1. Introduction

Threads and gears are critical machine elements used for power transmission and fastening. Accurate measurement ensures:

• Proper fit and interchangeability
• Smooth operation
• Reduced wear and failure

Measurement involves checking dimensions, form, and accuracy using specialized instruments.

PART A: Measurement of Threads

2. Important Thread Parameters

• Major Diameter – Largest diameter of thread
• Minor Diameter – Smallest diameter
• Pitch Diameter (Effective Diameter) – Most important parameter
• Pitch – Distance between adjacent threads
• Thread Angle – Angle between thread flanks

3. Measurement of Major Diameter

Method

• Measured using:
  • Micrometer
  • Vernier caliper

4

Accuracy

• Simple but not very precise for threaded surfaces

4. Measurement of Minor Diameter

Methods

• Using vernier caliper
• Using small ball method

Internal Threads

• Measured using inside micrometer

5. Measurement of Pitch

Method

• Using thread pitch gauge

4

Working

• Match gauge teeth with thread profile
• Read pitch value

6. Measurement of Pitch Diameter (Effective Diameter)

(A) Two-Wire Method

• Two wires placed in thread grooves
• Measurement taken using micrometer

(B) Three-Wire Method (Most Accurate)

Principle

• Three precision wires placed in thread grooves
• Measurement over wires gives pitch diameter

4

Advantages

• Highly accurate
• Widely used standard method

7. Measurement of Thread Angle

Method

• Using Tool Maker’s Microscope

Working

• Magnified image used to measure angle

8. Measurement of Internal Threads

• Using:
  • Thread plug gauges
  • Optical methods
  • Coordinate measuring machines (CMM)

PART B: Measurement of Gears

9. Important Gear Parameters

• Pitch Circle Diameter (PCD)
• Module (m)
• Number of Teeth (Z)
• Tooth Thickness
• Pressure Angle
• Backlash

10. Measurement of Gear Tooth Thickness

(A) Gear Tooth Vernier

Description

• Special vernier caliper designed for gears

4

Measures

• Chordal thickness
• Tooth height

11. Measurement by Constant Chord Method

Principle

• Measures chordal thickness at a fixed distance from tooth top

Advantages

• Simple and widely used

12. Measurement by Base Tangent Method

Working

• Measurement taken across several teeth using micrometer

Advantages

• More accurate than single tooth measurement

13. Measurement of Pitch

(A) Circular Pitch

$$\text{Circular Pitch} = \frac{\pi D}{Z}$$Circular Pitch=ZπD​

(B) Measurement

• Using gear tooth vernier
• Using optical comparator

14. Measurement of Gear Profile

Methods

• Optical comparator (profile projector)
• Involute measuring machine

4

15. Measurement of Runout

Definition

• Deviation of gear from true circular motion

Method

• Using dial indicator

16. Measurement of Backlash

Definition

• Clearance between mating gear teeth

Method

• Measured using dial gauge or feeler gauge

17. Measurement of Tooth Spacing Errors

• Measured using:
  • Indexing devices
  • Optical methods

18. Modern Methods

• CMM (Coordinate Measuring Machine)
• Laser scanning
• Digital gear analyzers

19. Sources of Errors

• Manufacturing defects
• Tool wear
• Improper alignment
• Temperature variation
• Measurement instrument errors

20. Applications

• Automotive industry
• Machine tools
• Aerospace components
• Precision engineering

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