Micrometer Screw Gauge
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flagWhat you'll discover
- arrow_forwardRelate pitch and number of circular divisions to least count
- arrow_forwardClose the spindle correctly using the ratchet
- arrow_forwardCombine main scale and circular scale into one reading
- arrow_forwardCorrect readings for positive and negative zero error
Pitch, least count and the magic of screws
A screw converts big rotations into tiny advances. If one full turn of the thimble moves the spindle 1 mm (the pitch) and the thimble edge carries 100 divisions, then rotating by just one division advances the spindle 1/100 mm. Least count = pitch / number of circular divisions = 1 mm / 100 = 0.01 mm.
That is ten times finer than vernier calipers, which is why the screw gauge is the right tool for wire diameters, paper thickness and hair. Total reading = linear (main) scale reading + circular scale division on the reference line × least count.
Using the ratchet and reading the scales
Place the wire between the stud and spindle and turn using the ratchet cap, never the bare thimble. The ratchet slips at a fixed gentle pressure, so every student squeezes the wire identically — this kills the over-tightening error that plagues beginners.
Read the last fully visible millimetre mark on the sleeve, then the circular division aligned with the reference line. For a wire, take readings in two perpendicular directions at three places along the length, because drawn wires are slightly oval and tapered. Average everything.
Zero error and backlash
Close the gap fully with nothing inside: if the circular zero stops below the reference line, the error is positive (subtract); above it, negative (add). For example, if division 96 sits on the line with 100 divisions on the thimble, zero error = −4 × 0.01 = −0.04 mm and the correction is +0.04 mm.
The second hidden enemy is backlash: worn screws have slack, so the spindle lags when you reverse direction. Always approach the final position rotating the same way. Write both of these in the error analysis section of your practical copy — they are standard NEB viva questions.