Screw Gauge Viva Questions – Measuring Diameter and thickness
Screw Gauge Viva Questions – Measuring Diameter and thickness
Aim
To measure diameter of a given wire and thickness of a given sheet using screw gauge.
Basic Concepts
1. What is a screw gauge?
An instrument used to measure very small lengths (diameter of wires, thickness of sheets) accurately, up to 0.01 mm or even 0.001 mm.
2. What is the principle of a screw gauge?
It works on the principle of a screw: the linear distance moved by the screw is directly proportional to the number of rotations given to it.
3. What is another name for a screw gauge?
Micrometer screw gauge, or simply micrometer.
4. Name the parts of a screw gauge.
U-shaped frame, stud (anvil), spindle (screw), sleeve/barrel (with pitch scale), thimble (with circular scale), and ratchet.
Pitch and Least Count
5. What is the pitch of a screw gauge?
The distance moved by the spindle per one complete rotation of the thimble.
6. What is the least count of a screw gauge?
The smallest length that can be measured accurately with it; L.C. = Pitch / Number of divisions on the circular scale.
7. What is the usual least count of a screw gauge?
0.01 mm (when pitch = 1 mm and circular scale has 100 divisions).
8. How can the least count be reduced further?
By increasing the number of divisions on the circular scale, or by decreasing the pitch.
9. If the pitch is 0.5 mm and the circular scale has 50 divisions, what is the least count?
L.C. = 0.5/50 = 0.01 mm.
10. Why do we need a least count at all?
Because no instrument can measure to infinite precision; the least count tells us the smallest reliable reading, and hence the possible error in every measurement.
Zero Error
11. What is zero error?
The error that occurs when the zero of the circular scale does not coincide with the reference line of the pitch scale when the anvil and spindle are in contact.
12. What are the types of zero error?
Positive zero error and negative zero error.
13. When is the zero error said to be positive?
When the zero of the circular scale lies below the reference line when the jaws are closed.
14. When is the zero error said to be negative?
When the zero of the circular scale lies above the reference line when the jaws are closed.
15. How is the zero error corrected?
Corrected reading = Observed reading − Zero error (using the proper sign).
16. If zero error is +0.03 mm, is it added or subtracted from the observed reading?
Subtracted (since correction = −Zero error, so we subtract +0.03 mm).
17. Why must zero error be noted before every set of readings?
Because the screw threads can wear out with use, changing the zero error over time; also different screw gauges have different zero errors.
Construction and Working
18. What is the anvil (stud)?
The fixed end of the screw gauge against which the object is pressed.
19. What is the spindle?
The movable screw that advances toward the anvil when the thimble is rotated.
20. What is the sleeve or barrel?
The fixed cylindrical part on which the pitch scale (linear scale) is engraved.
21. What is the thimble?
The rotating cap that carries the circular scale (head scale) and moves the spindle when turned.
22. What is the function of the ratchet?
It ensures uniform pressure is applied on the object each time, by slipping (clicking) once a set force is reached, preventing damage to the wire/screw or a false compressed reading.
23. Why shouldn’t we rotate the thimble directly instead of the ratchet?
Because applying force directly through the thimble can exert unequal, excessive pressure, compressing the wire and giving an inaccurate reading, and it can also damage the screw threads.
24. What is backlash error?
The error that arises when the screw does not move backward immediately on reversing the direction of rotation, due to wear between the screw and nut.
25. How is backlash error avoided?
By always rotating the screw in the same direction while taking a particular set of readings.
26. What is the function of the lock nut (if present)?
To fix the spindle in position so the reading can be taken/checked without disturbance.
Taking Readings
27. What is the Pitch Scale Reading (P.S.R.)?
The reading on the linear scale just before/visible up to the edge of the thimble.
28. What is the Circular Scale Reading (C.S.R.)?
The division on the circular scale that coincides with the reference line of the pitch scale.
29. What is the formula for the total (observed) reading?
Observed Reading = P.S.R. + (C.S.R. × Least Count).
30. Why do we take readings at different points and in mutually perpendicular directions?
Because a wire or sheet may not have a perfectly uniform/circular cross-section, so multiple readings give a more reliable average and also help detect irregularities (like ellipticity).
31. Why do we take the mean of several readings?
To minimize random errors and get a more accurate and reliable value.
Precautions and Errors
32. Why should parallax error be avoided while reading a screw gauge?
Because viewing the scale at an angle can cause the reading to appear shifted from its actual position, leading to error.
33. What precautions should be taken while handling a screw gauge?
Move the screw only in one direction while measuring, use the ratchet (not the thimble) to close the gap, avoid parallax, note zero error carefully, and keep the instrument clean and free from dust/rust.
34. What error can arise due to a bent wire?
An incorrect (usually higher) diameter reading, so the wire must be held straight and taut.
35. Can a screw gauge measure the diameter of a wire that is not uniform throughout?
Not a single true value — readings must be taken at several points and averaged to get a representative diameter.
36. Why is the object placed gently between the anvil and spindle, not forced?
Excess force can compress the wire/sheet or damage the screw threads, giving a false (smaller) reading.
Applications and Comparisons
37. What are the uses of a screw gauge?
Measuring the diameter of thin wires, thickness of sheets/glass plates/coins, and diameter of small spheres or ball bearings.
38. Compare a screw gauge and vernier caliper.
A screw gauge has a smaller least count (0.01 mm vs 0.01 cm for vernier caliper) and is used for smaller objects like wires and thin sheets; a vernier caliper measures larger lengths like the diameter of a cylinder or depth of a beaker.
39. Can a screw gauge measure the diameter of a thick rod (say, 5 cm)?
No, because the gap between the anvil and spindle in a standard screw gauge is limited (usually a few cm at most), so it is used only for small dimensions.
40. What is the range of a typical laboratory screw gauge?
0 to 25 mm, though this varies by instrument.
Numerical/Conceptual
41. If P.S.R. = 3 mm, C.S.R. = 25, and L.C. = 0.01 mm, what is the observed reading?
Observed Reading = 3 + (25 × 0.01) = 3.25 mm.
42. If zero error is −0.05 mm and observed reading is 2.40 mm, find the corrected reading.
Corrected Reading = 2.40 − (−0.05) = 2.45 mm.
43. Why is it important to record the least count before starting the experiment?
Because every subsequent calculated reading depends on it, and it also tells us the maximum possible precision/error range of our final result.
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