Calipers & Micrometers

Micrometer Screw Gauge

• A micrometer consists of two scales: the main scale on the sleeve (barrel) and the thimble scale on the rotating thimble.
• Key components: spindleThe precision screw that advances towards the anvil when the thimble is rotated. (advances towards the anvil), anvil (stationary surface), ratchet (ensures consistent tightening), C-frame (body).
• The object is clamped between the spindle and anvil by rotating the ratchet -- never tighten using the barrel directly, only the ratchet.

Reading a Micrometer

• Step 1: Read the main scale -- each division represents 0.5 mm. Note the number of visible divisions to the nearest 0.5 mm.
• Step 2: Read the thimble scale -- each division represents 0.01 mm. Read the number that aligns with the main scale.
• Step 3: Add the main scale and thimble readings together. For example: $2.5 + 0.47 = 2.97$ mm.
• Readings should always be recorded to 2 decimal places (e.g. 1.40 mm, not 1.4 mm).

Vernier Calipers

• Key components: upper jaws (external measurements), lower jaws (internal measurements), depth blade, main scale, vernier scale, and locking screw.
• The object is clamped between the jaws and the sliding vernier scale is locked in position.

Reading Vernier Calipers

• Step 1: Read the centimetre mark to the left of the vernier scale zero.
• Step 2: Read the millimetre mark to the left of the vernier scale zero.
• Step 3: Find where a vernier line matches a main scale line -- this gives tenths of a millimetre.
• Step 4: Add all readings together. For example: $1\text{ cm} + 3\text{ mm} + 0.3\text{ mm} = 13.3$ mm.
• Readings should be recorded to at least 1 decimal place.

Comparison

• Micrometers have a smaller measuring range (typically 0-25 mm) but better resolution (0.01 mm).
• Vernier calipers have a larger range (typically 0-150 mm) but lower resolution (0.1 mm).
• For many quick measurements, calipers are easier; for very precise small measurements, a micrometer is preferred.

Practical Considerations

• Both instruments must be checked for zero errorsWhen the instrument gives a non-zero reading when the true value is zero. Subtract the zero error from all subsequent readings. before use -- the reading should be zero with nothing between the jaws.
• Ensure the object is clean and free from debris before measuring.
• Apply consistent pressure when closing the jaws (use the ratchet on a micrometer).
• Take multiple readings at different positions and calculate a mean to account for irregularities.
• When measuring cylindrical objects, measure at several points along the length and at different orientations (e.g. at 90 degree intervals).