clearance between an air nozzle and the work surface changes, the flow resistance changes. This changes either the back pressure in the system or the flow rate through the nozzle. The resulting signal is related to the workpiece dimension over a selected operating range.

For a simple conceptual model, flow resistance Rₚ may be expressed as:

where Δp is pressure difference and Q is volumetric flow rate. This is a model for a specified operating regime, not a universal law for all pneumatic systems. Real nozzle flow can be affected by turbulence, compressibility, temperature, nozzle geometry and supply pressure.

Pneumatic comparators are particularly effective for internal diameters, bores, taper, roundness, straightness and surface-related conditions. They can measure without damaging the workpiece and often average irregularities over several jets. This averaging is useful for production control but may hide localized defects.

Advantages include high sensitivity, low contact force, rapid operation and suitability for automatic gauging. Limitations include dependence on clean, dry air; sensitivity to nozzle alignment and contamination; restricted measuring range; and the need for a carefully calibrated master. The air supply should be filtered and regulated, and nozzle-to-surface geometry should be kept consistent.

6. Optical Comparators

Optical comparators use light, lenses and an image or shadow to compare a workpiece profile with a standard. In a profile projector, the component is illuminated and its enlarged silhouette is projected onto a screen. A chart, template or digital overlay can then be used to assess angles, radii, contours, thread forms and small features.

The nominal optical magnification is:

where M is magnification, hᵢ is image size and hₒ is object size. For example, if a 2 mm feature appears as 20 mm on the screen, the nominal magnification is 10×. This equation assumes that the image is within the designed optical system and that distortion is negligible.

Optical methods reduce contact forces and are useful for delicate, miniature or complex profiles. Their limitations include errors from lens distortion, focus, screen alignment, illumination, edge definition and operator interpretation. Digital vision systems can automate edge detection and comparison, but they require validated lighting, calibration artefacts and software settings.

7. Selection of a Comparator

Selection should begin with the inspection characteristic rather than the instrument available. The following questions are important:

1. Is the feature external, internal, angular, profile-based or non-contact?
2. What is the total tolerance and what measurement uncertainty is acceptable?
3. Is the inspection for laboratory verification, setup, in-process control or final acceptance?
4. What are the production rate, operator skill and environmental conditions?
5. Is a numerical result needed, or is a simple accept/reject decision sufficient?
6. Must the result be recorded for traceability or statistical control?

Mechanical instruments are often suitable for general comparative work. Electrical systems suit automation and data collection. Pneumatic systems are excellent for high-volume bore inspection. Optical systems are appropriate for profiles and delicate components. No technology is universally superior: the best choice provides adequate discrimination, reliability and economic value for the inspection purpose.

8. Limit Gauges and the Taylor Principle

A limit gauge checks whether a feature lies between a maximum and a minimum permissible size. It does not normally provide the actual dimension. Typical examples include plug gauges for holes, ring gauges for shafts, snap gauges for external dimensions and thread plug or ring gauges for threaded features.

For a hole specified as:

Dₘᵢₙ is the lower limit and Dₘₐₓ is the upper limit. For a shaft, the same inequality applies to the shaft diameter, although the functional interpretation of GO and NO-GO members is reversed in relation to material condition.

Taylor’s principle states that:

• the GO gauge should check the maximum material condition and should check the full form of the feature; and
• the NO-GO gauge should check only one element or dimension at a time and should not enter when the feature is outside its limit.

For a hole, maximum material condition corresponds to the smallest permitted hole, Dₘᵢₙ. Therefore, the GO plug gauge is made approximately equal to Dₘᵢₙ and checks size, form and often alignment through the full length of the hole. The NO-GO plug gauge is related to Dₘₐₓ and should not enter the hole.

For a shaft, maximum material condition corresponds to the largest permitted shaft, Dₘₐₓ. The GO ring or snap-gauge member checks the shaft at this condition and normally embraces the complete feature. The NO-GO member checks the minimum limit and must not pass over the shaft.

Taylor’s principle is functional because it prevents acceptance of a component that has the correct local size but an unacceptable form or assembly condition. The principle must be applied with care to features such as interrupted surfaces, thin walls, keyways and complex profiles, where a complete-form gauge may not be practicable.

