A Checklist for Gaging Devices

Originally titled 'Making a List and Checking It Twice'

Don’t assume your gage is correct just because it has a calibration sticker. If it hasn’t been in frequent use, follow a measurement checklist to make sure.

You are probably able to more easily relax when you fly knowing that there is a checklist that needs to be completed by the pilot and crew before your plane can take off. Just about every facet of an aircraft’s operational systems must be checked and verified before it is allowed to go wheels up.

It’s pretty obvious why this is done: Lives are at stake. But the need to check also stems from the risk of familiarity: Humans tend to assume that everything is good when they repeat tasks over and over. The pre-flight checklist removes this assumption and can catch problems before they occur.

This same concept should be adopted when you take a gage or measuring instrument out of the gage room, and before the tool is put to use. Maybe it’s not needed for a life-critical measurement (although there are parts used in medical and aircraft applications that are), but preventing bad parts from being accepted is critical enough in terms of saving lost time and costs.

Just as there is a methodology for checking an aircraft, there should be a methodology of measurement for the gage itself. Whenever a gage has not been in frequent use, the following points may be used as a checklist to ensure that the instrument’s results will be as you would expect:

• Ensure that the readout device, whether it be a dial or digital indicator, or electronic probe, has been recently calibrated and certified, and is repeating and functioning as expected. Any detectable looseness in its mounting should 
be corrected. 

• Completely inspect the whole gage for any sign of looseness. Every joint (the post to the base, fine-adjust mechanisms, contact, anvil mounting and so on) is a potential source of looseness, which shows itself in gage non-repeatability. Also, when using a portable gage, do the same with the adjustable or interchangeable contacts, as they tend to come loose and will contribute to errors.

• Many gages, either portable or bench-mounted, employ a backstop. Check to make sure that it is set to the correct height for the part and that it is firmly locked into place.

• The sensitive contact points on many portable gages and bench comparators may be hardened steel, but some are tipped with wear-resisting tungsten carbide, sapphire or diamond inserts. Test these tips to see that they haven’t become loose in previous use. Also, it may be wise to examine them under a magnifying glass or even a microscope. If they are cracked, chipped or badly scored, their surface conditions may prevent accurate or repeatable readings. They may even scratch the work.

• If opposing anvils are supposed to be flat or parallel, check them with a wire or ball. By positioning the precision wire or ball between the anvils and simply moving it front to back and side to side, you can read parallelism on the indicator.

• One of the easiest chores to neglect is regular cleaning of indicating gages and bench comparators. If the system is an optical-based one, cleaning the optics and lenses is critical, especially when the instrument has been in storage for a while. As we often have noted in this column, dirt is the No. 1 enemy of accuracy. Dirt, dust, grit, chips, grease, scum and coolant will interfere with accuracy of optics, gage blocks, indicators and precision comparators. Clean all such instruments thoroughly at each use. Pay special attention to rust preventatives that may have been employed prior to storing the gage, as these can dry out over time and become virtually invisible, yet they still represent a substantial coating if not removed.

• Take the same steps to ensure the reliability of the master, whether it is a master disc, master ring, gage block or qualified part. Examine for nicks and scratches and the scars of rough handling. Ensure they also have been recently calibrated and certified, and handle them as you would any critical and important standard. After all, they are designed to provide the basis for the precision measurement of the gage.

• Double-check that the correct gage is being pulled for the application. Many gages look alike, but they each may have different characteristics based on the part being measured. Does the gage have the correct contact, gaging force and location of measurement that is required for the part being measured?

Finally, if you see a sudden shift in your process during the day, these same basic steps should be part of your troubleshooting routine. In a situation such as this, don’t automatically assume your gage is correct just because it has a calibration sticker. Strange things do happen, and you will do well to investigate all possibilities, especially the ones that familiarity or habits can make us overlook. You wouldn’t want the pilot of your flight to assume that a strange noise or sudden bump was acceptable. If something seems not quite right, take a step back and find out what’s happening and why.