answers to common gage questions

What is a ring gage?

Why are ring gages used?

What is the purpose of ring gages?

What is a calibration ring gage?

How to thread ring gages?

Ring gages are metrology tools that are used for gaging the outer diameters of pins, shafts, splines, fasteners, dowels, studs, and other machined cylindrical parts. These gauges are typically cylindrical in shape and are fabricated from a stable material (usually tool steel) with a highly precise (very high-tolerance) bore in their center that functions as the gauge for the outer diameter of the parts. The primary function of ring gauges is to establish on a go/no-go basis whether the part being gaged falls within the dimensional tolerance that was specified for it. The use of ring gauges enables inspections and quality decisions about machined parts to be made very quickly without the need to resort to the use of micrometers, calipers, thickness gauges, or other measurement instruments that could take longer to yield a result.

It important to note that ring gauges are used to quickly establish if a part falls within the accepted tolerances and yields a yes/no condition based on the gaging. It is not a measurement instrument in the sense that it is not providing an actual value of the part’s diameter – it simple gages whether the part falls within permissible limits and therefore can be accepted from a quality control perspective.

This article will provide information about the types of ring gauges, the various configurations that exist, the tolerance sizes, and the uses of these instruments. Note that there is an alternative context of the phrase ring gauge which is used in the jewelry industry and refers to tools (also called ring mandrels) that are used to gauge the sizes of rings such as wedding bands. These types of gauges are not addressed in this article.

To learn more about the other varieties of gauges, see our related guide covering the different types of gauges.

Types of Ring Gauges

There are three primary types of ring gauges from the functional perspective:

  • Go ring gauges
  • No-go ring gauges
  • Master or set ring gauges

Go ring gauges are designed to verify the upper tolerance limit of the outer diameter of the part. If the go ring gauge fits over the outer diameter of the part, then this indicates that the maximum allowable material condition for that part has not been exceeded, meaning that the diameter of the part falls within the upper limit of its tolerance, and therefore, the part is acceptable.

No-go ring gauges check the lower tolerance limit on the outer diameter of the part. If the no-go ring gauge does not fit over the outer diameter of the part, then this indicates that the minimum allowable material condition for the part has been satisfied, meaning that the diameter of the part exceeds the minimum or lower limit of its tolerance, and therefore, the part is acceptable.

If the go ring gauge will not fit over the part being tested, the part diameter is too large, meaning that it contains excess material and should be rejected. Similarly, if the no-go ring gauge fits over the part being tested, then too much material has been removed and the part falls below its minimum tolerance value and similarly should be rejected.

When ring gauges are used to directly assess the acceptability of parts, the practice is known as direct gaging.

Master ring gauges, also called set ring gauges, are used for the mastering of other metering instruments. These gauges may be used to set other measuring instruments such as bore gauges or internal micrometers, for example, by providing a measurement standard or reference standard for calibration purposes. The practice is known as indirect gaging.

Ring gauge tolerance classes

Ring gauges are available in a variety of tolerance classes. They are typically graded to the following levels, with XXX representing the highest-grade gauge that has the tightest tolerances and proceeding to looser tolerances as the grades progress towards ZZ :

  • XXX
  • XX
  • X
  • Y
  • Z
  • ZZ

The applicable standard that establishes these grades is Measurement of Plain External Diameters for use as Master Discs or Cylindrical Plug Gages, B89.1.5 – 1998, published by the American Society of Mechanical Engineers.

The gauge tolerance chart that defines the tolerance values as a function of class for different size ranges is shown in Table 1 below. Note that as you proceed from left to right in the table along a given row, the tolerance values increase (moving from class XXX to class ZZ). Also, absolute tolerance values rise with larger values of ring gauge sizes.

Table 1 – Ring gauge tolerances as a function of tolerance class and ring gauge size.

Tolerance Chart(English units, Total tolerance)
Ring Gauge Size Range (Inches)Tolerance by Class (micro-inches)
< XXXXX XYZZZ
0.0100.82510204070100200
0.8251.51015306090120240
1.5102.510204080120160320
2.5104.5102550100150200400
4.5106.5103365130190250500
6.5109.0104080160240320640
9.01012.01050100200300400800
12.01015.010751503004506001,200
15.01018.0101002004006008001,600
18.01021.0101252505007501,0002,000

Table data credit: Pratt & Whitney Measurement Systems, Inc.

Ring gauge configurations and material options

There are several configurations of ring gauge available, which are used for checking different types of parts. Plain ring gauges feature a smooth inner diameter and are designed for checking shafts and cylindrical parts that do not have threads or a spline cut along the surface. There are also thread ring gauges that are used to verify parts that have been threaded along their outer diameter. These gauges will have a corresponding thread cut on the inside diameter of the gauge that matches the thread type and pitch on the part. Ring gauges are available for all the standard thread types, sizes, and pitches used, including BSPT, NPFT, NPSM, NPT, NPTF, UNC, UNEF, UNF, UNS, and Whitworth.

Within the threaded ring gauges, special configurations exist for various types of thread including ACME Thread Ring Gages, Taperpipe Thread Ring Gages, Straight Pipe Thread Ring Gages, ANPT Thread Ring Gages, and Pg Conduit Thread Ring Gages (for use on fittings used in electrical conduit work).

Adjustable ring gauges are also available, which contain a set of three slots cut at 120o intervals which enable the gauge to be calibrated against a setting plug. This capability reducing production costs by extending the useful life of a gauge by permitting one that is no longer within tolerance due to use and wear to be tuned back to tolerance and reused.

There are also miniature thread ring gauges available for use in watches and other miniaturized mechanisms.

Ring gauge materials are typically tool steel, with options for chrome plated and carbide available that provide for greater wear resistance.

This data credit to Thomasnet.com

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