9. Gauge Tolerance and Wear Allowance

A gauge is itself manufactured with a tolerance. If the gauge were made exactly at the work limit, small manufacturing errors could cause incorrect decisions. Gauge tolerance is therefore allocated around the nominal gauge size according to the applicable standard, company specification or design policy.

Let T be the workpiece tolerance:

Gauge-maker tolerance is commonly represented as T_g. A frequent practical policy is to limit T_g to a specified fraction of the work tolerance, such as 10%, although the actual value must be selected from the relevant gauge standard and risk assessment. The fraction is not a universal law.

Wear allowance is additional material provided on the gauge surface that is expected to wear during service. It is normally applied to GO gauges because they are used more frequently and contact the maximum-material condition. It is not normally applied to NO-GO gauges, since wear could permit an oversize or undersize component to be accepted.

The direction of wear allowance is critical. For a GO plug gauge checking a hole, wear reduces the plug diameter. Since the GO plug represents the minimum hole size, a worn plug becomes smaller and may enter holes that are too small. To delay this condition, the new GO plug is made slightly larger than the basic GO size by the permitted wear allowance. For a GO ring gauge checking a shaft, wear increases the effective ring diameter, so the new ring is made slightly smaller than the basic GO size.

Gauge design must distinguish:

• workpiece limits;
• gauge manufacturing limits;
• wear limits; and
• actual service condition.

A generic allocation for a hole GO plug may be written conceptually as:

where W is a positive wear allowance, before the selected gauge tolerance is distributed about the gauge mean. This expression describes the direction of allowance, not a complete standard design formula. The final upper and lower gauge limits must be obtained by applying the chosen gauge tolerance symmetrically or asymmetrically as required by the governing standard.

Worked example: basic limit-gauge reasoning

A hole has limits 20.000 mm and 20.030 mm. Thus:

The GO plug corresponds to the 20.000 mm minimum hole, while the NO-GO plug corresponds to the 20.030 mm maximum hole. If a company policy permits a gauge-maker tolerance of 10% of work tolerance, then:

This 0.003 mm is the total permitted manufacturing variation for the relevant gauge dimension under that policy. It must not be confused with the workpiece tolerance of 0.030 mm. If a 0.002 mm GO wear allowance is specified, it is directed so that initial GO-gauge wear does not immediately reduce its ability to reject undersize holes. A detailed drawing would then state the gauge limits and datum convention explicitly.

10. Use and Control of Gauges

Before inspection, the gauge should be clean, identified and within calibration. The workpiece and gauge should be near the reference temperature, commonly 20 °C unless another standard is specified. Thermal expansion can be estimated for a uniform material by:

where α is the coefficient of linear expansion in °C⁻¹, L₀ is the reference length, and ΔT is temperature change in °C. The result ΔL has the same length unit as L₀. The relationship assumes small temperature changes, uniform temperature and a constant α.

The operator should avoid forcing a gauge, rocking it excessively, or using it to remove burrs. GO members should be applied with the prescribed technique; NO-GO members should be applied only as specified and should not pass. Gauges should be stored to prevent corrosion, impact and contamination. Calibration records should include identification, standard traceability, results, date, environmental conditions and next due date.

11. Inspection Planning

Inspection planning is the systematic determination of what to inspect, how to inspect it, when to inspect it, how many items to inspect and what decision to make from the results. A plan should be linked to functional requirements and manufacturing risks, not merely to available instruments.

A practical inspection plan includes:

• characteristic and drawing requirement;
• nominal size and specification limits;
• measuring equipment or gauge;
• resolution and demonstrated capability;
• sampling frequency or sample size;
• inspection method and fixturing;
• operator responsibility;
• reaction plan for nonconforming results; and
• records and traceability requirements.

11.1 Risk-based inspection

Critical-to-safety, critical-to-function and legally regulated characteristics normally require greater control than cosmetic or low-risk characteristics. Risk analysis can consider severity, occurrence and detectability. A high-severity feature may justify 100% inspection even when its occurrence is low. Conversely, a stable, low-risk process may be monitored by sampling.

11.2 Measurement-system capability

Before relying on measurements, the measurement system should be evaluated. A simple discrimination guideline is that instrument resolution should be substantially smaller than the tolerance, often one-tenth as a target, although the correct requirement depends on the application and measurement-system analysis.

