Other Dimensions: Dumbing Down Calibration Reports
May 27, 2009
The last time I checked, most folks have gages calibrated to determine what sort of dimensional state they are in. A properly presented calibration report will tell you if there is a possibility that some bad work was shipped because of a worn gage. Now, I know that wouldn’t happen at your company, but it does at some. Alternatively, the information provided could be a warning that this could happen in the not-too-distant future.
I understand that all of this may appear to be pretty radical stuff, but being old school, I believe that a calibration report should have data that can be reviewed. Okay, maybe I’m pushing the envelope here.
In my other job I chair the technical committee for the American Measuring Tool Manufacturers Association (AMTMA), and I sometimes get involved in measuring disputes between a member and another calibration laboratory. First, I want to see the calibration reports issued by each of the combatants to see who is guessing or who doesn’t know any better. Too often one of these reports essentially says nothing of any value so, as a source of enlightenment, it is a very dim piece of work.
An example that keeps popping up is a report with one line per gage that essentially identifies the gage with no data shown but will state “pass” or “fail.” Another is where a single reading is given for pitch diameter or outside diameter on a plain plug gage, for example. While a single reading is better than none, it raises more questions than it answers. Was only one reading taken? If more than one was taken, where was the reported reading taken? If more than one was taken, why not report them all?
Even worse is the reporting of an average size-a mythical number that can’t be applied to anything useful. I wonder if that means they took two readings, or perhaps they went off the deep end and took three. The beauty in providing so little information is that it makes it difficult for anyone to question your data.
The AMTMA technical committee has reviewed this problem and determined what should be received in the way of data in gage calibration reports. Here are some examples:
The go member is measured at the front, center and back, and rotated 90 degrees to take a further set of readings. The largest reading at each position is reported for a total of three. Because the no-go member is usually much shorter than the go, measurements are made at two positions over its length. Size variations that indicate gage roundness exceeds the applicable tolerance will be reported. An actual roundness plot is available on request.
When thread plug gages are involved, the same sequence of measurements is taken for both the major diameter and pitch diameter. In addition, the minor diameter is checked for clearance and the thread form inspected, but in some cases data is not presented unless a problem is discovered. Linear pitch will be measured on request.
Plain Ring Gages
Plain ring gages are calibrated in a similar fashion to plain plug gages, with only the smallest reading at each of the three positions reported. Because the no-go ring is the same length as the go ring, three sets of readings are taken.
Thread Ring Gages
Adjustable thread ring gages are made to fit a truncated setting plug designed for that purpose. Calibration of them means checking the fit between the setting plug and the ring. If the setting plug used is not the one used by the gagemaker, there will be some differences and the ring adjusted to match the plug. The report should indicate what was done and list the calibrated values of the setting plug used and its tool number. The minor diameter will be checked after any adjustments are made and will be reported if it is out of limit.
There you have it. Reports with this type of information can give you a good picture of any given gage. If you’re not getting this type of reporting, you’re getting a sketch rather than a picture. You should get a new artist because the one you have hasn’t advanced beyond crayons.