Micrometer or screw gauge

Screw Gauge

1. MICROMETER OR SCREW GAUGE Talha Javed Comsats University Islamabad

2. Screw Gauge Invented in 1638 by  William Gascoigne , an English astronomer. Precision upto 0.01mm

3. Parts of screw gauge Anvil: The shiny part that the spindle moves toward, and that the sample rests against Sleeves: The stationary round component with the linear scale on it, sometimes with vernier markings. lock : The lever that one can tighten to hold the spindle stationary, such as when momentarily holding a measurement.

4. Continued… Spindle : The shiny cylindrical component that the thimble causes to move toward the anvil. Ratchet knob : Device on end of handle that limits applied pressure by slipping at a calibrated torque Thimble : The component that one's thumb turns. Graduated markings. Frame : The C-shaped body that holds the anvil and barrel in constant relation to each other.

5. Two Scales Are Better Than One The micrometer has two scales: the main or linear scale and the rotating scale. The main scale or linear scale is graduated into 0.500 mm marks. The rotating scale is composed of 50 divisions, each graduated into 0.010 mm marks.

6. Linear scale divided into 0.500 mm marks

7. Whole Marks: 0.000, 1.000, 2.000, 3.000, etc Half Marks: 0.500, 1.500, 2.500, 3.500 etc

8. Rotating scale with 50 divisions, each equal to 0.010 mm

9. Pitch of screw gauge Pitch = Distance moved on Linear Scale(S) Number of Rotation(N)

10. Least Count Least Count = Pitch Number of Division on circular scale

11. Micrometer closed = 0.000 mm means no zero error

12. After first rotation = 0.500 mm

13. After second rotation = 1.000 mm

14. After third rotation =1.500 mm

15. Calculating Pitch Number of Rotations = 3 Distance moved on Linear Scale= 1.500 mm So, Pitch=S/N Pitch = 1.500/3 Pitch= 0.5 mm

16. Calculating L.C Now we are going to find Least Count (L.C) Pitch= 0.5 mm No. of div. on circular scale= 50 L.C = L.C= 0.5/50 L.C = 0.01mm Pitch Number of Division on circular scale

17. Linear Scale: 3.000 Rotating Scale: 0.270 Total Reading=3+0.270 mm = 3.270mm To get the final reading, simply add the readings on the linear and rotating scales

18. Linear scale: 6.500 Rotating scale: 0.170 Total Reading : 6.670 mm

19. Linear scale: 1.000 Rotating scale: 0.205 Total R eading : 1.205 mm

20. Reading no. Main S cale Reading M (mm) Rotating Scale Reading N (mm) R = N * Least count (mm) Final Reading = M + R (mm) 1 2 3 Observations And Calculations Sources of Errors : Zero error may not be removed Reading may not be noted properly Thimble may be rotated hardly beyond its limits

21. Zero Error If the zero mark of the circular scale comes on the reference line. In this case, the zero error and the zero correction, both are nil . If zero error is positive then subtract it from total or measured reading If zero error is negative then add it to the total or measured reading.

22. Zero Error