Tools

Bench Work

1. Introduction to Metrology

Metrology is the science of measurement. Accurate measurement ensures mass production, quality control, and interchangeability.

1.1 Fundamental Concepts

  • Interchangeability: Ability of parts to be replaced without custom fitting.
  • Accuracy: Closeness of measured value to the true value.
  • Precision: Closeness of repeated measurements to each other.
  • Least Count (Resolution): Smallest measurable value of an instrument.
  • Parallax Error: Error caused when the scale is viewed from an angle.

2. Linear Measurement Tools

2.1 Steel Rule

  • Material: Spring steel / Stainless steel
  • Metric Graduation: 0.5 mm or 1 mm
  • English Graduation: 1/64 inch

Note: Do not measure from worn edges. Start from 1 cm or 1 inch mark.

2.2 Vernier Caliper

Used to measure Outside Diameter (OD), Inside Diameter (ID), and Depth.

Metric Least Count Calculation:

1 Main Scale Division (MSD) = 1.0 mm

1 Vernier Scale Division (VSD) = 0.98 mm

Least Count = 1 MSD − 1 VSD

Least Count = 1.0 − 0.98 = 0.02 mm

General Formula:

Least Count = Value of 1 MSD ÷ Total Vernier Divisions

English Vernier Least Count: 0.001 inch

2.3 Micrometer (Screw Gauge)

Works on screw-nut principle.

Least Count Calculation:

Pitch = 0.5 mm

Number of Thimble Divisions = 50

Least Count = Pitch ÷ Number of Divisions

Least Count = 0.5 ÷ 50 = 0.01 mm

  • Outside Micrometer
  • Inside Micrometer
  • Depth Micrometer

2.4 Vernier Height Gauge

Used with a surface plate for marking precise heights.

3. Angular Measurement Tools

3.1 Engineer’s Try Square

Used to check squareness: 90°

3.2 Universal Bevel Protractor

Accuracy: 5′ (minutes)

1° = 60′

5′ = 1/12°

3.3 Sine Bar

L = Distance between rollers

H = Height of slip gauges

θ = Angle

sin θ = H / L

θ = sin−1(H / L)

4. Transfer & Comparison Tools

  • Outside Caliper
  • Inside Caliper
  • Jenny (Hermaphrodite) Caliper
  • Telescopic Gauge
  • Small Hole Gauge
  • Feeler Gauge
  • Dial Indicator

5. Layout (Marking Out) Theory

5.1 Datum

Two adjacent surfaces filed flat and square at 90°.

5.2 Layout Tools

  • Surface Plate (Granite preferred)
  • Layout Dye (Prussian Blue)
  • Scriber
  • Surface Gauge
  • V-Block (90° groove)
  • Angle Plate

6. Punches & Hammers

Prick Punch

Point Angle: 30°

Center Punch

Point Angle: 90°

Ball Peen Hammer

  • Flat face – Striking punches
  • Peen face – Riveting

7. General Procedure for Bench Work

  1. Clean workpiece
  2. Create datum at 90°
  3. Apply layout dye
  4. Mark dimensions
  5. Punch lines and centers
  6. Verify dimensions before machining