GOOD GAGES GET REPLACED DUE TO INCORRECT ACCEPTANCE CRITERIA.
No one in their right—or left—mind would knowingly set something up to fail but too often they do just that without realizing it. If you’re like most folks and know an audit is coming, you will check many details beforehand to avoid being written up for one oversight or another. These checks will include making sure that your gage and instrument calibration is current and you’ve got the reports to support it. However, if that’s all you review in this area, you could be in trouble.
Too often reports are filed after a cursory glance at them for red flags but, as is often said, the devil is in the details and those details can cause trouble. The data in those reports may look satisfactory but an auditor will want to know what your acceptance criteria is for the set. The easy way out is to specify a grade but that is what can trip you up later when the set no longer meets that grade. The spec may have been valid for a new set but once the blocks have been used may no longer meet it.
Your work tolerances may be such that the blocks could be several times the new tolerance and still be satisfactory but having that grade as the acceptance criteria prevents it. Similarly, if you are going to be working with the calibrated size of each block it doesn’t matter if they don’t meet a particular grade.
The way to avoid this situation is to set your acceptance criteria to the coarsest level you can live with. Rather than a grade, specify dimensional limits. An auditor will not cite you for using finer limits if that turns out to be the case.
Many folks who specify fixed limit gages such as plain plugs or rings make the same mistake but since these items are used on the shop floor with wear being a regular concern, re-calibration frequency cycles can be quite short. These ‘go/no-go’ gages are made to one of a number of standard gage maker tolerances for ease of selection. Those tolerances belong to the gage maker for the gage when it is new but far too many people use those tolerances as the acceptance criteria for used gages.
This can work but it means the gages will be rejected earlier in their life than need be and the tighter tolerance may require more frequent calibration. You pay more for the tighter tolerance and you pay more in calibration costs. Your overall gaging cost is higher than it need be since the gages are being replaced earlier than necessary.
Once again, this means perfectly good gages get replaced due to incorrect acceptance criteria.
Where go/no-go gages are concerned the user has the option of specifying the gage sizes and these should be used to give you a wear allowance and thus, acceptance criteria that is practical as well as economical. Yes, this means the gages will use up some of your component tolerance but at least the whole game is under your control.
Setting masters such as discs or plain rings are gages that are regularly specified to limits that are expensive to buy and maintain due to unrealistic acceptance criteria that automatically comes with the use of finer than necessary tolerances. And in the case of setting rings, too often the measurement uncertainty attached to their calibration makes the requested tolerance impossible to prove one way or the other.
The simple way out of this is to set instruments to the calibrated value of a lower accuracy master to reduce purchase and maintenance costs.
Another area that can cause trouble is calibration frequency. Arbitrary time periods are the most used method and once established, are rarely reviewed or changed. Again, I’m going off the deep end here and assuming you review the data on the calibration reports as well as the previous one. This can reveal situations requiring a shortening of a cycle as the gage wears close to an acceptance limit so you aren’t accepting parts with a worn out gage. Similarly, it can alert you to a situation where the cycle could be expanded beyond your normal ‘rule’ for such matters.
Specifications are useful for determining if your instruments and gages appropriate for their assigned tasks. But if they were not properly thought out, they’ll come back to haunt you.