Screw gauge

Screw Gauge

A screw gauge, also known as a micrometer screw gauge, is a precision measuring instrument used to measure dimensions of small objects with high accuracy. It is commonly used in mechanical engineering and metallurgy to measure the thickness, diameter, or length of parts with minute dimensions.

Principle of Operation

The screw gauge operates on the principle of a screw. When the screw is rotated, it advances by a small, known distance per rotation, which can be measured against a scale.

Main Parts of a Screw Gauge

  1. Frame: The C-shaped body that holds the anvil and the barrel.
  2. Anvil: The fixed measuring surface against which the object to be measured is placed.
  3. Spindle: The movable screw that the thimble causes to advance or retract.
  4. Sleeve or Barrel: The part with the main scale engraved on it.
  5. Thimble: The part that is rotated to move the spindle. It has a circular scale engraved on it.
  6. Ratchet: A device that ensures uniform pressure is applied during measurement.

How to Use a Screw Gauge

  1. Clean the object and the measuring surfaces of the screw gauge.
  2. Rotate the ratchet to bring the spindle close to the anvil.
  3. Place the object between the anvil and the spindle.
  4. Rotate the ratchet until the object is firmly held between the anvil and the spindle.
  5. Read the measurement from the main scale and the circular scale.

Reading a Screw Gauge

The measurement is obtained by adding the main scale reading and the circular scale reading.

  • Main Scale Reading (MSR): The reading on the main scale just before the zero of the circular scale.
  • Circular Scale Reading (CSR): The reading on the circular scale that lines up with the datum line on the sleeve.

The total measured value is calculated as:

[ \text{Total Measured Value} = \text{MSR} + (\text{CSR} \times \text{Least Count}) ]

Where the Least Count (LC) is the smallest measurement that can be taken and is given by:

[ \text{Least Count} = \frac{\text{Pitch of the Screw}}{\text{Number of Divisions on the Circular Scale}} ]

Example of Reading a Screw Gauge

Suppose the pitch of the screw is 0.5 mm and there are 50 divisions on the circular scale. The least count would be:

[ \text{Least Count} = \frac{0.5 \text{ mm}}{50} = 0.01 \text{ mm} ]

If the main scale reading is 5 mm and the 25th division on the circular scale lines up with the datum line, the total measured value would be:

[ \text{Total Measured Value} = 5 \text{ mm} + (25 \times 0.01 \text{ mm}) = 5.25 \text{ mm} ]

Differences and Important Points

Feature Description
Use Used for measuring small dimensions with high precision.
Accuracy Highly accurate, with measurements typically in the range of hundredths of a millimeter.
Least Count The smallest measurement that can be taken, usually 0.01 mm or 0.001 mm.
Zero Error A common error that occurs if the zero on the circular scale does not line up with the datum line when the anvil and spindle are in contact.
Positive Zero Error Occurs when the zero of the circular scale is below the datum line when the jaws are closed.
Negative Zero Error Occurs when the zero of the circular scale is above the datum line when the jaws are closed.

Zero Error Correction

If there is a zero error, it must be accounted for in the final measurement. If the zero error is positive, subtract it from the total measured value. If the zero error is negative, add it to the total measured value.

Conclusion

The screw gauge is an essential tool for precision measurement in various fields. Understanding its operation, how to read it, and how to correct for zero error is crucial for obtaining accurate measurements. With practice, using a screw gauge becomes a straightforward task, allowing for the precise measurement of small objects.