Define Pitch and Least Count — Screw Gauge and Vernier Caliper

Definition of Pitch: Pitch is the linear distance advanced by the tip of the screw along the axis of the screw in one complete rotation of the screw head.

In other words: • When a screw makes ONE full turn (360°), it moves a fixed linear distance forward or backward • This linear distance = PITCH

Pitch formula: Pitch = Distance moved by screw / Number of rotations = Linear distance on main scale / 1 rotation

For Screw Gauge: • Pitch = 1 mm (for most standard screw gauges — the screw advances 1 mm per revolution) • Some precision gauges have pitch = 0.5 mm

For Vernier Caliper: • 'Pitch' concept applies to screw-type verniers • The main scale (MS) division = pitch of the main scale

Example: A screw gauge advances 2 mm in 4 rotations. Pitch = 2/4 = 0.5 mm

Definition of Least Count: Least Count is the smallest reading/measurement that a measuring instrument can accurately measure.

For Screw Gauge: Least Count (LC) = Pitch / Total number of divisions on the Circular Scale (CS)

Example: • Pitch = 1 mm, Circular scale has 100 divisions • LC = 1/100 = 0.01 mm = 10 µm (micrometres)

For Vernier Caliper: Least Count = 1 Main Scale Division (MSD) - 1 Vernier Scale Division (VSD) = 1 MSD - (n-1/n) MSD (where n = number of Vernier divisions)

Standard values: • Standard Screw Gauge: LC = 0.01 mm • Standard Vernier Caliper: LC = 0.1 mm

Reading formula for Screw Gauge: Measurement = Main Scale Reading (MSR) + (Circular Scale Reading × Least Count) = MSR + (CSR × LC)

Example: MSR = 5.0 mm, CSR = 45 divisions, LC = 0.01 mm Measurement = 5.0 + (45 × 0.01) = 5.0 + 0.45 = 5.45 mm

Parts of a Screw Gauge:

1. Anvil — fixed end
2. Spindle — movable end (connected to screw)
3. Sleeve (Main scale) — linear scale
4. Thimble (Circular scale) — rotating scale
5. Ratchet stop — applies standard pressure
6. Lock nut — locks the reading

How to use a Screw Gauge:

1. Check for zero error
2. Place object between anvil and spindle
3. Tighten using ratchet until it clicks
4. Read Main Scale Reading (MSR) from sleeve
5. Read Circular Scale Reading (CSR) from thimble
6. Apply formula: Reading = MSR + (CSR × LC)

Zero error: • Positive zero error: circular scale reads above 0 when fully closed → subtract from reading • Negative zero error: circular scale reads below 0 → add to reading

Difference between Screw Gauge and Vernier Caliper: Property | Screw Gauge | Vernier Caliper Principle | Screw (rotary motion) | Sliding scale Typical LC | 0.01 mm | 0.1 mm Precision | More precise | Less precise Measures | Diameter, thickness | Length, inner/outer diameter, depth Range | 0–25 mm (usually) | 0–150 mm or more