Think About It: Gage Calibration Reliability
You’ve made the threaded parts and are confident they are okay because you’ve checked them with your gages. Then you get the call from the customer advising you that their gages have rejected the parts and they are demanding re-work or replacements ASAP.
Explaining to the customer that you’re certain the parts are good because your gages had been recently calibrated and found to be okay does not impress. Your customer advises that his gages have been calibrated recently as well and were found to be within spec. Now what?
This problem comes up quite frequently and the usual reaction is to blame the gages because, as we all know, no one makes bad parts. Of course, if you’re the buyer of the parts you know they’re probably bad and also blame it on the supplier’s gages.
In one situation a review of both calibration reports indicated that both gages were good but how could that be when one accepts the parts while the other does not? A search for answers now focuses on the reliability of their calibration.
The first thing you have to look at is the actual calibrated sizes of the gages involved. They may be within the specification they were made to but where they are dimensionally can be enough to create the problem. For example, the gages are made to published specifications which means that they could be different in size by .0003” for example and be within the specification. However, if one is at the top of the limit for the gage and the other is at the bottom limit, this spread is enough to create a problem when the product they are inspecting is at one of those limits. Nine times out of ten this is the reason for the situation outlined.
The solution going forward is for the maker of the parts to stop the practice of keeping the product close to one of the limits or the situation will continue to arise. The product size limits are much greater than those of the gages and is there to be used. Some processes are relatively fixed on where the thread they will produce will be such as when they are tapped.
If the threads were produced by single point turning, the size can be adjusted in the machine setup to keep it at mid-limit. This will prevent variations in thread geometry from having a major impact unless the buyer of the product has put limitations on variations in this regard.
When external threads are involved, the adjustable thread ring gages used to check them may be set on setting plugs that are different in size and create the same problem outlined earlier. A difference in size of less than .0001” is enough to create such problems. If this is the case, you can match the two rings by having the same setting plug used to calibrate them.
To double check everything you may be able to directly measure the product to confirm its size. If the rejection is caused by thread pitch problems in the product, you may have to get a calibration lab to provide this information because few companies have the right equipment to do the measurements. And since the effects on the functional size of the product is magnified when it’s a pitch problem, a small variation can make a big difference.
I don’t want to beat this to death but pitch problems can be the cause of problems when there is a variation in pitch between the gages or when the length of the gages is different so the effects of pitch variation may not be evident due to the shorter length of engagement of one of the gages.
Similar problems arise when two different methods of doing the product inspection are used and a simple plain diameter is involved. For example, a plain ring gage or plug gage may be used to inspect the product in one case while the other party to the dispute is using direct measurement. Or, both parties are using direct measurement but one is using mechanical contact while the other is using noncontact measurement such as an air gage or optical system.
The bottom line in these situations is to think about all of the details in the overall picture to sort it out.