Ultra-Precise Measurement of Wire Diameter with 0.01mm Accuracy
✅ FREE Experiment • 📏 Interactive Micrometer • 🎓 NEB Class 11 Practical
The Screw Gauge (also called a Micrometer Screw Gauge) is a precision measuring instrument used to measure very small dimensions with an accuracy of 0.01 mm. It works on the principle of a screw and is essential for measuring the diameter of thin wires, thickness of paper, metal sheets, and other small objects.
In this experiment, you'll learn how to use a screw gauge to measure the diameter of a wire, understand concepts like pitch, least count, and zero error, and perform accurate measurements with proper error correction.
🔄 Use the controls to rotate the thimble and take measurements
Main Scale Reading (MSR): Read the highest visible mark on the main scale (sleeve)
Circular Scale Reading (CSR): Read the division on thimble aligned with reference line
Total Reading = MSR + (CSR × LC)
| S.No. | Main Scale Reading (mm) |
Circular Scale Reading |
Observed Diameter (mm) |
Zero Error (mm) |
Zero Correction (mm) |
Corrected Diameter (mm) |
|---|---|---|---|---|---|---|
| No readings recorded yet. Take measurements and click "Record Reading" | ||||||
Mean Observed Diameter: 0.00 mm
Mean Zero Error: 0.00 mm
Mean Zero Correction: 0.00 mm
Mean Diameter of Wire: 0.00 mm
A screw gauge is a precision measuring instrument that uses a calibrated screw for measurement. It can measure dimensions as small as 0.01 mm, making it ideal for measuring the diameter of wires, thickness of sheets, and other small objects.
The main parts of a screw gauge are:
The distance moved by the spindle when the thimble is rotated by one complete rotation (360°). In standard screw gauges, pitch = 0.5 mm or 1.0 mm.
The smallest measurement that can be accurately measured by the instrument. It is calculated as:
For a screw gauge with pitch = 0.5 mm and 50 divisions on circular scale:
LC = 0.5 / 50 = 0.01 mm
When the spindle and anvil are just in contact (without any object), if the zero of the circular scale does not coincide with the reference line, the instrument has a zero error.
Types of Zero Error:
A screw gauge (micrometer screw gauge) is a precision measuring instrument used to measure very small dimensions with high accuracy (0.01 mm). It's used to measure the diameter of thin wires, thickness of metal sheets, paper, and other small objects.
A screw gauge works on the principle of a screw. When the screw rotates in a fixed nut, it moves forward or backward along its axis. The distance moved is proportional to the number of rotations. By having a fine thread (small pitch), very small movements can be measured accurately.
Pitch is the distance moved by the spindle along the main scale when the thimble is given one complete rotation (360°). In most standard screw gauges, the pitch is 0.5 mm or 1.0 mm. It can be determined by: Pitch = Distance moved on main scale / Number of complete rotations.
The least count is the smallest measurement that can be accurately measured by the instrument. It is calculated as: LC = Pitch / Number of divisions on circular scale. For a screw gauge with pitch 0.5 mm and 50 divisions, LC = 0.5/50 = 0.01 mm.
When the spindle and anvil are just in contact (without any object between them), if the zero of the circular scale does not coincide with the reference line on the main scale, the screw gauge is said to have a zero error. This error must be corrected from all readings.
When the zero of the circular scale lies below the reference line (has crossed it) when spindle and anvil are in contact, it's called positive zero error. In this case, the observed reading is more than the actual value. Zero correction = -|zero error|.
When the zero of the circular scale lies above the reference line (hasn't reached it) when spindle and anvil are in contact, it's called negative zero error. The observed reading is less than actual value. Zero correction = +|zero error| = +(100 - observed reading) × LC.
Corrected Reading = Observed Reading - Zero Error
Or: Corrected Reading = Observed Reading + Zero Correction
Where: Observed Reading = MSR + (CSR × LC)
The ratchet is a safety device at the end of the screw gauge that prevents over-tightening. When the object is properly gripped, the ratchet starts slipping with a clicking sound, ensuring uniform pressure and preventing damage to both the instrument and the object being measured.
The main parts are: (1) U-shaped frame, (2) Anvil (fixed end), (3) Spindle (moving end), (4) Sleeve/Barrel (contains main scale), (5) Thimble (contains circular scale), and (6) Ratchet (for uniform pressure).
Multiple readings are taken at different positions along the wire because: (1) The wire may not be perfectly uniform, (2) It reduces random errors, (3) The mean value is more accurate than a single reading, (4) It helps identify any systematic errors or outliers.
Backlash error occurs due to wear and tear in the screw thread, causing a gap between the screw and nut. When the direction of rotation of the thimble is reversed, there's a small play before the spindle starts moving. This can be minimized by always approaching the final reading from the same direction.
Screw Gauge: Least count 0.01 mm, measures very small dimensions (0-25 mm range), higher precision, used for wire diameter, sheet thickness. Vernier Caliper: Least count 0.1 mm or 0.02 mm, measures larger dimensions (0-15 cm range), lower precision, used for external/internal diameters and depths.
A screw gauge has a smaller least count (0.01 mm) compared to a vernier caliper (0.1 mm or 0.02 mm). The screw mechanism allows for finer divisions and more precise measurements, making it suitable for measuring very small dimensions with high accuracy.
(1) Always use the ratchet to prevent over-tightening, (2) Clean anvil and spindle before use, (3) Check for zero error before starting, (4) Avoid parallax error by proper eye positioning, (5) Handle gently as it's a precision instrument, (6) Take multiple readings, (7) Store in protective case, (8) Ensure object is perpendicular to spindle axis.
No, standard screw gauges have a limited range (usually 0-25 mm). For larger objects, you would need screw gauges with larger ranges (25-50 mm, 50-75 mm, etc.) or use other measuring instruments like vernier calipers.