Measuring with Calipers, Micrometer and Gauge

A caliper measurement gives a reliable result when the instrument is clean, shows zero with the jaws closed, and you read it straight on without pressing hard. Many rejects start right at measurement: wrong pressure, reading at an angle or an instrument past its calibration date lets a bad part through or rejects a good one.

What to check before the first measurement

Before you measure the first piece on a shift, go through four steps:

1. Wipe the jaws or measuring surfaces and the part itself. A chip, oil or a burr changes the result.
2. Close the caliper or micrometer and check the zero. On a vernier caliper, the zeros of the vernier and the main scale must line up.
3. Look at the sticker or the instrument card. The date of the next calibration is set by the plant.
4. Check the workstation instruction to see which instrument is assigned to a given dimension and how often to measure pieces.

If an instrument does not show zero or has an expired sticker, do not measure with it and report it to the team leader.

How to measure with calipers step by step

Calipers measure three kinds of dimensions:

• outside (shaft diameter, sheet thickness): with the large jaws, the part as close to the beam as possible;
• inside (hole, groove): with the small jaws at the top, spread until they touch the walls;
• depth: with the depth rod at the end of the beam, set square against the bottom.

You close the jaws lightly until they touch the part, without pressing. Hold the instrument square to the measured surface, because a tilted caliper shows more than the real dimension. Read it with your eyes at the level of the scale.

How to read a 0.02 mm vernier

A 0.02 mm vernier has 50 divisions over a length of 49 mm. The reading has two parts:

1. Read the full millimetres from the main scale, before the vernier zero.
2. Find the vernier line that lines up with a line on the main scale and multiply its number by 0.02 mm.

Example: the vernier zero sits past 21 mm and the third line lines up. The result is 21 + 3 x 0.02 = 21.06 mm. A 0.05 mm vernier has 20 divisions and a 0.1 mm one has 10 divisions; the reading rule is the same.

On the shop floor people talk in "hundredths": one hundredth is 0.01 mm, two hundredths is 0.02 mm. A micrometre (µm) is 0.001 mm.

Calipers, micrometer or gauge

Each instrument has a different job. Accuracy depends on the specific model, so the table gives typical values.

Which instrument and which tolerance apply to a given dimension is set by the plant in the instruction and on the drawing. A dimension without its own tolerance falls under the general tolerance written near the title block, e.g. "ISO 2768-m". For a dimension from 6 to 30 mm in class m, that is ±0.2 mm.

How to measure with a micrometer and what the ratchet is for

You read a micrometer in two places. Whole millimetres and halves are on the sleeve (the longitudinal scale), hundredths of a millimetre on the thimble (the circular scale). Example from the manufacturer's manual: the sleeve shows 7, the thimble 0.37, the result is 7.37 mm.

You bring the measuring faces together only with the ratchet, the knob at the end of the thimble. The ratchet limits the pressure, so the result does not depend on the strength of your hand. Turning the thimble alone until it stops squeezes the part and gives a reading that is too low. How many turns of the ratchet to make depends on the model and the plant instruction.

You check the micrometer zero with the faces closed, because it can shift after an earlier measurement or after a temperature change.

GO / NO-GO limit gauge

A plug gauge for holes has two ends. The GO end must enter the hole, the NO-GO end must not. The part is good only when both conditions are met.

You insert the gauge without force. Forcing in the NO-GO end gives a false result and ruins the instrument. The GO end of a working gauge has a wear allowance, because it is used most often; a worn gauge goes to calibration.

Where rejects come from during measurement

The most common sources of error are:

• pressure: jaws pressed too hard or a thimble turned without the ratchet;
• parallax: reading at an angle instead of straight on;
• temperature: a part straight from machining or an instrument warmed in the hand;
• dirt and burrs on the part or on the measuring surfaces;
• positioning: a tilted caliper, measuring at the tips of the jaws;
• wear of the instrument and no current calibration.

If a dimension comes out at the edge of the tolerance or a second measurement gives a different result, do not "nudge" the instrument toward the expected number. Clean the part, check the zero, measure again and report it to the team leader. The decision about the part is made by quality control according to plant rules. More about the rhythm of work at machines is in the text about working in production, and about accuracy requirements in the automotive industry.

Frequently asked questions

What should I do if the caliper does not show zero?

Clean the jaws and close them again. If the zero still does not line up, put the instrument aside and report it to the team leader; you do not measure parts with a caliper whose zero is shifted.

What is calibration of a measuring instrument?

It is a comparison of the instrument's readings with a standard; the certificate gives the indication error and the measurement uncertainty. The interval between calibrations is set by the plant, and on the shop floor you see it on the sticker or the instrument card.

Do I have to measure every piece?

It depends on the workstation instruction: the plant specifies in the inspection card which dimensions to check and how often. Ask about the measuring rules at your workstation already on the first day at a new job.

What is the difference between calibration and legal verification?

Legal verification (legalizacja) is a statutory check of instruments used, for example, in trade or health protection. Calipers, micrometers and gauges used in quality control on the shop floor are usually subject to calibration.

This text describes the general rules. What you are entitled to depends on your type of contract: an employment contract (umowa o pracę) or a contract of mandate (umowa zlecenie). Your own contract and payslip are what count.