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How do you set a dial bore gage?

The level of complexity in setting a dial bore gage all comes from the method you have chosen to use. Today, we are going to walk you through the general processes by which you can set a dial bore gage, and the associated advantages and disadvantages of each method. We think having the knowledge of how to accurately set a dial bore gage is an important skill for any machinist or metrologist.

In general, the process of setting a dial bore gage is important to maintain an accurate reference point of zero. The reference contact on a dial bore gage is used to set the nominal size, and when the bore gage has been properly set, it will produce a reading of zero when measuring that nominal size. In other words, you need a zero point from which you measure beyond. Setting the zero point, or the nominal size can be accomplished via a number of methods.

A common way to set a dial bore gage involves using an outside micrometer. The steps to accomplish this are: 1) set your micrometer to the size of the bore you are testing, 2) using the extensions needed for the specific bore diameter, set up your dial bore gage with the proper extensions, 3) put your gage between the micrometer spindle and anvil, 4) using a rocking motion, settle the gage into the micrometer, and 5) zero the indicator to the minimum reading and that is your nominal size. This method is definitely the fastest and most convenient method, given the availability of the tools involved. However, when using a micrometer to set your dial bore gage, you run the risk of including the measurement errors of the micrometer itself, and of misaligning the spherical measuring points of the gage to the micrometer.

Another way to set a dial bore gage is to utilize a master ring. In order to set your dial bore gage in this way:1) lay your master ring on a smooth, hard surface, like a granite surface plate 2) set your bore gage in the ring, thus supporting it, and 3) adjust the sensitive contact point of your bore gage until the indicator reads zero. The main benefits of this method are that the actual measurement goal is duplicated and master rings are easily made to very closely match the size of a part. The major disadvantage of using master rings to set your dial bore gage is that they are expensive tools, and collecting one for each of the potential bore sizes you require could become quite costly.

A third method of setting a dial bore gage involves the use of a gage block assembly. By setting gage blocks into a clamp with jaws at both ends, you are able to create a highly accurate reference master by which to set your gage. Gage blocks are readily available, and most workshops or warehouses will already have them on hand. Additionally, gage blocks provide multiple sizes which can result in more flexibility of measurement and savings in cost. The central disadvantage of using gage blocks in order to set your dial bore gage is the time that it takes to assemble the gage blocks to match the required nominal size or zero point. Furthermore, you run the risk of having potential errors resulting from the wringing process that may be tricky to pick up on.

Check out our video demo on the Fowler Bore Gage Setting Master Kit: Click Here

Whatever method or tools that are used when setting a dial bore gage, there are some key concepts to always keep in mind. Make sure that your tools are clean from dust and debris. Any small intrusion can have ramifications on the accuracy of your zero point. When done well, accuracy can be 0.0001inch to the nominal size. Take into consideration the tools you have available, the funds you are willing to spend, and any individual preferences or skills when choosing the best way to set your dial bore gage.


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