Measurement and tolerances — Engineering, 11–13 years
Real parts are never made with infinite precision. Engineers decide which small differences are safe, useful or unacceptable, so parts fit and products work reliably.
A small allowed difference
A tolerance is the small range around a target measurement that a part may have and still work. A bolt labelled 10 millimetres might be allowed to measure from 9.9 to 10.1 millimetres. The tighter the fit or safety requirement, the smaller the permitted range usually needs to be.
Why allow variation?
Factories cannot make every part exactly identical: tools wear, materials expand and measurements have limits. Rejecting every tiny difference would waste time and material. Tolerances solve this by stating what variation still works, while important features can receive much tighter limits.
Checking a pin
A hole is designed to be 20.0 millimetres wide, with a tolerance of plus or minus 0.2 millimetres. Step 1: calculate the lower limit: 20.0 − 0.2 = 19.8 millimetres. Step 2: calculate the upper limit: 20.0 + 0.2 = 20.2 millimetres. Step 3: a measured hole of 20.1 millimetres passes; 20.4 millimetres does not.
The sensible mistake
It is easy to read “10 millimetres ± 0.1” as a single exact size, because the target number stands out. But the ± sign describes a range, not extra decoration. A part measuring 9.9, 10.0 or 10.1 millimetres meets that specification; 9.8 does not.
Where it appears
Tolerances let replacement bicycle parts, phone connectors and toy bricks fit parts made at different times. They matter in medicine too: a syringe must deliver a safe amount, not merely an amount close to the target. Engineers set tighter tolerances where a small error could cause failure or harm.
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