I have received a couple of emails from readers recently concerning what does or does not have to be calibrated within a quality system. In both cases, the companies already have a program in place to ensure their measuring equipment, masters, etc., are calibrated on a regular basis but an odd item has popped up leading to debate within the company on whether that odd item has to be included in their calibration program as well.
The general rule of thumb in these situations is: if it is used to qualify something, it has to be calibrated on a regular basis. As I’ve noted in a previous column, occasionally things get mixed up when a gage is being used to identify something rather than qualify it. Screw pitch gages are one example.
Most machinists or inspectors are familiar with screw thread pitch gages. Typically, they have the form of various thread pitches, a different one on each blade/leaf or thin piece of metal that makes up a set of them. In use, they are held up to a bolt or other threaded product until one is found to match the product thread. They are handy gages to have when you have to identify a thread but of little use in determining whether a thread meets any kind of specification.
Too often custom-made devices are included in calibration programs but are not properly calibrated. They may have a master supplied for them that is used for this purpose but it does not cover the measuring range of the device. The master involved is basically a setting master which tells the user it is good at one point but necessarily as good anywhere else in the range. Many people use setting rods supplied with micrometers in a similar fashion which is not acceptable as a means of calibration for the same reason.
Some companies ensure that company owned equipment is calibrated on a regular basis but overlook employee owned instruments that are used in their work. This can lead to uncalibrated instruments being used to qualify product by accident or design. If your company permits or encourages the use of employee owned tools, they must be included in your calibration program. This means your calibration status stickers need to be placed on them which may not sit well with the owner of the tool. This should be explained at hiring so there is no misunderstanding later. Also, your quality department should be made aware of when personally owned tools may not be available for calibration. This can be due to their owner being on vacation while they are locked up in a toolbox only the owner has a key for. If you operate several shifts in your company you could run into similar problems where the owner is on a different shift. Avoid these problems by ensuring only company owned equipment is used at your facility.
Occasionally a company will try to reduce calibration costs by using a method guaranteed to increase the risk of bad quality getting out the door. They cut costs by only calibrating specified tools that are used for final inspection of the product. This can work as long as no other tools are used but sooner or later an uncalibrated instrument or gage can end up being used—usually on the day a quality auditor from the customer is on hand checking things out. This method also assumes that every workpiece has every feature verified at final inspection which may not be provable.
Trying to lower costs this way can actually increase them dramatically when final inspection reveals an incorrect dimension was produced early in the manufacturing cycle due to the use of uncalibrated equipment. All of the follow up operations will have been wasted. A sampling plan with calibrated equipment could have avoided this and the money spent on calibration would be petty cash compared to a few thousand finished products now destined for the scrap bin.
One valid area for calibration cost cutting is ignored by most companies. I’m referring to the calibration reports issued on their equipment. Too many people quickly check the reports for red flags and then file them if none are found. They should be comparing the results over time with a view to possibly lengthening the calibration cycle.
If you can’t prove how good an instrument or gage is, a quality auditor will assume it is no good and things will go downhill from there.