• Tags: screw-gauge, micrometer, backlash
• Difficulty: Foundation

The screw gauge (micrometer) has LC = $\frac{pitch}{number}$ of circular scale divisions. Pitch = distance advanced per complete rotation (typically 0.5 mm). With 50 circular divisions: LC = 0.5/50 = 0.01 mm = 0.001 cm — ten times more precise than a vernier caliper. Reading = MSR + (CSR x LC), where CSR is the circular scale division aligned with the reference line. Zero error: when the screw is fully closed, if the zero of circular scale is below the reference line, the error is positive (subtract); if above, negative (add). Backlash error occurs due to wear in the screw threads — gaps between the screw and nut. Eliminated by always rotating the screw in the same direction when approaching the measurement. For cylindrical objects, measure diameter at several positions and orientations, then average. The ratchet stop ensures consistent pressure for reproducible readings.