1. What is linear measuring instruments ?
Linear measuring instruments are devices used to measure length, diameter, thickness, height, and distance in engineering and manufacturing. These instruments are very important in mechanical engineering, production, and quality control to ensure accuracy and precision in components.
They are used in industries like automobile, aerospace, manufacturing, and fabrication to maintain dimensional accuracy and interchangeability.
2 Types of Linear Measuring Instruments
Linear measuring instruments are broadly classified into:
- Non-precision instruments
- Precision instruments
- Comparators
2.1 Non-Precision Linear Measuring Instruments
Non-precision linear measuring instruments are tools used when a high degree of accuracy (typically better than 0.5 mm or 1/64 inch) is not required. These are common in workshops for quick measurements, marking, and layout work.
(a) Steel Rule
The most fundamental measuring tool in any shop. Unlike a common desk ruler, a steel rule is made of tempered spring steel and starts its graduation exactly at the end of the rule.
Features
- Made of hardened stainless steel
- Graduated in mm and inches
- Length ranges from 150 mm to 1 meter
Least count = 0.5 mm or 1 mm
Applications of steel rule
- Rough measurement
- Marking workpieces
- Workshop and fabrication work
(b) Measuring Tape
Features
- Flexible and long
- Used for measuring large distances
- Made of steel, fiberglass, or cloth
Applications
- Construction
- Layout and large machine measurement
2.2 Precision Linear Measuring Instruments
These provide high accuracy and reliability.
(a) Vernier Caliper
A precision instrument used to measure external, internal, and depth dimensions.
Parts
- Main scale
- Vernier scale
- Inside jaws
- Outside jaws
- Depth rod
Least Count
0.1 mm, 0.02 mm, or 0.01 mm
Types
- Mechanical
- Dial
- Digital
Advantages
- Multipurpose
- Portable
- Easy to use
(b) Micrometer Screw Gauge
Definition
Used for measuring small thickness and diameters with high accuracy.
Parts
- Frame
- Anvil
- Spindle
- Sleeve
- Thimble
- Ratchet
Least Count
0.01 mm or 0.001 mm
Types
- Outside micrometer
- Inside micrometer
- Depth micrometer
Advantages
- Very high precision
- Reliable for small dimensions
(c) Height Gauge
Definition
Used for measuring height and marking lines on a surface plate.
Features
- Used with surface plate
- Provides accurate vertical measurement
Applications
- Layout work
- Inspection
(d) Depth Gauge
Definition
Used for measuring depth of holes, slots, and recesses.
2.3 Comparators
Comparators measure deviation from standard dimensions rather than direct measurement.
(a) Dial Indicator
Features
- High sensitivity
- Used to detect small variation
- Magnifies small movement
Applications
- Checking flatness
- Alignment
- Runout of shafts
(b) Slip Gauges (Gauge Blocks)
Definition
Highly accurate blocks used as standard for calibration.
Material
- Steel
- Ceramic
- Tungsten carbide
Applications
- Calibration
- Precision measurement
- Tool setting
3. Characteristics of Linear Measuring Instruments
- Accuracy – Closeness to true value
- Precision – Repeatability of measurement
- Sensitivity – Ability to detect small change
- Range – Maximum measurable dimension
- Least Count – Smallest measurable value
4. Sources of Error
- Parallax error
- Temperature variation
- Wear of measuring surfaces
- Improper handling
- Zero error
5. Advantages of Linear Measuring Instruments
- Ensure dimensional accuracy
- Improve product quality
- Reduce rejection
- Essential in mass production
- Help maintain interchangeability
6. Applications
- Automobile and aerospace industries
- Manufacturing and machining
- Quality control
- Research and development
- Inspection and maintenance