In manufacturing process control is key. Without it you may as well spin around 7 times, turn off the lights and play darts. Sometimes you will hit the dart board (and maybe even get a bulls eye), but more likely than not, your dart will be off the board completely. The same can be said in a manufacturing environment that does not have adequate process control. Sometime your quality will be spot on, but more times than not your parts will not be in spec. From this it’s easy to see that process control is not about quality. In fact, process control has just as much to do with quality as fast food has with good flavor. Process control is about consistency, not quality.
Process control systems are created to control process parameter to one extent or another. The control system itself doesn’t care what the process is or how it’s defined, its responsibility is simply to ensure the process parameters are kept within reasonable limits as defined by the process. Again, what those limits are is of no concern to the control system whatsoever. Once the limits are given to the control system, its job is to simply maintain them. And who determines what the process parameters are and what limits should be kept? Someone who knows what the overall quality the control system should maintain.
So what about quality? Quality is simply a subjective measure of the object’s properties, that when taken wholly is determined to be good, bad or somewhere in between. Figuring out how to process something to a certain degree of quality requires setting specific process parameters. This is usually done by trial and error; take a first stab at it based on an educated guess, assess the results, make a change, rinse and repeat. If the control system is working correctly, you should eventually have a process that results in the quality you need. