Calibrating Snap Gages

Calibrating Snap Gages

Snap gages are metrology tools that are used to determine the correct diameter or thickness of a part. These tools are specifically designed to precisely check the size of various types of parts produced by machining by giving immediate go/no-go decision. The assessment made by the snap gauges is on the outside dimension of the parts, which may be shaped as spheres, grooves, cylinders, rods, etc.

The main purpose of a snap gauge is as a tool for inspection and quality control. It can provide a quick way to determine if the outside dimension of a part lies within the specified tolerances or not, without actually measuring its value.

There are many advantages of using a snap gage in a manufacturing or industrial environment:

  • A large number of parts can be checked by a snap gage in a short time.
  • It does not require a high degree of skill of a worker to determine the size of a part
  • Snap gauges are much less expensive than other measuring devices.


Types Of Snap Gages

Snap gages have different shapes and sizes. They can be “C” type, “A” type, “U” type. They may include a digital or dial indicator. There are also double-ended snap gages, progressive snap gages, and adjustable snap gages.

The snap gage is designed with a frame with jaws and two anvils. One anvil is fixed but the other is under spring tension. When a part is placed within the jaws to measure it, there is a spring action which “snaps” the part into the correct alignment. That’s is why it is called a “snap gage”.

A snap gage may have a variety of anvil shapes and sizes that depend on the dimensions of the part being checked. Anvils are generally narrower than the part that is being checked. The edges of anvils are chamfered for easy insertion of parts to be checked. Some snap gages have only one anvil on each face of the gauge, while other designs have more than one per side. Anvils should be inspected and checked to ensure that they are parallel for accuracy as they may wear over time.

There are both fixed type snap gauges and also adjustable snap gauges. Initially, there were only fixed type snap gauges that allowed go/no-go checking. Fixed means that the size of the part these gages could measure and their tolerances were fixed and the gage could not be adjusted for different sized parts.

In such cases, in a production line, where parts of different dimensions are being produced a different gage was required for each part. This meant a large set of snap gauges had to be maintained and that too needed to be replaced as soon as a new part with a different dimension came into the production line.

Further improvements led to the addition of an indicator to the adjustable anvil which increased to usability and measurement range of these devices.


How To Use A Snap Gage?

Place the part to be measured within the frame of the snap gage and if it goes through the go range, then it will mean that the part meets the higher limit of the part’s size specifications. Now the check the part by passing it through the no-go section of the snap gage. The part should be accepted if it does not pass through that opening as it will ensure that the lower dimension specification is met.

Snap gages are indirect measurement reading devices. They do not have an absolute scale for measurement. They are calibrated off-site in a certified calibration laboratory elsewhere. So in reality the snap gage does not measure the dimension but only compares the difference against a master dimension.


Accuracy Of Snap Gages

Snap gages have it made possible to do extremely accurate measurements without any complex device nor highly skilled worker capabilities. Even with a good caliper measuring with ±0.001 inch tolerance is rather difficult even by a skilled worker. But snap gages allow measurements of tolerances of ±0.0005 inch or even better rather easily.

Making measurements by a snap gage is very quick as many parts can be measured rapidly. The operator needs to check just the last one or two digits on a display, instead of counting all the units and decimals up to the measured dimension.

Accuracy is the key advantage of a snap gage. Some gages can measure up to 0.0000001 inches of precision. However, such tools need to be handled very carefully as even slight damage will reduce its measuring accuracy.


How To Calibrate ASnap Gage?

Use a set of gauge blocks for precise and fast calibration of a snap gage. You may also use an accurate digital micrometer to make very precise measurements. Calibrate both the upper and bottom anvils. First set the block and then you should compare it with the reading of the micrometer. The gauge reading and calibration standard’s reading should match exactly. Make the required adjustment to bring them to the same levels.

Procedure For Calibrating Snap Gage

  1. Visually inspect under test gage for any wear and tear, abrasion, or damage that may compromise the function of the gage.
  2. Make sure that the identification data is correct.
  3. Use selected gage blocks and do gradual build-up to the required dimension for the gage. Make sure that the blocks enter the gage with even pressure. You will need one stack of gage blocks for “go” and another stack of blocks for the “no go” dimension.
  4. If the dimension does not match then check with another gage block stacks, keeping within the gage tolerance.
  5. Confirm that anvils are parallel by checking with gage blocks stack in different positions near the edges of the gaging surfaces.
  6. Accept or reject the gage as per check accordingly.
  7. Update gage records.

e2b calibration is an ISO/IEC 17025 accredited calibration and repair laboratory providing a full scope of services that are traceable to NIST. We provide calibration services of your snap gages in our climate-controlled, state-of-the-art laboratory. Contact us for more information about our on-site services.


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