Test & Inspection

Other Dimensions: Functional Calibration

At first glance, the title of this month’s column seems like a no-brainer. After all, isn’t that what calibration is all about? The answer is yes, that’s what it is supposed to be about. But too often the calibration process that is followed does not produce the desired result because the “what’s it” question has not been answered.

The “what’s it” rule of calibration produces the question that calibration is supposed to answer. In the case of dimensions for gages and masters, it is: What’s it supposed to be? When instruments are involved: What’s it supposed to do? If the appropriate question is not answered, all the data on the calibrated item will be of little or no practical use. 

Before you announce to the world that you now have proof I’m running a few bricks short of a full load, let’s take a look at some examples.<br><br>
Dial, digital and vernier calipers are very popular general purpose measuring tools that are regularly the subject of calibration. Gage blocks are most often chosen for this work so what could go wrong? Nothing, if the calipers are used to measure over flat parallel surfaces all the time because the blocks become the perfect masters to use since they closely duplicate what the instrument will be measuring. Any areas of wear on the measuring faces will be bridged by the blocks and won’t cause any problems. However, when measuring diameters, areas of wear will be incorporated in the readings taken so the calibration results won’t reflect variations in performance. 

Applying the “What’s it supposed to do?” question would avoid this problem and be more meaningful. In our example, the caliper will spend most of its life measuring diameters so the masters used for the calibration should be cylindrical—or gage blocks should be used on a minimal contact area basis. Whichever masters are used, checks should be made at several positions along the measuring faces to detect geometrical irregularities that will add error to measurements. Alternatively, the measuring faces should be calibrated on their own with a view to determining the extent of wear, damage, or lack of parallelism. 
Many standards for such instruments give information about calibrating them; however, in most cases, they relate to a new instrument straight out of the box. Verifying how well it does what is expected of it can be quite revealing and bring peace to disputes over measurements. <br><br>
Keeping the “What’s it supposed to do?” question in mind during calibration means that calibration will be relevant whether the instrument involved is brand new or used.

Similar situations occur with many devices having mechanics to one extent or another within their operating mechanism. This includes indicators that may be used with their spindle axis horizontal while calibration is done with it vertical.

The “What’s it supposed to be?” question is worth asking when doing calibration on fixed limit gages and masters. The answer may indicate that the device you intend to use for their calibration will not be suitable. 

Consider a simple outside diameter calibration of a plain plug gage. It is usually assumed that if it is a parallel type gage, it will be the same diameter wherever it is measured. Alas, like so many things in life, appearances can be deceiving. This is particularly so when differences are not visible to the unaided eye. <br><br>
You have to take several measurements along and around the gage to declare a diameter size. And if you’re measuring it with a bench micrometer type device, the large diameter anvils prevent you from getting a reliable reading at the end of the gage. If you are using a comparative setup your sensitive contact will be good for getting close to the end of the gage, but the lower anvil contact area and flatness problems may reduce most of that advantage. <br><br>
The advantage of fixed limit gages in checking product becomes a curse when it’s calibration time. The reason for this is any deviation in size or geometry will affect its functional size. I tried to avoid ruining your day with geometry problems, but if we’re going to answer the “What’s it supposed to do?” question for a gage, we can’t ignore anything that will affect its performance. 

Hill Cox is president of Frank J. Cox Sales Ltd. (Brampton, Ontario, Canada). He may be reached at askquality@bnpmedia.com.

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