One Hundredth of a Millimetre

Why it matters

Turn a screw one full circle and it advances exactly one millimetre. Mark 100 steps around its head, and each step is a hundredth — a millimetre, divided by rotation.

The idea in plain words

The rotating ruler. Tap the parts.

reading = MS + (circular division × L.C.)

MS reads 4 mm; circular scale shows 67. Wire diameter?

4.67 mm

• mm/turn: pitch — distance the spindle advances per full rotation (usually 1 mm or 0.5 mm)
• the precision: L.C. — pitch ÷ number of circular divisions = 0.01 mm typically
• the safeguard: ratchet — clicks to stop you squeezing the wire — equal pressure, honest readings
• the quirk: backlash error — looseness when reversing rotation — always approach the reading turning one way
• 4 + 67 × 0.01

Predict first

A screw gauge has pitch 1 mm and 100 circular divisions. Its least count is…

L.C. = pitch ÷ circular divisions = 1/100 = 0.01 mm — ten times finer than the vernier.

• 0.01 mm — correct
• 0.1 mm
• 1 mm

What you do

Twist the thimble to both target readings and hold each one steady: main scale gives whole millimetres, the drum gives the hundredths.

Check yourself

The screw gauge is the right tool for…

Reserve the 0.01 mm instrument for 0.01 mm jobs; a rule handles the pencil fine.

Pitch 0.5 mm, 50 divisions. L.C. = ?

0.5 ÷ 50 = 0.01 mm — different design, same precision.

The ratchet's job is to…

Squeeze a wire hard and you measure a squashed wire — the ratchet clicks before that happens.

• a wire's diameter and a paper sheet's thickness — correct
• the length of a pencil
• a room's width
• 0.01 mm — correct
• 0.1 mm
• 0.025 mm
• apply the same gentle pressure every time — correct
• lock the reading in memory
• count the rotations