Taking measurements with a screw gauge involves reading two scales and combining them, often accounting for any initial error.
A screw gauge is a precision instrument used to measure small distances, thicknesses, or diameters with accuracy typically up to 0.01 mm. It works on the principle of a screw and nut. The key components involved in taking a measurement are the main scale and the circular scale.
Understanding the Components
- Anvil and Screw: The object to be measured is placed between the fixed anvil and the movable screw tip.
- Sleeve (Pitch Scale): This is the main scale engraved along the sleeve. It typically measures in millimeters (mm) and half-millimeters (0.5 mm). The reference states that when the screw is worked, the sleeve moves over the pitch scale. This pitch scale shows the main reading.
- Thimble (Headscale): This is the circular scale on the rotating thimble. The reference specifically mentions that on the thimble there is a circular scale known as headscale which is divided into 100 equal parts. This scale is used to read fractions of the smallest division on the pitch scale.
- Pitch: The reference mentions the pitch of the screw gauge. The pitch is the linear distance moved by the screw (and thus the thimble along the sleeve) for one complete rotation of the thimble (360 degrees). It's usually 0.5 mm or 1 mm.
- Least Count (LC): This is the smallest distance that can be accurately measured by the screw gauge. It's determined by the pitch and the number of divisions on the headscale (circular scale).
- LC = Pitch / Number of divisions on the headscale.
- Based on the reference: If the pitch is 0.5 mm and the headscale has 100 divisions, the LC = 0.5 mm / 100 = 0.005 mm. If the pitch is 1 mm and the headscale has 100 divisions, the LC = 1 mm / 100 = 0.01 mm. The least count is essential for converting the headscale reading into a linear measurement.
- Ratchet: A mechanism that clicks when sufficient pressure is applied, ensuring uniform pressure for consistent measurements.
Steps for Taking a Measurement
Here's how to use a screw gauge to measure an object:
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Clean and Check Zero Error:
- Ensure the anvil and screw surfaces are clean.
- Bring the anvil and screw tip into contact using the ratchet (don't overtighten).
- Check the zero error. Ideally, the zero mark of the headscale should coincide with the zero mark of the pitch scale.
- Positive Zero Error: If the headscale zero is below the pitch scale zero line.
- Negative Zero Error: If the headscale zero is above the pitch scale zero line.
- Record the zero error value (the headscale reading that aligns with the pitch scale line when the jaws are closed, with appropriate sign).
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Place the Object: Open the screw gauge by rotating the thimble and place the object (e.g., a wire, a thin sheet) between the anvil and the screw tip.
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Tighten Gently: Rotate the thimble until the object is held firmly but gently. Use the ratchet to apply consistent pressure and stop when it clicks a few times.
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Read the Pitch Scale (Main Scale Reading - MSR):
- Note the last visible division on the pitch scale (sleeve) just before the edge of the thimble. This is your Main Scale Reading (MSR).
- For example, if the last visible mark is 5 mm and a 0.5 mm mark is also visible just after it, the MSR is 5.5 mm.
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Read the Headscale (Circular Scale Reading - CSR):
- Look at the headscale on the thimble. Identify the division on the headscale that coincides (aligns perfectly) with the pitch scale reference line (the horizontal line on the sleeve). This is your Headscale Coinciding Division.
- Based on the reference, the headscale has 100 equal parts. Let's say the 45th division aligns.
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Calculate the Total Observed Reading:
- Multiply the Headscale Coinciding Division by the Least Count (LC) of the screw gauge. This gives you the Headscale Reading (HSR).
- HSR = Headscale Coinciding Division * LC
- Add the MSR and HSR to get the Total Observed Reading.
- Total Observed Reading = MSR + HSR
- Example: If MSR = 5.5 mm, LC = 0.01 mm, and CSR = 45, then HSR = 45 * 0.01 mm = 0.45 mm. Total Observed Reading = 5.5 mm + 0.45 mm = 5.95 mm.
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Apply Zero Correction:
- Subtract the zero error from the total observed reading to get the final, accurate measurement.
- Final Reading = Total Observed Reading - Zero Error
- Example: If the zero error was +0.02 mm, Final Reading = 5.95 mm - (+0.02 mm) = 5.93 mm.
- Example: If the zero error was -0.03 mm, Final Reading = 5.95 mm - (-0.03 mm) = 5.95 mm + 0.03 mm = 5.98 mm.
Summary of Reading Calculation
| Reading Component | How to Read |
|---|---|
| Main Scale Reading | Last visible division on the sleeve before the thimble edge. |
| Headscale Coinciding Division | Division on the thimble's headscale that aligns with the pitch scale line. (Reference mentions 100 equal parts on the headscale located on the thimble). |
| Least Count (LC) | (Pitch / Number of headscale divisions). Pitch is the distance the sleeve moves per thimble rotation. |
| Headscale Reading (HSR) | Headscale Coinciding Division * LC |
| Total Observed Reading | Main Scale Reading + Headscale Reading |
| Zero Error | Reading when jaws are closed (can be positive or negative). |
| Final Reading | Total Observed Reading - Zero Error |
By following these steps and correctly identifying the readings on the pitch scale (sleeve) and the headscale (thimble with its 100 equal parts as per the reference), you can accurately measure the dimensions of small objects using a screw gauge.
