# Calipers, Micrometers & Vernier Scales ## Micrometer Screw Gauge - A micrometer (or micrometer screw gauge) is a tool used for measuring small widths, thicknesses or diameters - For example, the diameter of a thin metal rod - It has a resolution of **0.01 mm**

Resolution is the smallest change in a quantity that an instrument can detect

### Components of a Micrometer - The micrometer is made up of **two scales**: - The **main scale** - this is on the sleeve (sometimes called the barrel) - The **thimble scale** - this is a rotating scale on the thimble - Key components include: - **Spindle** - the precision screw that advances towards the anvil - **Anvil** - the stationary surface that the object rests against - **Ratchet** - the rotating mechanism used to close the jaws consistently - **C-frame** - the U-shaped body that holds the components - **Sleeve (with main scale)** - the fixed barrel showing the main measurement - **Thimble** - the rotating cylinder with the thimble scale ### Using a Micrometer - The spindle and anvil are clamped around the object being measured by rotating the **ratchet** - This should be tight enough so the object does not fall out but not so tight that it is deformed

Never tighten the spindle using the barrel, only using the ratchet - this reduces the chances of overtightening and zero errors

### Reading a Micrometer - The value measured from the micrometer is read where the thimble scale aligns with the main scale **Steps to read a micrometer:** 1. Read the **main scale first** - each division represents 0.5 mm 2. Note how many divisions are visible to the **nearest 0.5 mm** (e.g. 2.5 mm) 3. Next read the **thimble scale** - each division represents 0.01 mm 4. Read the number that aligns with the main scale and multiply by 0.01 mm (e.g. 17 × 0.01 = 0.47 mm) 5. Finally, **add the main scale and thimble reading together** to get the final measurement (e.g. 2.5 + 0.47 = 2.97 mm)

This should always be recorded to 2 decimal places (e.g. 1.40 mm not just 1.4 mm)

## Vernier Calipers - Vernier calipers are another distance measuring tool that uses a sliding vernier scale - They can also be used to measure diameters and thicknesses, just like the micrometer - However, they can also measure the length of small objects such as a bolt or the depth of a hole - Vernier calipers generally have a resolution of **0.1 mm**, however, some are as small as 0.02 mm - 0.05 mm ### Components of Vernier Calipers - The calipers are made up of **two scales**: - The **main scale** - The **vernier scale** - Key components include: - **Upper jaws** - for external measurements (e.g. diameter of a cylinder) - **Lower jaws** - for internal measurements (e.g. inside diameter of a tube) - **Main scale** - the fixed scale on the main body - **Vernier scale** - the sliding scale that moves along the main scale - **Locking screw** - to hold the measurement in place - **Blade for depth measuring** - extends from the back for measuring depths ### Using Vernier Calipers - The two upper or lower jaws are clamped around the object - The sliding vernier scale will follow this and can be held in place using the **locking screw** ### Reading Vernier Calipers - The value read from the caliper when the vernier scale aligns with the main scale **Steps to read vernier calipers:** 1. Read off the **centimetre mark to the left** of the vernier scale zero - here it is 1 cm 2. Read off the **millimetre mark to the left** of the vernier scale zero - here it is 3 mm 3. Find the point where the line **matches up with the line on the bar scale** - this tells you the number of tenths of a millimetre, here it is 0.3 mm 4. **Add the reading together** to get your measurement: 1 cm + 3 mm + 0.3 mm = 13.3 mm OR 1.33 cm

This should always be recorded to at least 1 decimal place (e.g. 12.1 mm not just 12 mm)

## Comparison of Micrometers and Vernier Calipers | Feature | Micrometer | Vernier Caliper | |---------|-----------|----------------| | **Measuring range** | Smaller (typically 0-25 mm) | Larger (typically 0-150 mm) | | **Resolution** | 0.01 mm | 0.1 mm (sometimes 0.02-0.05 mm) | | **Accuracy** | Better (due to better resolution) | Lower | | **Speed of use** | Slower (involves rotating thimble) | Quicker to use | | **Best for** | Very precise small measurements | Quick measurements over wider range | - In general, the micrometer has a smaller measuring range than a vernier caliper - However, the micrometer has a better accuracy (due to better resolution) - The vernier caliper is quicker to use, whilst the micrometer involves rotating the thimble - Therefore, to take many measurements, a caliper would be easier to use ## Practical Considerations - Both instruments should be checked for **zero errors** before use - With nothing between the jaws, the reading should be zero - If not, this zero error value must be subtracted from all measurements - For the most accurate measurements: - Ensure the object is clean and free from debris - Apply consistent pressure when closing the jaws - Take multiple readings at different positions and calculate a mean - Keep instruments clean and stored properly when not in use - When measuring cylindrical objects: - Measure at several points along the length - Take readings at different orientations (e.g. measure diameter at 90° intervals) - This accounts for any irregularities in shape