Guides

Gauges

Gauges are dimensional inspection tools used to establish quickly whether a part falls inside its specified tolerance limits. They normally display no numerical value. They return an accept or reject result, according to whether the part mates correctly with the gauge.

A gauge is not a measuring instrument

A caliper or a micrometer measures a dimension and hands you a number, say 24.982 mm. A gauge works the other way round. It embodies a defined size, a limit or a particular geometric form, and is used to check whether the part conforms to it.

That is why most gauges carry no scale. The operator never reads a size. They check whether the gauge enters, does not enter, fits or fails to fit as intended.

The method earns its place in production, where many identical parts have to be checked fast and with as little operator judgement as possible.

The question it answers

A gauge does not answer “what is the size?”. It answers “is the size within the permitted limits?”.

Master and reference gauges

A master gauge embodies a known size to a high degree of accuracy. It is used to set comparative instruments, to check other gauges, or to verify a measuring system as a whole.

Its job is not usually to inspect large numbers of production parts directly. It is the reference against which some other device is set or checked.

Set of Johansson gauge blocks in a wooden case — Johansson gauge blocks are the most common form of length reference on the shop floor and in the lab.

Limit gauges

Limit gauges embody the permitted limits of a dimension. They are used in pairs, or combine two gauging surfaces in a single body. Either way they confirm that the dimension lies between the lower and the upper permitted limit, with no need to measure its actual value.

Must mate with the part over the whole specified length, under the weight of the gauge or with light hand pressure. Excessive force is never permitted.

NO GO

Must not mate with the part beyond the amount allowed by the applicable procedure or standard.

Exactly how the two sides behave depends on whether an internal or an external dimension is being checked.

Taylor’s principle

The governing design rule for limit gauges was set out by William Taylor. The GO gauge checks the maximum material condition and should, as far as is practical, check every feature that affects the fit at the same time. That is why it normally has full form and engages the whole gauged length.

The NO GO gauge checks the least material condition. It is usually shorter, or restricted in its contact area, so that it checks one feature at a time.

A long cylindrical GO gauge, for instance, does not merely check a local diameter of the bore. It can also reveal form or alignment errors that would prevent the parts going together in service. Taylor’s principle is the reason the GO side is normally the longer one and the one that does most of the work.

Checking a bore with plug gauges

For a bore to be accepted, the GO plug must enter over the whole specified length while the NO GO plug must not enter. If the GO side will not go in, the bore is either too small or has a form defect that would block assembly. If the NO GO side does go in, the bore is larger than the upper limit.

The reason lies in how much material the part carries. In a bore, material is at its greatest when the diameter is at its smallest permitted value. So the GO plug corresponds to the lower limit of the diameter, and the NO GO plug to the upper limit.

Double ended Ø12H7 plug gauge with GO and NOGO ends, the NOGO end carrying a red band — Double ended plug gauge Ø12 H7. GO on the left, NOGO on the right with its red marking.

Gauges for plain cylindrical bores are known as plug gauges. They come as single ended, double ended or progressive. On a double ended gauge the GO and NO GO ends sit on either side of the same handle. The GO end is usually the longer of the two, because it checks the whole functional length of the bore. The NO GO end is shorter and often carries a red band.

On the progressive type both limits sit on the same end. The GO section enters first, and the NO GO section behind it must not. Plug gauges for large diameters may be lightened or built in sections to keep the weight down.

Checking a shaft with snap and ring gauges

On a shaft, the GO gauge corresponds to the upper limit of the diameter and must pass over the part. The NO GO gauge corresponds to the lower limit and must not pass. If GO will not pass, the shaft is above the permitted limit or has a form error. If NO GO passes, the shaft is below the permitted limit. This is because on a shaft the maximum material condition is the largest permitted diameter.

Ring gauges

Ring gauges enclose the entire external cylindrical surface of the shaft. The GO and NO GO limits are normally handled by two separate rings. The full circular surface of the GO ring gives a functional check of the diameter and can reveal form errors that would prevent assembly.

Snap gauges

Snap gauges are applied to the shaft from the side, which makes them much faster over a large batch. They come as single, double or progressive. On the progressive type the GO and NO GO limits sit on the same side.

Double ended snap gauge for shafts with GO and NoGo jaws — Double ended snap gauge. The two pairs of jaws carry the GO and NO GO limits separately.

