Metrology, Measurement and Inspection

Metrology, Measurement, and Inspection

Introduction

Metrology is the science of measurement. For a machinist, it forms the very foundation of the trade: manufacturing a part to the correct dimension, angle, and surface finish requires absolute mastery of measuring instruments, their reading, their uncertainty, and the standards that govern them. This chapter covers all the knowledge required for the Red Seal exam in Machining, including units, instruments, measurement techniques, inspection, and geometric tolerances.

In Canada, machinists must comply with national standards, notably CSA B149.1 (Canadian Electrical Code, Part I) for the electrical aspects of equipment, and especially ISO and ANSI/ASME standards for tolerances and threading. The Red Seal exam evaluates your ability to select the right instrument, use it correctly, and interpret results against the specifications on the technical drawing.

Units of Measurement and Systems

International System (SI) vs. Imperial System

Canada officially uses the International System (SI), but the machining industry still commonly uses the imperial system (inches, fractions, feet). You must be fully bilingual in measurement.

Golden rule: in machining, you almost always work in millimetres (mm) and micrometres (µm) for fine tolerances. 1 µm = 0.001 mm = 0.00003937 in.

Fractions, Decimals, and Tolerances

A machinist must instantly convert a fraction to a decimal:

Common trap: do not confuse 0.001 in (one thousandth of an inch) with 0.01 mm (one hundredth of a millimetre). 0.001 in = 0.0254 mm. An instrument graduated in 0.001 in is not equivalent to an instrument graduated in 0.01 mm.

Measuring Instruments: Classification and Use

Rules and Squares

Caliper (Vernier Caliper)

The caliper is the most versatile instrument. It comes in analog (vernier), dial, and digital versions.

Typical accuracies:

Reading the vernier (method):

Example: if the main scale reads 23 mm and the 7th vernier graduation coincides, the measurement is 23 + (7 × 0.02) = 23.14 mm.

Common errors:

Micrometre

The micrometre is the standard precision instrument for outside, inside, and depth measurements. Its accuracy is 0.01 mm (0.001 in) for standard models, and 0.001 mm (0.0001 in) for precision models.

Principle: a micrometric screw with a pitch of 0.5 mm (or 0.025 in). One full turn of the thimble moves the spindle 0.5 mm. The sleeve is graduated in 50 divisions (0.5 mm ÷ 50 = 0.01 mm).

Reading:

Example: sleeve = 12.5 mm, thimble = 28 graduations → 12.5 + (28 × 0.01) = 12.78 mm.

Zero check: use the standard gauge supplied with the instrument. While turning the ratchet, the spindle must touch the surface without forcing. If the zero is not aligned, adjust or note the error.

Trap: the ratchet is used to apply constant force. Never tighten directly by the thimble, as the force varies and skews the measurement.

Dial Indicator

The dial indicator measures variations in dimension, not an absolute dimension. It is used for:

Types:

Use: position the plunger perpendicular to the surface being measured. Pre-set the instrument with a standard gauge or reference block. Readings are taken in revolutions (each revolution = 1 mm or 0.1 in) and graduations.

Limit Gauges (Go / No-Go)

Limit gauges (plug, pin, ring) check whether a dimension is within tolerance without providing a numerical value.

Rule: the GO side must pass under light pressure (its own weight), the NO-GO side must not pass. Never force a gauge.

Gauge Blocks (Precision Blocks)

Gauge blocks are rectangular blocks made of steel or tungsten carbide, ground and lapped to an accuracy of ±0.0001 mm. They are used to:

Combining blocks: to obtain a given dimension, choose the smallest number of blocks possible (ideally 3 or 4). Start with the last decimal place.

Example: to obtain 37.485 mm, choose:

Total = 37.485 mm.

Maintenance: blocks must be cleaned, degreased, and stored with a rust inhibitor. Never touch them with bare hands (corrosion from skin acids).

Tolerances and Fits

Definitions

ISO Tolerance System (ISO 286 Standard)

The ISO system defines tolerance grades (IT01, IT0, IT1 … IT18) and positions (uppercase letters for bores, lowercase letters for shafts).

Example: a 25 H7 bore means:

Common fits:

Geometric Tolerances (GD&T)

Geometric tolerances (ASME Y14.5 standard) control the form, orientation, and position of features, not just their dimensions.

Essential symbols:

Example: a hole with the callout ⌖ ⌀0.05 A means the axis of the hole must lie within a cylinder of 0.05 mm diameter, centred on the theoretical position, relative to datum A.

Trap: true position is always a cylindrical zone (⌀), even if the hole is square. Do not confuse it with a simple X/Y coordinate tolerance.

Threading: Measurement and Inspection

Thread Standards

Thread Measurement

Major diameter: measured with a special micrometre with anvils or a thread gauge.

Minor diameter: measured with a pointed caliper.

Pitch diameter: measured using the three-wire method.

Three-wire method:

D_eff = M - 3 × d_wire + 0.866025 × P

Where:

Example: for an M20 × 2.5 thread, with wires of 1.5 mm, measurement M = 21.35 mm:

D_eff = 21.35 - 3 × 1.5 + 0.866025 × 2.5 = 21.35 - 4.5 + 2.165 = 19.015 mm.

The theoretical pitch diameter for M20 × 2.5 is 18.376 mm. The part is therefore out of tolerance (too large).

Thread Gauges

GO/NO-GO gauges for threads check:

Rule: the GO gauge must screw on by hand along the full length of the thread. The NO-GO gauge must not exceed 2 turns.

Surface Finish

Roughness Parameters

Surface roughness is measured in Ra (arithmetic mean deviation) and Rz (average height of the 5 highest peaks and valleys).

Finish symbols: the basic symbol is a triangle (✓). One triangle = roughing, two triangles = finishing, three triangles = grinding. The Ra value is indicated above the symbol.

Measurement: the roughness tester (profilometer) with a diamond stylus scans the surface over a sampling length (cut-off). Typical cut-off values are 0.8 mm (for Ra 0.4 to 3.2 µm) and 2.5 mm (for Ra > 3.2 µm).

Trap: roughness is not the same as waviness. Roughness is measured over a short length, waviness over a longer length. A poor cut-off choice gives erroneous results.

Inspection and Quality Control

Control Plan

Before machining, the machinist must:

The 10% Rule (Uncertainty Rule)

The instrument's uncertainty must be at most 10% of the part tolerance. If the tolerance is ±0.05 mm, the instrument must have a resolution of at least 0.005 mm (5 µm).

Measurement Conditions

Statistical Process Control (SPC)

The machinist may be required to fill out control charts (X-bar, R). Key points:

Applicable Canadian Standards

CSA B149.1 (Canadian Electrical Code, Part I)

Although this code primarily concerns gas and electricity, it applies to machining equipment that uses electric motors and controls. Key rules:

CSA B44 (Elevator Safety Code)

Applicable if the machinist works on elevator components (rare, but possible in maintenance).

ISO 9001 (Quality Management Systems)

Although not mandatory for the exam, knowledge of traceability and instrument control principles is assessed. Instruments must have a calibration certificate traceable to a national standard.

Traps to Avoid

Summary

Final exam tip: read each question twice. Identify the required unit (mm or in), the tolerance, and the appropriate instrument. Trick questions often involve conversions, instrument zeroing, and the 10% rule. Practice reading verniers and micrometres on varied examples until reading becomes automatic.