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In some recent columns I dealt with common mistakes that are made when calibrating gages. The columns dealt with the picky details, but often a look at the bigger picture can help you get to the specific details faster or alert you to a bigger, unseen problem that won’t go away no matter how much fiddling you do.
I attended a presentation by a gage user that outlined his experience with a high-tech thread gage calibration device he had spent a bomb on. No more reviewing reports as the machine spewed out the numbers and made the decisions. Calibration was done in a fraction of the time. When you’ve got hundreds of gages to do, this thing had to be the greatest invention since they put erasers on pencils. Or so this gage user thought.
To prove his point, he quoted results from a batch of several hundred gages from different makers that the machine had “calibrated.” It rejected every one.
The fact that not one of these gages was considered acceptable should have set off an alarm. Even the worst gage makers actually get some gages right, a thought that even the greatest skeptic will agree with. Others attending the presentation had already run tests on this device and found it inaccurate for gage calibration and were not surprised by its performance.
The lesson to be learned here is that when every measurement is identical, something is probably wrong, and it doesn’t matter whether the device is accepting or rejecting everything. Repeatability of a process or calibration device is required to obtain reliable measurements-but you can have perfect repeatability of the wrong answer.
Whether it’s gages or component parts measurements, when they all lean in the same direction compared to someone else’s readings, someone has got a measurement problem. Here are some elements to consider:
Resolution of the equipment is too coarse.
The temperature is off.
Incorrect setting masters are being used or correct ones are used incorrectly.
Worn contacts and/or anvils can put a consistent error into the readings.
Indicators and/or probes that are not correctly mounted on comparator stands will result in nonlinear response from them-all of the time.
People problems can never be ruled out.
Measuring forces are different.
Check the batteries.
To solve any mysteries in measurement you need to have something that is stable to start with or you’ll be running around in circles forever. Whether the readings are considered right or wrong, if they are repeatable, you can use a process of elimination to determine what’s going on. If they don’t repeat, there are a number of things that should be checked to stabilize whatever mess you’re trying to sort out. Here are some of them:
The instrument is physically damaged.
Contact points are loose on mechanical equipment.
Unstable power source to digital equipment.
Hands-on analog indicators are loose.
Normalizing times are not being observed when it’s a calibration dispute.
Check the batteries.
Any one of these situations can cause unstable readings. It’s easiest to check the batteries. You should have spares on hand because they always go down on the night shift on a weekend when all local suppliers are closed. Switch those in the instrument with unused ones, and if the problem persists, you know that’s not the cause. Alternatively, you could put the instrument on an AC adapter to see if that will correct the problem.
Yes, I know most battery-powered instruments have warning symbols to indicate a low battery situation, but this is often overlooked and may not be working properly.
Air gages are another instrument that requires a stable supply but in this case, besides electrical power to a display system, a stable supply of clean, dry compressed air is required for them to work properly. Repeatability problems arise when the air is dirty and crud accumulates around the sensing jets. If the air is not dry, it can condense in the pneumatic side of the instrument putting water in the air circuit that will cause fluttering pointers, flickering digital displays and wrist-slashing frustration.
I hope these notes will help you sort out these problems when they arise. Of course, I’ve left the most critical tip to the last: don’t forget to check the batteries.