Fix pitch = linear advance per full rotation Pitch is defined as the distance moved on the main scale per complete rotation of the circular scale (thimble). For a standard screw gauge, pitch = 0.5 mm (the screw advances 0.5 mm per revolution). LC = Pitch / n, where n = number of divisions on circular scale. For 50 circular divisions: LC = 0.5/50 = 0.01 mm. The trap: confusing pitch with LC — pitch is 0.5 mm, LC is 0.01 mm, a factor of 50 difference.

Identify zero error before reading When thimble is closed on zero (no object), check whether the circular scale zero coincides with the main scale datum line. If it does not, read the circular division that coincides — that is the zero error. If the circular zero is above the datum line, zero error is positive; if below, negative. Corrected reading = Total reading − zero error.

Read linear scale and circular scale separately, then combine Total reading = main scale reading (to nearest 0.5 mm) + circular scale division × LC. The linear scale advances in 0.5 mm steps (for standard pitch); each complete revolution exposes a new 0.5 mm mark. Circular scale gives the fractional extra beyond the last 0.5 mm mark. For example: main scale = 4.5 mm, circular scale = 22nd division, LC = 0.01 mm → total = 4.5 + 22 × 0.01 = 4.72 mm.

Know backlash error and how to eliminate it Backlash error arises from play (mechanical looseness) in the screw mechanism — when the rotation direction is reversed, the thimble moves without linear advance for a short arc. NEET may ask: 'To eliminate backlash error, the user should always rotate the screw in the same direction.' The trap: thinking zero error and backlash error are the same — zero error is a fixed offset from the calibration mark; backlash is a mechanical hysteresis when direction is reversed.