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.
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+0.270mm
= 3.270mm
To get the final reading, simply
add the readings on the linear
and rotating scales
20. Reading no. Main Scale
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 :
1. Zero error may not be removed
2. Reading may not be noted properly
3. 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.
