A real practical bench | Elementa Labs Africa

ChemistryMicrometer screw gauge

1 of 8 · Level 1: Identify

Wind the thimble shut, then turn it open exactly one complete revolution and watch the spindle.

01Turn the thimble and watch the spindle move along the sleeve.This is the whole mechanism: the thimble turns a screw, and a screw turns rotation into a small, exactly-known linear movement. One revolution is one pitch, every time.
02Wind the thimble back until the gap is wider than the object.Forcing an object into a gap that’s too small damages the anvil and spindle faces, and a damaged face is a zero error you can’t correct away.
03Rest the object against the anvil, the fixed face, and bring the spindle to meet it.The anvil doesn’t move. The object sits against it and the screw brings the other face up; you don’t chase the object with both.
04Turn the thimble down until the faces are nearly touching, then STOP using the thimble.The thimble is the fast way across the gap. It isn’t the way to make contact: it can apply as much force as your grip allows, and it will spring the frame.
05Finish with the ratchet until it clicks and slips.The ratchet slips at a fixed torque, so every measurement is taken at the same pressure. Closed by hand you tighten it differently every time, and a soft object is squashed by however hard you happened to grip.
06Use the lock if you want to take the gauge off the work before reading it.The locking ring holds the spindle so the reading can’t creep while you move the instrument to the light. It isn’t needed if you read it in place.
07Read the sleeve first: whole millimetres above the datum line, half-millimetres below it.The thimble’s edge tells you which marks are uncovered. Read the thimble first and you have nothing to add its hundredths to.
08Then read the thimble division level with the datum line.That is a COUNT, not a length. It becomes a length when you multiply it by 0.01 mm.
09Add them: sleeve + thimble × 0.01 mm.7.5 + 37 is 44.5 and is nonsense. 7.50 + 0.37 is 7.87 mm and is the reading.
10Before all of it, shut the gauge on nothing and see whether it reads zero.It often doesn’t. Whatever it reads shut is carried into every measurement you take with it, and it has to come off the end with its sign.

Laboratory

thimble rotation

0.00 rev

spindle movement

0.00 mm

Drag down the thimble to wind the spindle in, up to wind it out; roll slowly for the last hundredth. Close on the work with the ratchet, the small knurled cap at the end, so it slips at the right pressure. The lock is the ring on the sleeve.

Touching, but closed by hand, at unknown pressure.

Read the sleeve first: the last mark the thimble’s shoulder has uncovered, whole millimetres above the datum line and half-millimetres below it. Then read the thimble division lying on that line. One division is 0.01 mm, one revolution is 0.5 mm, and the gauge opens to 25 mm in 50 turns.

Turn the thimble exactly one full revolution. The "+1 turn" step does it precisely. Watch how far the spindle backs off the anvil as you do.