Adjustable bore gages are comparative instruments. This means that the readout, whether it be a dial, digital indicator or other type of electronic readout, will show the amount and direction of variation in the test bore from nominal size. In this case, the gage needs to be set to the required nominal size to which the actual bore is to be compared.
The gaging components of an adjustable bore gage are mainly in the head of the gage. These include a sensitive contact through which the measurement is transferred mechanically to the readout device; a pair of centralizing contacts that align the gage radially to the bore; and a reference contact that is used to set nominal size. Setting the bore gage consists of adjusting this reference contact to produce a zero reading on the indicator when the gage is measuring the nominal size.
There are a number of ways that the adjustable bore gage can be set to the nominal size. Some are good for a quick setup; some are more expensive but very precise; and some provide a balance of reliability and versatility.
Using an outside micrometer may be the quickest and most available method. However, there are issues with the accuracy of this setup. Micrometers have inherent errors that can be passed along to the gage. Another issue is accurately locating and aligning the spherical measuring points of the gage on the micrometer contacts. While readily available and inexpensive, this setup method probably offers accuracy of no better than 0.002 inch. This accuracy must be compared to the part tolerance to determine what portion is consumed by inherent error.
Probably the best method for setting the adjustable bore gage is with a master ring. This duplicates the actual measurement, and master rings can be made very close to the part size. Most bore gages have a flat surface on the head that is parallel to the reference and sensitive contact. Using this method, the ring is laid on a granite surface plate; the bore gage is set in the ring that will support it; and the sensitive contact is adjusted until the indicator reads zero. It is also possible to use a ring gage that is not exactly the nominal size, as the offset can be incorporated into the set of the dial indicator. A ring gage is the preferred method for setting repetitive sizes or when an adjustable bore gage is going to be dedicated to a particular size. The downside of this method is the potential cost. Rings can be expensive, and if one is required for each of many sizes on the shop floor, the total cost may be prohibitive.
Using a gage block assembly in a clamp with jaws at both ends will also provide a highly accurate reference master. When multiple sizes are required and flexibility is key, this can be the preferred method. Gage blocks are the most basic reference standard. They are readily available and provide high accuracy. The only drawback is the time required to assemble the gage block stack to the nominal size. Also, because only a single stack is used, there is no verification that wringing errors have not been made in assembling the stack. Though small, these could affect the performance of the gage.
As with setting any gage to its nominal size, care needs to be taken. Setup of the masters or gage blocks is key. This means they must be clean and free of dirt. Also, they need to be temperature stable, as does the gage.
Once the reference contact is set to the desired size, the locking screw should be made snug, and the gage should be tested again for zero and repeatability. If nothing has changed, keep snugging down the locking screw until it is locked into position. Then check repeatability again until you are confident that the gage is performing accurately.
Both the master ring and the gage block method provide the best setting performance; they can provide overall accuracies of 0.0001 inch to the nominal size.