Smooth Ring Calibration

A Smooth Ring Gage, sometimes called a Plain Ring Gage or Master Ring Gage, is a cylindrical gage with an internal bore with a precise dimension.

Smooth Ring Gages are typically used as a Go/No-Go gage to quickly inspect the outside diameters of inspected parts without needing to take measurements of the parts. The gages are also often used as a master gage for setting other measuring instruments to a known value or zero position, such as electronic bore gages, air tooling gages, internal micrometers and other length measuring instruments.
Smooth Ring gages are typically made from steel, chrome, and carbide. Steel rings are the most prevalent, chrome rings are used for areas where durability and corrosion resistance are important and carbide gages provide extreme wear resistance.

Accuracy Classification

Smooth Ring Gages are classified by their level of accuracy per the ‘Gage Maker’s Tolerance Chart for Pin and Ring Gages’. The classification ranges from Class ZZ with the widest tolerances to Class XXX with the tightest tolerances. The size of the ring gage will determine the exact tolerance within each classification.

The tolerances can be bilateral for use in setting variable gages, or unilateral for use as go/no-go gages. For smooth ring gages, Go gages are built with a minus tolerance and No-Go gages are built with a plus tolerance. The Go and No-Go gages are often distinguished from each other by the use of a groove cut into their knurled outside surface. Smooth Ring gages are also typically marked with their size, type and tolerance classification.

Smooth Ring Gage Calibration

ANSI/ASME B89.1.6 is the main standard for the calibration of smooth ring gages. It establishes the methods for the calibration using horizontal comparison methods and includes the requirements for the calibration standards, the environmental conditions, and the techniques to ensure that the measurements are made with an acceptable level of accuracy.

An internal type comparator is the most often used device for calibrating smooth ring gages. They can be super-micrometers that contain a flat table and contacts that protrude above the table surface to contact the bore of the ring or super-micrometers that have internal measurement attachments that contact the interior of the ring. For the most accurate measurement, the contacts or attachments are set to the nominal value of the smooth ring gage using a master gage block stack for each size to be checked. The smooth ring is then compared against the measured value of the gage block stack.

Preliminary Setup

A master gage block stack is normally used as a reference value for most internal comparators. The gage blocks must be the same material as the measured smooth ring or additional corrections will need to be made to the measurements.

When building the gage block stack, the gage blocks and any internal accessory jaws must be properly wrung together. Just clamping the gages together with a rod or cage can lead to an incorrect reading as the measuring contacts on the internal comparator will push on the internal accessory jaws and push the gage block stack apart slightly.

A thermal soak plate needs to be used to allow both the master gage block stack and the test gage to reach the same temperature before beginning the calibration. The soak plate is typically made from either steel or granite. The amount of time required to soak will depend on the mass of the gages, where 2 hours may be enough for smaller gages while soaking overnight may be required for larger gages. The gages should be kept as close together on the plate as possible and the items should be shielded from any noticeable drafts and/or vents during the soaking period. Gloves should be used at all times when handling the gage blocks or test gages to minimize temperature fluctuations.

Calibration Process

The gage block master should first be measured and its value either recorded or zeroed out. Then the smooth ring is measured and its value compared to the gage block master to determine its corrected measurement value.

Smooth ring measurements should be taken at three points along one axis and repeated again after the gage has been rotated 90° for a total of six measurements. The measurements should be taken at distances approximately 1/16 inch from the bottom and the top surface plus one at the midpoint. Smaller rings may only require two readings per axis.

When measuring the gage, the contact force should be the lowest force possible for which repeatable readings are obtained or as otherwise indicated in the internal comparator’s operating manual. The gages should be measured as quickly as possible to keep the temperatures stabilized. The total deviation of all of the readings should be within the Gage Maker’s tolerances for the nominal value of the gage.

The data provided with the calibration certificate should show visually or clearly state the positions where the measurements were taken on the gage.

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