A standard screw gauge typically features 50 divisions on its circular scale. This number is crucial for accurate measurements and determines the instrument's precision.
A screw gauge is a precision instrument used for measuring the diameter of thin wires, the thickness of sheets of metal, and other small lengths with high accuracy, often down to a hundredth of a millimeter. Its design relies on the principle of a screw, which converts small rotations into linear movement.
Understanding the Scales of a Screw Gauge
To fully grasp the role of divisions, it's essential to understand the two primary scales present on a screw gauge:
- Main Scale (or Pitch Scale): This is a linear scale etched along the barrel of the screw gauge, similar to a ruler, usually marked in millimeters (mm). It measures the main length or diameter.
- Circular Scale (or Thimble Scale): This scale is marked on the thimble, which rotates as the screw gauge is adjusted. It provides the fractional part of the measurement, allowing for very precise readings.
The Circular Scale: The Key to Precision
The number of divisions on the circular scale is a defining characteristic of a screw gauge's precision.
- Standard Divisions: As per common specifications, a typical screw gauge often has 50 divisions on its circular scale. Each full rotation of the thimble moves the spindle by a fixed distance, known as the pitch.
- Pitch of the Screw: In many screw gauges, the pitch is 1 mm. This means that when the circular scale completes one full rotation (moving all 50 divisions past the main scale line), the main scale reading changes by 1 mm.
The Significance of Divisions: Calculating the Least Count
The number of divisions on the circular scale, combined with the pitch of the screw, directly determines the least count of the instrument. The least count is the smallest measurement that can be accurately made with an instrument.
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Definition: The least count (LC) is calculated by dividing the pitch of the screw by the total number of divisions on the circular scale.
$$ \text{Least Count} = \frac{\text{Pitch of the screw}}{\text{Number of divisions on the circular scale}} $$
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Example Calculation:
- If the pitch of the screw gauge is 1 mm (meaning one full rotation advances the spindle by 1 mm).
- And there are 50 divisions on the circular scale.
Then, the Least Count (LC) = $\frac{1 \text{ mm}}{50 \text{ divisions}} = 0.02 \text{ mm}$.
This means that each division on the circular scale represents 0.02 mm of linear movement, allowing for measurements with a precision of two hundredths of a millimeter.
Reading a Screw Gauge: Practical Application
When taking a measurement, the reading is obtained by combining the main scale reading with the circular scale reading:
- Main Scale Reading (MSR): Note the reading on the main scale just before the edge of the thimble.
- Circular Scale Reading (CSR): Identify the division on the circular scale that coincides with the main scale's reference line.
- Total Reading: Total Reading = MSR + (CSR × Least Count).
Variations in Divisions
While 50 divisions are common, some screw gauges may have 100 divisions on their circular scale. A screw gauge with 100 divisions would have a least count of 0.01 mm (if the pitch is 1 mm), making it even more precise. However, for many educational and practical purposes, a 50-division circular scale with a 0.02 mm least count is standard.
Summary of Key Characteristics
| Feature | Description |
|---|---|
| Main Scale | Measures in millimeters (mm). |
| Circular Scale | Rotational scale providing fractional measurements. Typically has 50 divisions, though 100 divisions are also found. |
| Pitch of the Screw | The linear distance moved by the spindle for one complete rotation of the circular scale. Often 1 mm. |
| Least Count | The smallest measurable distance, calculated as Pitch / Number of Circular Scale Divisions. For a 1mm pitch and 50 divisions, the LC is 0.02 mm. |
| Purpose | To measure small lengths, such as the diameter of wires or thickness of sheets, with high precision (e.g., read more about screw gauges on Wikipedia). |
The divisions on the circular scale are fundamental to the screw gauge's ability to provide accurate and precise measurements of small dimensions.
