What is a Micrometer?
A micrometer is a precision measuring tool that uses a screw mechanism to measure small dimensions with an accuracy of up to 0.01mm. It is 10 times more precise than a standard caliper.
Types of Micrometers
- Outside (Spindle) Micrometer: Most common, used to measure external dimensions. Ranges: 0-25mm, 25-50mm, 50-75mm. Accuracy: 0.01mm.
- Inside Micrometer: Measures hole diameters, interchangeable measuring tips, ranges from 5mm upwards.
- Depth Micrometer: Measures depths of holes and recesses, flat reference base.
- Digital Micrometer: LCD display, accuracy 0.001mm, zeroing function, higher price.
Parts of an Analog Micrometer
- Anvil - fixed reference surface
- Spindle - movable part touching the object
- Sleeve - main scale with 0.5mm divisions
- Thimble - rotating part with 0-50 scale (each line = 0.01mm)
- Ratchet - ensures constant measuring pressure (clicks)
- Lock - locks the measurement
How to Read an Analog Micrometer
- Read full millimeters: Check the sleeve for visible full millimeter markings before the thimble edge.
- Read half millimeters: If the 0.5mm line is visible, add 0.5mm.
- Read hundredths of a millimeter: Read the thimble value aligned with the sleeve center line. Each line = 0.01mm.
- Combine values: Total = full mm + 0.5mm (if visible) + thimble × 0.01mm.
Example Readings
Example 1: Sleeve: 7mm, 0.5mm line not visible, thimble: 23 → 0.23mm → Total: 7.23mm
Example 2: Sleeve: 12mm, 0.5mm line visible, thimble: 37 → 0.37mm → Total: 12.87mm
Proper Measuring Technique
- Clean anvil and spindle surfaces
- Zero the micrometer
- Open micrometer wider than object
- Insert object between anvil and spindle
- Rotate thimble until spindle lightly touches object
- Use ratchet until 3 clicks are heard
- Lock measurement
- Read dimension
Importance of the Ratchet
Ensures consistent measuring pressure. Too tight or too loose contact will lead to inaccurate results or damage.
Common Errors
| Error | Effect | How to Avoid |
|---|---|---|
| Too much force (without ratchet) | Undersized reading, micrometer damage | Always use ratchet (3 clicks) |
| Dirty surfaces | Incorrect reading +0.01-0.05mm | Clean object and micrometer |
| High temperature | Thermal expansion distorts reading | Wait 15 minutes for temperature equalization |
| No calibration | Systematic error | Check zero before each measurement |
| Measuring edge | False reading | Measure at middle height |
Calibration and Zero Check
For 0-25mm micrometers: close using ratchet and check for 0.00mm. For higher ranges, use calibration standard. Some micrometers have adjustment keys.
Digital Micrometer Usage
- Turn on (ON/OFF)
- Close with ratchet
- Press ZERO
- Place object
- Use ratchet to 3 clicks
- Read LCD
- Switch mm/inches
Maintenance
After use: clean surfaces, leave slight gap, store in case. Monthly: lubricate spindle. Store in dry place away from dust and vibrations.
Measurement Ranges
Mikrometers are produced in 25mm ranges: 0-25, 25-50, 50-75, etc. Longer screw would cause errors due to bending.
Micrometer vs Caliper Accuracy
| Tool | Accuracy | Use |
|---|---|---|
| Analog caliper | ±0.02mm | General measurements, IT7-IT10 tolerance |
| Digital caliper | ±0.01mm | Standard measurements, IT6-IT8 |
| Analog micrometer | ±0.01mm | Precision measurements, IT4-IT6 |
| Digital micrometer | ±0.001mm | Ultra-precision, IT1-IT4 |
When to Use a Micrometer
- Required accuracy ±0.01mm or better
- IT6 or higher tolerances
- Measuring precise shafts and holes
- Quality control of rotating parts
- Checking tool wear
Knowledge Test
Micrometer shows 8mm on sleeve (0.5mm line visible), thimble 42 → dimension?
Solution: 8 + 0.5 + 0.42 = 8.92mm
Summary
Micrometer requires precision and care. Key points: use ratchet, check zero, proper maintenance, clean surfaces, mastering micrometer usage is essential in precision metalworking.