Because a snap gauge does not enclose the whole surface of the shaft, the check may need repeating at several angular positions in order to catch a departure from roundness.

Adjustable gauges

Adjustable gauges have movable contact faces and can be set to different limit sizes within a given range. They are set using suitable reference standards. Their advantage is that one gauge covers more than one dimension, and that wear can be compensated for.

Go and NoGo micrometer with two adjustable spindles for limit gauging — A GO and NO GO micrometer. The two spindles are set to the lower and upper limit, so the instrument works as an adjustable snap gauge.

Watch out

Adjustability demands more careful verification. An adjustable gauge must not be used unless it has been set and checked against a suitable reference standard.

Taper gauges

Taper gauges are used to check internal and external conical surfaces. They come as taper plug gauges and as taper ring gauges.

Checking a taper is never about a single diameter. It examines the relationship between diameter, angle and axial position. Depending on the design, acceptance may be judged from limit marks, from how far the gauge enters, or from the contact pattern between the surfaces.

Thread gauges

Threads call for dedicated gauges, because a thread fit never hangs on a single diameter. It is governed by the pitch diameter, the pitch, the flank angle and departures from the helical form.

Metric thread plug gauge with GO and NO GO ends — Thread plug gauge, for internal threads.

M16x2 6g thread ring gauge marked GO, for external threads — Thread ring gauge, for external threads.

The GO thread gauge has the full profile and checks whether the thread will actually assemble. It must run on by hand over the whole specified length, without undue force.

The NO GO gauge usually has a shorter engagement length and a modified profile. Its job is mainly to confirm that the pitch diameter has not passed its limit. How far it may engage is set by the applicable standard and the type of thread. For routine checking of metric threads, engagement of up to two full turns is the criterion commonly used.

Checking with a thread plug or thread ring gauge does not always take the place of checking the major and minor diameters separately. Where the drawing calls for those features, plain gauges or numerical measurements are needed as well. More in the guide to thread gauges.

What is marked on a gauge

Markings vary with the type of gauge and the standard applied. Typically you may find:

The nominal size.

The tolerance or fit class.

Whether it is the GO or the NO GO side.

The limit deviations, or the actual size of the gauge.

The manufacturing standard.

The maker and an identification number.

A red band on the NO GO side.

What each marking means must be confirmed against the certificate and the instructions for that particular gauge.

Using a gauge properly

The reliability of the check rests as much on how the gauge is used as on the gauge itself.

Clean the gauge and the part. Swarf, oil, burrs and dust can all change the result.

Inspect the gauging surfaces for wear, corrosion or knocks.

Align the gauge properly with the dimension. Entering it askew can jam it or cause a false reject.

Use normal hand pressure only. A gauge must never be forced, tapped in or driven with a tool.

Do not spin a plain gauge inside the part for no reason. Friction only accelerates wear.

Take temperature into account, particularly on large sizes or tight tolerances.

Record the result, the identity of the gauge and its calibration status.

Gauge tolerance and wear

A gauge is not a theoretically perfect size. It has a manufacturing tolerance of its own, and an uncertainty of its own.

The GO side is used more often and comes into fuller contact with the parts, so it wears faster. Depending on the system applied, a wear allowance may be provided for it. The NO GO side is used less and usually never enters the part fully, so it wears less.

Wear can shift the real limit being applied and let unsuitable parts through. That is why gauges need a unique identity, a defined checking schedule, and a documented calibration or verification status.

Gauge tolerances and wear allowances are not a fixed percentage for every application. They follow from the type of gauge, the tolerance class and the standard applied.

What a gauge cannot tell you

An accept result does not mean you know the actual size of the part. A gauge will not tell you:

How close the dimension is sitting to a limit.

Which way the process is drifting.

What correction the machine tool needs.

The numerical value of a form error.

Whether drawing requirements are met that the form of the gauge does not cover.

Setting up the process, tracking it statistically and investigating a reject all call for numerical measurement with a suitable instrument.

In short

Gauges carry the tolerance limits off the drawing and put them at the point of inspection. They allow a fast, repeatable accept decision with no number to read.

A simple output does not make a simple tool. A gauge is only as reliable as its design, its relationship to the limits on the part, Taylor’s principle, its wear, its calibration and the way it is handled.