Metrology

What is metrology?

The science of measurement, including all theoretical and practical aspects.

Why is metrology critical in modern manufacturing?

Ensures interchangeable parts meet specified dimensions and tolerances.

How did Henry Ford use metrology in Model T production?

He ensured all rear axles were made to the same dimensions and tolerances for interchangeability.

What is a key difference between mass production and custom manufacturing like Rolls-Royce?

Mass production uses interchangeable parts; custom parts are hand-fitted.

What does a manufacturing engineer use metrology for?

To measure parts and ensure they are within specified tolerances.

What are dimensions in manufacturing?

Linear or angular sizes of a component specified on a part drawing.

What is a tolerance according to ANSI Y14.5M-1982?

The total amount a specific dimension is permitted to vary.

Why are tolerances necessary?

Variations occur in all manufacturing processes, affecting part size.

What is a bilateral tolerance?

Variation permitted in both positive and negative directions from the nominal dimension.

What is an example of an unbalanced bilateral tolerance?

2.500 +0.010, -0.005.

What defines a unilateral tolerance?

Variation permitted in only one direction from the specified dimension.

How are limit dimensions specified?

By stating the maximum and minimum allowable dimensions.

What happens if tolerances are too small?

Manufacturing costs increase significantly.

According to NIST, how do tolerances change over time?

They shrink by a factor of 3 every 10 years.

What factors cause parts from the same machine to differ?

Speed, temperature, lubrication, incoming material variation.

What is geometric tolerancing?

Tolerances involving shape features of a part.

What is an allowance in metrology?

The specific difference in dimensions between mating parts.

What does MMC stand for?

Maximum Material Condition, where a feature contains the maximum material within limits.

What is a clearance fit?

A fit allowing rotation or sliding between mating parts.

What is an interference fit?

A fit with negative clearance, requiring force to assemble.

What is a transition fit?

A fit with small clearance or interference for accurate location.

What is a datum in metrology?

A theoretically exact axis, point, line, or plane.

What is a nominal surface?

The designer’s intended, geometrically perfect surface contour.

What determines the actual surface of a part?

The manufacturing processes used to make it.

Why are surfaces important for appearance?

They affect how the surface feels, looks, and behaves for coating or sealing.

How do surfaces impact function?

They influence wear, friction, lubrication retention, and load-bearing capacity.

Why do smooth surfaces improve electrical contacts?

They enhance electrical and thermal conductivity.

What are the four elements of surface texture?

Roughness, waviness, lay, flaws.

What is roughness in surface texture?

Small, finely-spaced deviations from the nominal surface.

What causes waviness on a surface?

Work deflection, vibration, or heat treatment.

What does 'lay' refer to in surface texture?

The predominant direction or pattern of the surface texture.

What are examples of surface flaws?

Cracks, scratches, inclusions.

What is surface roughness?

A measurable characteristic based on roughness deviations.

How is surface roughness (Ra) calculated?

As the arithmetic average of absolute vertical deviations from the nominal surface over a length.

What is the difference between Ra and Rq?

Rq (RMS) is the root mean square of deviations, always greater than Ra due to larger deviations’ impact.

How is surface roughness typically measured?

With a stylus tracing the surface profile.

What is the purpose of a cutoff length in surface measurement?

To filter out waviness and focus on roughness deviations.

What does the surface texture symbol on a drawing indicate?

Roughness, waviness, and lay specifications.

What is surface integrity?

The study and control of the subsurface layer and its changes affecting performance.

What are the four energy forms affecting surface integrity?

Mechanical, thermal, chemical, electrical.

What surface change is caused by mechanical energy?

Residual stresses (e.g., from bending sheet metal).

What is an example of a thermal energy surface change?

Heat-affected zone in welding.

What chemical energy change affects metal surfaces?

Intergranular attack (corrosion or oxidation).

What does electrical energy cause on a surface?

Craters from short circuits (e.g., in arc welding).

Which manufacturing process has tighter tolerances: machining or sand casting?

Machining (±0.05 mm vs. sand casting’s ±0.5-1 mm).

Why do tolerances vary with part size?

Larger parts are harder to control precisely in manufacturing.

What process is noted for superior surface finishes?

Honing, lapping, polishing, or superfinishing.

Why does improving surface finish increase processing cost?

It requires additional operations and more time.

What surface feature can act as a stress concentration point?

Surface flaws.

How does surface roughness affect adhesive bonding?

Slightly rough surfaces increase bond strength.

