Other Dimensions: The Other Stuff
October 1, 2007
Being in the calibration business for such a long time means we’ve had our share of weird requests, as have many of our customers. While providing a laugh from time to time, lately we seem to be getting more of them and that’s downright scary.
I mention this because a faithful reader recently asked a question that reminded me of problems that pop up during an audit. Too often auditors ask for calibration reports on things that do not need to be calibrated and occasionally customers overlook what should be. Herewith, a rundown on the other stuff that can become the subject of such requests.
The rule: If you use it to qualify a feature, it must be calibrated. If you use it to identify a feature, then you usually don’t need to calibrate it. For example, if you use a ruler to determine a nominal hole size or a length but use other devices to verify that the feature in question is within tolerance, you don’t have to calibrate the ruler. But it should be marked accordingly so it is not used in a situation that would require it to be calibrated.
Transfer devices such as telescoping or small hole gages fall into a gray area. While they don’t measure anything, they are transferring a part feature so it can be measured. If they do that inaccurately, the measurement will be in error. What can go wrong? Flats on their spherical surfaces. If the tolerances involved are large, you may not have to worry about it, but if they’re not, you will. If in doubt, it’s wise to check for them.
Instruments such as height transfer stands, surface gages and planer/shaper gages can’t be calibrated but should be checked periodically to ensure their settings don’t shift after locking and that they contact the surface plate correctly.
The design and stability of some height gages is such that they are ideal for use as a transfer stand. Many companies keep them after their measuring accuracy is gone just for that purpose-and get caught by an auditor demanding they be calibrated. If they are being used as a transfer stand only, they do not need to be calibrated, but should be clearly labeled to indicate they are not to be used for directly measuring parts on their own. If the instrument is of the vernier scale type design, use paint or nail polish to cover the vernier scales so they can’t be used accidentally for measurement. If it is an electronic version, pull the batteries and put a calibration sticker over the battery cover so the auditor cannot demand it be calibrated.
Comparator stands on which an indicator is usually perched and is used for checking parts have nothing to calibrate other than the indicator. However, the anvil or worktable that may be part of the stand should be inspected for wear, particularly if you are using the stand to check cylindrical parts after setting it with gage blocks. The tighter the tolerances involved, the more frequently such reference surfaces should be checked for flatness.
Thread pitch gages are used to identify the pitch of a given thread but should never be used to qualify a thread; therefore, they don’t need calibration. Radius gages are another matter if they are used to ensure a minimum radius is present or a maximum radius is not exceeded. Granted, the eyeball of the operator is probably in greater need of verification, but the gages should be checked when new. Then you can coat the gaging surfaces using a felt-tipped marker and only check those pieces that no longer have the coating, say a year down the road.
Those cheap pin gage sets also need to be calibrated. Yes, I know it will cost more than you paid for them but if using them to qualify a hole size, you have no choice. If you use drill or reamer blanks for the same purpose, they too require calibration.
Last but not least, that “master” bolt you may have been using to check some threaded holes needs calibration. Unfortunately, you have to remember that while you have been using it in place of a gage, it is not one and probably cannot be measured to any decent level of accuracy to justify its use as one. I’ll leave you to sort that one out with the auditor.