Your GO plug enters the bore. Your NO-GO plug does not start. The part is accepted and moves to assembly, where it is found to be out of round and gets returned. Nothing went wrong with the gauges or the operator. The inspection simply answered a narrower question than the one assembly was asking.
Limit gauging returns one bit of information per feature: inside the limits, or outside them. That bit is cheap, fast and repeatable, which is why it survives on shop floors that own perfectly good measuring machines. What it is not is a measurement.
A GO gauge represents the maximum material condition of the mating part. For a hole, that is the smallest permitted size, and the gauge is made to enter over the full engagement length. Because it engages the whole feature at once, it checks size and form together. If the bore is bent, tapered, oval or lobed enough to violate the envelope, the GO gauge will not pass, even when every individual diameter measures inside tolerance.
A NO-GO gauge represents the minimum material condition, the largest permitted size for a hole. It is designed to check local two-point size at any single position, so it is deliberately short. It must not enter. If it does, the feature is oversize somewhere.
This split is Taylor's principle, and it is why the pair exists. IS 3455 sets out the gauging practice and the tolerances that follow from it.
| Question | GO gauge | NO-GO gauge |
|---|---|---|
| Is the feature at least the minimum size? | yes | no |
| Is the feature within the maximum size? | no | yes |
| Does the feature assemble over its full length? | yes | no |
| Is form error inside the tolerance envelope? | yes | no |
| What is the actual dimension? | no | no |
A number. Attribute data has no magnitude. You cannot compute Cp or Cpk from pass and fail counts, you cannot see the process drifting towards a limit, and you cannot tell whether a bore came in at the bottom of the band or a micrometre from the top. A process that has quietly moved 15 µm across a 21 µm tolerance looks identical to a centred one until the day it fails.
Where in the feature the problem sits. A GO gauge that will not enter tells you something is wrong, not whether the cause is at the mouth, the middle or the far end.
Surface finish. A bore with tall roughness peaks presents a smaller effective envelope than its measured size suggests, and the GO gauge feels those peaks.
Anything about the other features. Position, perpendicularity and concentricity are separate characteristics needing separate gauges or a measuring machine.
A gauge is a manufactured part with its own tolerance, and that tolerance is taken out of the work tolerance rather than added to it. The traditional gauge maker's rule allocates about 10% of the part tolerance to the gauge.
Take a 25 mm H7 bore. The band under ISO 286 is 0 to +21 µm. A gauge set made to roughly a tenth of that leaves you about 19 µm of usable band, before you account for wear. GO gauges also carry a wear allowance, since they take the heaviest use and wear in the direction that makes them pass parts they should reject.
That is the quiet arithmetic behind a gauge purchase. Buying a looser gauge class saves money on the order and spends it again on good parts rejected and bad parts accepted.
Use it when the tolerance is comfortable relative to your process spread, when volumes are high, when the check happens at the machine, and when an operator needs an unambiguous answer without interpretation.
Switch to variable measurement when the tolerance is tight enough that you need to see the process, when you owe a customer capability indices, when you are diagnosing a problem rather than sorting parts, or when first-article approval requires actual dimensions.
Most shops end up running both: a bore gauge or air gauge for setting and monitoring, plug gauges for the fast decision at the machine. The mistake is not choosing one. It is assuming the fast decision carried more information than it did.
If you are matching a gauge pair to a fit you are actually inspecting, our plain gauges are made to the class the job needs rather than a default, and we will tell you when a gauge is the wrong instrument for the tolerance you are holding.