Measurement uncertainty may be represented by a standard uncertainty u or expanded uncertainty U. If independent standard uncertainty components uᵢ are combined by the root-sum-square method:

and expanded uncertainty is often reported as:

where k is a coverage factor selected for the intended confidence convention. These equations assume appropriately characterized components and negligible or separately treated correlations. A measurement near a specification limit should be interpreted using the organization’s decision rule, including any guard band or risk policy.

11.3 100% inspection versus sampling

100% inspection is appropriate when failure consequences are severe, the process is unstable, production volume is manageable or automated gauging is economical. It does not guarantee quality if the measurement system is poor or the inspection itself is unreliable.

Sampling is economical when the process is stable and the cost of inspection is significant. Sampling plans may be based on statistical acceptance criteria, historical process performance or engineering judgment. The sample must be selected without bias. Inspecting only easily accessible parts or only the beginning of a batch can produce false confidence.

11.4 Process capability and inspection frequency

For a stable approximately normal process, capability indices are commonly used. The potential capability index is:

The centered capability index is:

USL and LSL are upper and lower specification limits, μ is the process mean and σ is the process standard deviation. These indices assume a stable process, meaningful estimates of μ and σ, and a distribution for which the interpretation is appropriate. Cₚ measures spread relative to tolerance but ignores centering; Cₚₖ includes centering. Capability indices should not replace engineering judgment or inspection of critical features.

A stable process with a high Cₚₖ may justify reduced frequency, provided that ongoing control detects drift. A new, adjusted or unstable process requires more frequent inspection, often including first-off inspection and short-interval checks.

11.5 Economic design of an inspection plan

Inspection planning balances inspection cost against the expected cost of escapes, internal scrap, rework, customer returns and failures. A simplified expected-cost expression is:

where each term is the expected cost over the selected production period. More detailed models may express inspection cost as a function of sample size n and escape cost as a function of detection probability. Such models require reliable estimates of failure probabilities and costs; false precision should be avoided.

Economical inspection does not mean minimizing inspection at any price. It means achieving the required level of assurance with the lowest justified total cost. Prevention, capable processes, mistake-proofing and automatic in-process feedback may be more economical than intensive final inspection.

12. Applied Case: Bore Inspection in a Production Cell

This is an illustrative case.

Background

A manufacturer produces 2,000 aluminium housings per shift. Each housing contains a precision bore specified as 25.000 to 25.025 mm. The bore affects the fit of a bearing. Previously, operators used a micrometer-type internal measurement at final inspection, creating a long inspection cycle and inconsistent results between operators.

Problem

The process occasionally produced undersize bores after tool wear. Final inspection detected some defects, but the delay allowed a large batch to be produced before corrective action. The company needed faster detection without sacrificing functional assurance.

Analysis

The feature is internal, cylindrical and produced in high volume. A pneumatic comparator is suitable because it provides rapid comparative measurement with low contact force. A certified master bore can establish the setting. The process engineer first conducted a measurement-system study, checking repeatability, reproducibility, linearity over the tolerance range and sensitivity to air pressure and contamination.

Historical data showed that the process was approximately stable but gradually drifted toward the lower limit as the cutting tool wore. The risk was therefore associated not only with random variation but also with time-dependent drift.

Approach and solution

The company adopted the following plan:

1. Use a pneumatic bore gauge for 100% in-process screening.
2. Set the comparator with a traceable master near the target size.
3. Check the first five parts after tool replacement or adjustment.
4. Inspect one part every 30 minutes during stable production.
5. Inspect an additional part after any tool-offset change.
6. Use a calibrated limit plug gauge at final audit intervals.
7. Stop the process and quarantine parts since the last confirmed acceptable result when a result approaches or crosses the action limit.
8. Clean air filters and measuring jets at a defined maintenance interval.

The action limit was placed inside the specification limit to allow investigation before nonconforming product was produced. Its exact value was established from process variation, measurement uncertainty and the organization’s decision rule rather than selected arbitrarily.

Outcome

The revised plan reduced final-inspection time and detected tool-wear drift earlier. The pneumatic comparator provided rapid feedback, while periodic reference checks and final audits protected against gauge drift. The plan was economical because frequent automated or semi-automated checks were concentrated on the known failure mechanism, rather than applying the same inspection intensity to every characteristic.

