What is a Caliper?
A caliper is a precision measuring tool used to measure external dimensions, internal dimensions, and hole depths. It allows measurements with an accuracy of 0.02mm (analog caliper) or 0.01mm (digital caliper).
Types of Calipers
- Analog (Vernier) Caliper: Reading using the main scale and vernier scale. Accuracy: 0.02mm or 0.05mm. No batteries required. Cheapest option.
- Digital Caliper: LCD display showing values in mm or inches. Accuracy: 0.01mm. Zeroing function at any position. Requires battery (SR44 or CR2032).
- Dial Caliper: Analog dial showing fractions of a millimeter. Accuracy: 0.02mm. Easier reading than vernier. Medium price.
Parts of a Caliper
- Upper jaws (external) – for measuring external dimensions
- Lower jaws (internal) – for measuring internal dimensions and holes
- Depth rod – a movable rod to measure depth
- Main scale – with millimeter divisions
- Slider – movable part with vernier or display
- Lock screw – to fix the slider in position
How to Read an Analog Caliper
- Read full millimeters: Check where the zero of the vernier aligns on the main scale. Note the full millimeters before the zero.
- Read fractions of a millimeter: Find the line on the vernier that exactly aligns with a line on the main scale.
- Add values: Result = full millimeters + (vernier reading × 0.02mm)
Example: Zero of vernier is between 34mm and 35mm → 34mm. Line 7 on vernier aligns → 7 × 0.02mm = 0.14mm. Result: 34.14mm
Measuring External Dimensions
- Open the jaws wider than the object.
- Place the object between the upper jaws.
- Gently close the jaws until they touch the surface.
- Ensure the caliper is perpendicular to the object.
- Lock the slider.
- Read the measurement.
Common mistakes: Too much pressure, measuring at an angle, measuring at edges instead of mid-height.
Measuring Internal Dimensions
- Insert the lower jaws into the hole.
- Open the jaws until they gently touch the sides.
- Ensure jaws are at the widest part of the hole.
- Lock the slider.
- Read the measurement and add the jaw thickness (usually 10mm – check your caliper).
Example: Reading: 35.00mm + jaw thickness 10mm = 45.00mm hole diameter
Measuring Depth
- Place the caliper base flat on the edge of the hole.
- Extend the depth rod to the bottom of the hole.
- Ensure the base is parallel to the surface.
- Lock the slider and read the measurement.
Using a Digital Caliper
- Turn on the caliper with the ON/OFF button.
- Close the jaws completely and press ZERO to calibrate.
- Measure as usual.
- Read the value on the display.
- Switch between mm/inches using the MM/INCH button.
ZERO mode advantage: You can zero the caliper at any position, e.g., set it to 10mm and measure differences from that value.
Maintenance and Care
- Clean the caliper with a soft cloth after use.
- Lubricate once a month with a drop of machine oil on the slider.
- Store in a case in a dry place.
- Check zero regularly (closed jaws = 0.00mm).
- Check calibration after a drop.
Measurement Accuracy – Best Practices
| Rule | Why it matters |
|---|---|
| Gentle pressure | Too much force bends jaws and distorts measurement |
| Perpendicularity | Measuring at an angle increases value |
| Clean object | Dirt adds error |
| Room temperature | Metal expands thermally |
| Repeat measurements | Three measurements eliminate random errors |
Caliper Measurement Ranges
- 150mm (0-150mm) – most popular, covers ~90% of tasks
- 200mm (0-200mm) – medium-sized objects
- 300mm (0-300mm) – large parts
- 500mm+ – specialized applications
When NOT to Use a Caliper
- Accuracy <0.01mm → use a micrometer
- Measuring small internal diameters → use a dial gauge
- Length >500mm → use a ruler or tape
- Measuring angles → use a protractor
Quiz
Task: Measuring a hole with internal jaws shows 25.46mm, jaw thickness is 10mm. What is the hole diameter?
Answer: 25.46mm + 10mm = 35.46mm
Summary
The caliper is a fundamental tool in any workshop. Mastering proper measurement technique ensures accuracy suitable for most industrial tasks. Remember regular maintenance and handle the tool gently.