LINEAR MEASURING INSTRUMENTS

1. Introduction

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:

  1. Non-precision instruments
  2. Precision instruments
  3. Comparators

3. Non-Precision Linear Measuring Instruments

These instruments are used where high accuracy is not required.

(a) Steel Rule

A steel rule is a simple measuring tool used to measure linear dimensions directly.

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

  • 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

4. 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.

5. 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

6. Characteristics of Linear Measuring Instruments

  1. Accuracy – Closeness to true value
  2. Precision – Repeatability of measurement
  3. Sensitivity – Ability to detect small change
  4. Range – Maximum measurable dimension
  5. Least Count – Smallest measurable value

7. Sources of Error

  • Parallax error
  • Temperature variation
  • Wear of measuring surfaces
  • Improper handling
  • Zero error

8. Advantages of Linear Measuring Instruments

✔ Ensure dimensional accuracy
✔ Improve product quality
✔ Reduce rejection
✔ Essential in mass production
✔ Help maintain interchangeability

9. Applications

  • Automobile and aerospace industries
  • Manufacturing and machining
  • Quality control
  • Research and development
  • Inspection and maintenance