Learning points

The case demonstrates that instrument selection, process knowledge and reaction planning must be integrated. A comparator is most valuable when its signal leads to timely process action. Inspection frequency should be linked to risk and process behaviour, not simply to production quantity.

13. Limitations, Governance and Contemporary Practice

Comparators and gauges can produce misleading decisions when surfaces contain burrs, dirt, form errors or temperature gradients. Operator technique, fixture deformation and incorrect datum selection are common causes of error. Digital systems introduce additional concerns: software configuration, cybersecurity, data integrity, access control and retention of calibration records.

Quality records should be truthful, traceable and protected from unauthorized alteration. Deliberately manipulating gauge settings or omitting failed inspections is both professionally unethical and potentially dangerous. Measurement laboratories and production departments should maintain traceability to recognized standards where required by contract or regulation.

Modern systems increasingly combine electronic comparators, machine vision, automated gauging and manufacturing execution systems. Artificial intelligence may assist with image segmentation, anomaly detection or prediction of tool wear, but it should not be treated as an unquestioned measurement authority. Training data, lighting changes, model drift, explainability and human override must be controlled. A validated conventional gauge may remain preferable where the decision is simple and the risk is high.

Best-Practice Checklist

• Define the functional characteristic and its specification before selecting equipment.
• Use the correct GO and NO-GO logic for holes, shafts and special features.
• Apply Taylor’s principle where complete-form and single-element checks are appropriate.
• Separate work tolerance, gauge-maker tolerance and wear allowance.
• Calibrate gauges and comparators using traceable standards.
• Control temperature, cleanliness, alignment and contact force.
• Confirm measurement-system capability before making production decisions.
• Link inspection frequency to risk, stability and process drift.
• Establish a clear reaction plan for borderline and failed results.
• Record inspection results honestly and preserve traceability.

Chapter Summary

Comparators determine dimensional deviation relative to a reference and are valuable for rapid, repeatable production inspection. Mechanical comparators use levers, gears or flexures; electrical comparators convert displacement into an electrical signal; pneumatic comparators relate air pressure or flow to dimensional clearance; and optical comparators use enlarged images or profiles. Each type has characteristic advantages, limitations and environmental requirements.

Limit gauges provide economical accept/reject decisions. Taylor’s principle requires the GO gauge to represent maximum material condition and check the complete feature, while the NO-GO gauge checks the opposing limit without entering or passing. Gauge-maker tolerance must be distinguished from workpiece tolerance, and wear allowance must be directed according to the way the GO gauge wears.

An effective inspection plan identifies characteristics, instruments, sampling, uncertainty, decision rules and reaction actions. Risk, process stability, capability and total cost should determine whether inspection is continuous, periodic, sampled or automated. Economical inspection is achieved not by reducing checks indiscriminately but by directing measurement resources toward features and failure mechanisms that matter most.

Key Terms

Comparator: An instrument that indicates deviation from a reference dimension.

Gauge: A device used to determine whether a feature conforms to specified limits.

Limit gauge: A gauge that gives an accept/reject decision at dimensional limits.

GO gauge: A member intended to check the maximum-material condition and functional form.

NO-GO gauge: A member intended to check the opposing limit without passing.

Taylor’s principle: The rule governing full-form GO inspection and single-element NO-GO inspection.

Sensitivity: Change in indication per unit change in input.

Resolution: The smallest distinguishable change in indication.

Repeatability: Agreement under the same measurement conditions.

Hysteresis: Difference in indication for the same input approached from opposite directions.

LVDT: A differential transformer used to sense linear displacement electrically.

Pneumatic comparator: A comparator using air pressure or flow to detect dimensional variation.

Optical comparator: A device that compares an enlarged image or silhouette with a standard.

Gauge-maker tolerance: Permitted manufacturing variation in a gauge.

Wear allowance: Extra material provided to compensate for expected gauge wear.

Measurement uncertainty: Quantified doubt associated with a measurement result.

Process capability: The ability of a stable process to meet specification limits.

Cₚₖ: Capability index considering both process spread and centering.

Inspection plan: A documented method defining what, how, when and how much to inspect.

