Go/No-Go Gauges: How Attribute Gauging Works

A go/no-go gauge checks a feature against its two size limits without giving a reading: the go member is made to the part's maximum material limit and must fit, while the no-go member is made to the minimum material limit and must not. If go goes and no-go stops, the feature is inside tolerance.

This is attribute gauging at its simplest, no number, no operator judgment, just pass or fail in a second. That speed and foolproofness is why go/no-go gauges have run high-volume inspection for a century, from thread checks to hole diameters. But there is real design behind the two members: Taylor's principle governs their shape, and wear allowance governs how they age. This guide covers how the gauges work, the principle that shapes them, and where a pass/fail check helps and where it hides the very data you need for statistical process control.

What is a go/no-go gauge?

A go/no-go gauge is a fixed-limit inspection tool with two ends, each machined to one boundary of the part's tolerance. On a hole, the go plug is sized to the smallest allowed hole and must enter; the no-go plug is sized to the largest allowed hole and must not enter more than a hair. If the go end passes and the no-go end is refused, the hole is somewhere between its limits, good, and you never learned or needed the exact size. The gauge embodies the tolerance in steel.

The two members represent the two material conditions. The go gauge is made to the maximum material condition of the feature, the most material, meaning the smallest hole or the largest shaft, because that is the limit the part must clear to assemble. The no-go gauge is made to the least material condition, the least material, the largest hole or smallest shaft, the limit past which the feature is too loose. Because the gauge is a physical object at those exact limits, it checks the part the same way every time, in every hand, which is the whole appeal of attribute gauging.

How does attribute gauging work?

Attribute gauging sorts parts into categories, go or no-go, accept or reject, instead of measuring a value. The operator presents the feature to each member and reads the result off the physical fit: entry or refusal, pass or stop. There is nothing to interpret, nothing to record, and almost nothing to get wrong, which is why it survives on fast lines where a micrometer reading per part would be impossible. The gauge does the judging; the operator just watches what it does.

Here is the standard way to check a part with a two-member gauge:

1. Clean the feature and the gauge. Wipe chips, burrs, and oil off both. A speck of debris can block a good go end or hold a no-go end off, turning a good part into a false reject or a bad part into a false accept.
2. Try the go member first. Present the go end and let it enter under its own weight or light, even pressure. It should pass fully. Never force it, forcing masks an undersized hole and wears the gauge.
3. Try the no-go member second. Present the no-go end. On a good part it should not enter, or should start and stop within a small allowed amount. If it goes in, the feature is over the limit.
4. Read the combined verdict. Go passes and no-go stops means good. Go fails means the feature is too small (or a hole too tight); no-go passes means the feature is too large (or a hole too loose). Only both results together confirm the part.
5. Segregate rejects immediately. Set failed parts aside where they cannot rejoin the flow. An attribute check gives no warning of a drift toward the limit, so a reject is your only signal.
6. Check the gauge on schedule. Verify the gauge against a master periodically, because a worn go member slowly accepts parts it should reject.

What is Taylor's principle?

Taylor's principle is the rule that shapes go and no-go members, and it says the two ends do different jobs. The go gauge should check the maximum material condition of as many related dimensions as possible at once, so a go plug is a full cylinder that checks the hole's size and its form and its straightness together, because a part only assembles if all of them are right simultaneously. The no-go gauge should check the minimum material condition of a single dimension only, so a no-go member is made to touch at just two points, checking one dimension in one place, because looseness has to be caught dimension by dimension.

The reason is functional. Assembly fails if the collective effect of size and form makes the feature too big to mate, so the go end has to see them together, the way the mating part will. But a feature can be too loose in one direction and fine in another, so the no-go end has to isolate each dimension rather than average them. In practice a go plug is a full-form cylinder and a no-go is a pin or a segment that contacts at points. Get this backward, a full-form no-go, and you can pass an out-of-round hole that no single-dimension check would allow.

What are gauge tolerance and wear allowance?

Gauges are manufactured parts too, so they cannot be made to a perfect size, they get their own tolerance, called the gaugemaker's tolerance. A common convention is to set it at roughly ten percent of the part's work tolerance, so a 0.010-inch part tolerance yields a gauge tolerance near 0.001 inch. That gauge tolerance is normally placed inside the work tolerance (the "unilateral" or maker's practice), so the gauge never accepts a bad part, the safety comes out of the plant's usable tolerance, not the customer's requirement.

Wear allowance handles the fact that the go member rubs against parts thousands of times and slowly wears toward accepting oversized work. So a small wear allowance, often around five percent of the work tolerance, is added to the go gauge in the direction of the maximum material limit, giving it room to wear before it starts passing bad parts. The no-go member gets no wear allowance, because it rarely engages the part and so barely wears. The result is a go gauge that is checked and retired as it wears past its limit, and a no-go gauge that holds its size far longer.

By the numbers. The go/no-go convention traces to William Taylor's 1905 patent on limit gauging, and the gaugemaker's-tolerance and wear-allowance percentages above, roughly ten percent of the work tolerance for gauge tolerance and about five percent for go-gauge wear, are widely used industry rules of thumb rather than a single universal law, so treat them as ranges set by your own standards. When you formally qualify a go/no-go gauge as a measurement system, the AIAG Measurement Systems Analysis reference manual covers attribute measurement studies, assessing how well an attribute gauge and its operators agree with the truth and each other (AIAG, Measurement Systems Analysis). The measurement fundamentals underneath, resolution, reference values, and traceability, are documented in the NIST/SEMATECH engineering statistics handbook (NIST/SEMATECH e-Handbook).

When should you use go/no-go gauges?

Reach for go/no-go gauges when you need speed and consistency and do not need the number. High-volume production, where recording a measurement on every part is impossible, is the classic case, thread inspection, hole and pin diameters, slot widths. They are also strong where operator skill varies, because the gauge removes judgment: there is no scale to misread. And they pair naturally with functional GD&T callouts at maximum material condition, where a functional gauge physically embodies the tolerance zone, bonus and all.

Avoid them, or supplement them, when you need to understand the process rather than just sort it. An attribute gauge tells you a part failed but not by how much or which way it is trending, so it gives no early warning. That is the trade behind attribute versus variable inspection: pass/fail is cheap and fast but blind to drift. If a dimension is critical, or the process runs close to a limit, a variable gauge that yields a real number is worth the extra time because it lets you catch trouble before it makes a reject.

How do go/no-go gauges fit with SPC and inspection?

Go/no-go gauges are excellent sorters and poor sensors, and that distinction sets where they belong. They shine at containment, keeping bad parts from moving forward, in incoming inspection and on the line. But because they produce no measured value, they cannot feed a control chart, so a process controlled only by attribute gauging is flying blind between rejects. The richest quality systems use both: go/no-go gauges to sort fast, and periodic variable measurements to run SPC and see the process drifting toward a limit before it crosses.

They also still need a trustworthy measurement system. A go/no-go gauge that has worn or is used with debris makes false accepts and rejects, which is why an attribute study, the go/no-go equivalent of a gage R&R and a routine gauge check matter as much here as with any instrument. A first article inspection confirms the gauges match the drawing before a run. The last piece is where the results land: when go/no-go outcomes and reject counts are logged live at the point of inspection instead of tallied on a clipboard, a rising reject rate that signals a process drifting toward a limit shows up the same shift, giving the floor the early warning the attribute check itself cannot. That live feedback is what Harmony gives a plant. CLS made that shift, from measurements found the next morning to results visible during the shift, so even a pass/fail gauge starts telling you something before the scrap piles up.