Chapter 2 of 8
Chapter 2: Dial Calipers
2.1 Introduction
Dial calipers are among the most common and versatile precision measuring instruments used in manufacturing, inspection, and engineering design. Compared to steel rules, dial calipers provide greater precision, improved repeatability, and multiple measurement capabilities using a single instrument.
In this chapter, students will learn how dial calipers work, how to identify their components, how to correctly read both inch and metric dial calipers, and how to apply them to real inspection tasks. This chapter builds directly on Chapter 1 by extending concepts of precision, discrimination, and greatest possible error into higher-resolution instruments.
2.2 Measurement Capabilities of Dial Calipers
Dial calipers can perform four primary types of measurements:
- Outside measurements — external diameter, thickness, width
- Inside measurements — internal diameter or slot width
- Step measurements — distance between two parallel surfaces
- Depth measurements — depth of holes or recesses
Understanding which measuring faces to use is essential for accuracy.
2.3 Major Components of a Dial Caliper
A standard dial caliper consists of the following parts:
- Outside measuring jaws — used for outside dimensions
- Inside measuring jaws — used for inside dimensions
- Depth rod — used for depth measurements
- Blade (beam) — the fixed, graduated scale
- Slider — moves along the blade
- Dial — displays fine measurements
- Pointer (needle) — rotates as the slider moves
- Reference edge — tracks large increments on the blade
- Rack and pinion — converts linear motion to dial rotation
Each component contributes to converting linear displacement into a precise numerical reading.
2.4 Precision, Discrimination, and GPE for Dial Calipers
Inch Dial Calipers
- Blade scale increment: 0.100 in
- Dial division: 0.001 in
- Typical precision: 0.001 in
- Greatest Possible Error (GPE): ±0.0005 in
Metric Dial Calipers
- Blade scale increment: 1 mm
- Dial division: 0.02 mm
- Typical precision: 0.02 mm
- Greatest Possible Error (GPE): ±0.01 mm
Dial calipers significantly reduce uncertainty compared to steel rules, but they still require proper technique and care.
2.5 Reading an Inch Dial Caliper
Reading an inch dial caliper involves two steps:
- Read the blade scale to determine the number of full inches and tenths (0.100 in increments).
- Read the dial to determine the additional thousandths (0.001 in).
Final Measurement = Blade Reading + Dial Reading
Example
Blade reading = 1.200 in
Dial reading = 0.036 in
Reported measurement: 1.236 ± 0.0005 in
2.6 Reading a Metric Dial Caliper
Metric dial calipers are read similarly:
- Read the blade scale in millimeters
- Read the dial in hundredths of a millimeter
Example
Blade reading = 24 mm
Dial reading = 0.38 mm
Reported measurement: 24.38 ± 0.01 mm
Try It: Caliper Reading Practice
A random inch caliper reading is generated below. Add the blade and dial values, type the final measurement in inches, and check your work.
Blade scale shows: —
Dial shows: —
2.7 Proper Use and Technique
To ensure accurate readings:
- Clean measuring faces before use
- Ensure jaws are square to the part
- Use light, consistent pressure
- Lock the slider before reading if needed
- Never force the caliper onto a part
Remember: Dial calipers are precision instruments — not clamps.
2.8 Using Dial Calipers with Gage Blocks
Gage blocks are commonly used to:
- Verify dial caliper accuracy
- Perform self-calibration checks
- Practice reading consistency
Students should measure known gage block sizes using both inch and metric calipers and compare results after unit conversion.
2.9 Data Analysis Concepts (Lab Integration)
When multiple measurements are taken:
- Mean represents the average value
- Range shows data spread
- Sample Standard Deviation estimates variation within a sample
In MET 135, sample standard deviation is used unless otherwise specified.
2.10 Chapter Summary — Study Points
- Dial calipers provide higher precision than steel rules
- Four measurement types are possible with one instrument
- Read blade scale first, dial second
- Precision and GPE must be reported correctly
- Proper technique prevents systematic error
- Gage blocks are used for verification, not estimation
Chapter 2 Exercises
Each exercise includes a solved example followed by a student practice problem.
Exercise 1: Inch Dial Caliper Reading
Example (Solved)
Blade reading = 0.900 in
Dial reading = 0.072 in
Final measurement: 0.972 ± 0.0005 in
Student Practice
Blade reading = 1.300 in
Dial reading = 0.046 in
Determine the final measurement.
Show answer
1.300 + 0.046 = 1.346 ± 0.0005 in
Exercise 2: Metric Dial Caliper Reading
Example (Solved)
Blade reading = 18 mm
Dial reading = 0.44 mm
Final measurement: 18.44 ± 0.01 mm
Student Practice
Blade reading = 32 mm
Dial reading = 0.16 mm
Determine the final measurement.
Show answer
32 + 0.16 = 32.16 ± 0.01 mm
Exercise 3: Outside Diameter Measurement
Example (Solved)
Outside jaws measure a shaft:
Blade = 0.500 in
Dial = 0.028 in
Final measurement: 0.528 ± 0.0005 in
Student Practice
A bolt diameter reads:
Blade = 0.750 in
Dial = 0.094 in
Determine the diameter.
Show answer
0.750 + 0.094 = 0.844 ± 0.0005 in
Exercise 4: Unit Conversion with Caliper Data
Example (Solved)
Measured diameter = 1.250 in
Converted to mm: 1.250 × 25.4 = 31.75 mm
Student Practice
A caliper reading shows 0.875 in. Convert to millimeters.
Show answer
0.875 × 25.4 = 22.23 mm (rounded to two decimal places).
Exercise 5: Percent Difference Between Instruments
Example (Solved)
English caliper mean = 1.000 in
Metric caliper mean = 25.42 mm = 1.001 in
%D = ((1.001 − 1.000) / 1.000) × 100 = 0.10%
Student Practice
English caliper mean = 2.500 in
Metric caliper mean = 63.60 mm
Convert and calculate percent difference.
Show answer
63.60 ÷ 25.4 = 2.504 in.
%D = ((2.504 − 2.500) / 2.500) × 100 = 0.16%