Traceability: Ability to relate a measurement result to recognized standards through an unbroken chain.

Review Questions

Short Answer Questions

1. Distinguish a comparator from a direct measuring instrument.
2. State the two main requirements of Taylor’s principle.
3. Why is wear allowance normally applied to GO gauges rather than NO-GO gauges?
4. Identify two advantages and two limitations of pneumatic comparators.
5. What information should an inspection plan contain?

Long Answer Questions

1. Explain the construction, operating principle, applications and limitations of mechanical comparators.
2. Compare electrical, pneumatic and optical comparators for high-volume production inspection.
3. Describe the design logic of plug, ring and snap gauges for holes and shafts.
4. Explain gauge-maker tolerance, wear allowance and the consequences of incorrect allowance direction.
5. Develop a risk-based inspection strategy for a precision component produced by a stable but slowly drifting process.

Analytical / Application Questions

1. A hole is specified as 30.000 to 30.040 mm. Determine the basic GO and NO-GO plug-gauge sizes and explain the functional purpose of each. State what additional information is needed before releasing a detailed gauge drawing.
2. A process has USL = 50.020 mm, LSL = 49.980 mm, μ = 50.006 mm and σ = 0.004 mm. Calculate Cₚ and Cₚₖ, then interpret the effect of process centering.
3. A production line experiences occasional bore undersize after tool wear. Propose an inspection plan using a comparator, including measurement frequency, calibration checks, action limits and quarantine logic. Justify the plan economically and technically.

Multiple Choice Questions with Answers

Question 1

What is the primary function of a comparator?

A. To determine material composition

B. To indicate deviation from a reference dimension

C. To measure cutting temperature

D. To identify surface colour

Correct Answer: B. To indicate deviation from a reference dimension

Question 2

For a hole, maximum material condition corresponds to:

A. The largest permitted hole

B. The smallest permitted hole

C. The nominal hole only

D. A hole with zero tolerance

Correct Answer: B. The smallest permitted hole

Question 3

According to Taylor’s principle, a GO gauge should generally:

A. Check only one isolated element

B. Check the complete functional form at maximum material condition

C. Be used only after the NO-GO gauge

D. Have no calibration requirement

Correct Answer: B. Check the complete functional form at maximum material condition

Question 4

Which comparator commonly uses a movable magnetic core and differential windings?

A. LVDT comparator

B. Mechanical dial indicator

C. Pneumatic plug gauge

D. Optical profile projector

Correct Answer: A. LVDT comparator

Question 5

A feature changes by 0.01 mm and the comparator indication changes by 1.0 mm. What is the displacement magnification?

A. 0.01:1

B. 1:1

C. 10:1

D. 100:1

Correct Answer: D. 100:1

Question 6

Which condition is especially important for pneumatic comparators?

A. Unregulated and contaminated air

B. Clean, dry and regulated air

C. A highly magnetic workpiece

D. A perfectly transparent workpiece

Correct Answer: B. Clean, dry and regulated air

Question 7

A work tolerance is 0.030 mm and a stated policy allows gauge-maker tolerance equal to 10% of work tolerance. What is the gauge-maker tolerance?

A. 0.0003 mm

B. 0.003 mm

C. 0.030 mm

D. 0.300 mm

Correct Answer: B. 0.003 mm

Question 8

Why is wear allowance normally not applied to a NO-GO gauge?

A. NO-GO gauges never contact workpieces

B. Wear could make the NO-GO gauge accept an out-of-limit feature

C. NO-GO gauges are always optical

D. Wear improves every NO-GO decision

Correct Answer: B. Wear could make the NO-GO gauge accept an out-of-limit feature

Question 9

For USL = 10.10 mm, LSL = 9.90 mm and σ = 0.02 mm, what is Cₚ?

A. 0.83

B. 1.00

C. 1.67

D. 3.33

Correct Answer: C. 1.67

Question 10

Which inspection strategy is generally most appropriate for a stable, low-risk process with a capable measurement system?

A. Unplanned inspection only after customer complaints

B. Risk-based sampling with ongoing process monitoring

C. No specification or records

D. Inspection using an uncalibrated instrument

Correct Answer: B. Risk-based sampling with ongoing process monitoring