What ancient unit was based on King Henry I's arm?

Yard (0.9144 m), from nose to thumb tip.

How was the meter originally defined in 1872?

1/10,000,000 of the distance from the North Pole to the equator.

What material was used for the original meter standard?

90% platinum, 10% iridium bar.

How is the meter defined since 1960?

1,650,763.73 wavelengths of orange light from krypton-86 in a vacuum.

What are the seven fundamental quantities in metrology?

Length, mass, time, electric current, temperature, light intensity, matter.

What is manufacturing metrology primarily concerned with?

Measuring length-related quantities like length, width, diameter, and roughness.

What does measurement compare?

An unknown quantity to a known standard using consistent units.

What is accuracy in metrology?

Closeness of a measured value to the true value, absent systematic errors.

What is precision in metrology?

Degree of repeatability, minimizing random errors.

What is the difference between high accuracy/low precision and low accuracy/high precision?

High accuracy/low precision: close to true value but scattered; low accuracy/high precision: consistent but off-target.

What is repeatability?

Variation in measurements by one individual using the same instrument.

What is reproducibility?

Variation in measurements by different individuals using the same instrument.

What is calibration?

Comparing a measurement device to a standard tied to national standards (e.g., NIST).

What is the 'rule of 10' in gage selection?

Instrument must be at least 10 times more accurate than the tolerance.

What is the standard measuring temperature?

20°C (68°F).

Why is inspection critical for product quality?

Ensures components fit, assemble properly, and are replaceable for mass production.

What are the two types of inspection?

Inspection by variables (measurement) and by attributes (gaging).

What is an example of inspection by variables?

Measuring a cylindrical part’s diameter.

What is an example of inspection by attributes?

Using a GO/NO-GO gage to check if a part is within tolerance.

Why is gaging faster than measuring?

It only checks if a part meets specs, not how much it deviates.

What is a disadvantage of manual inspection?

Time-consuming, boring, and prone to human error.

What is the risk of sampling inspection?

Defective parts may slip through due to incomplete measurement.

What does 100% inspection theoretically achieve?

Ensures 100% good quality by screening all defects.

Why doesn’t manual 100% inspection guarantee perfect quality?

Human errors occur despite inspecting every part.

What is a benefit of automated 100% inspection?

Parts sortation and feedback to adjust upstream processes.

Why is post-process inspection costly?

Expensive steps are completed before defects are found.

What is an advantage of in-process inspection?

Identifies defects early, reducing rejection/rework costs.

What are the three main inspection points?

Receiving, in-process, final inspection.

What is acceptance sampling?

Inspecting a sample and deciding to accept or reject a lot based on criteria.

What is a pro of acceptance sampling?

Inexpensive and suited for destructive testing.

What is a con of acceptance sampling?

Does not prevent poor quality, only detects it.

What does the economic model C1 = p*C2 determine?

When to use 100% inspection (if p > C1/C2) vs. doing nothing.

What defines contact inspection?

Uses mechanical probes or devices that touch the part.

Name a contact inspection technology.

Coordinate Measuring Machine (CMM).

Why are contact methods commercially important?

Widely used, accurate, reliable, and often the only option.

What is a non-contact inspection advantage?

Avoids surface damage and allows faster inspection.

What is an example of a non-optical non-contact method?

Ultrasonic inspection.

What are line-graduated instruments?

Devices with marked scales (e.g., rulers, calipers) for measurement.

What is the resolution of a typical vernier caliper?

0.001 inch (0.025 mm).

What is the sensitivity of a micrometer?

0.0001 inch (2.5 μm).

How do you read a vernier caliper showing 1.5 + 3×0.025 + 7×0.001?

1.582 inches.

What is an Abbe offset error in calipers?

Misalignment between measurement point and scale causing inaccuracy.

How do you read a micrometer with 0.200" (large), 0.025" (small), 0.016" (thimble)?

0.241 inches.

What are precision gage blocks used for?

As dimensional standards for calibrating other instruments.

What is 'wringing' in gage blocks?

Sticking blocks together to build precise stacks.

What does a GO gage check?

Maximum material condition (e.g., minimum hole size).

What does a NO-GO gage check?

Minimum material condition (e.g., maximum hole size).

What does a dial indicator measure?

Straightness, flatness, roundness, runout, etc., with amplification.

How does a sine bar measure angles?

Using the formula sin A = H/L with gage blocks.

What is a pneumatic gage used for?

Measuring inside/outside diameters using air pressure differences.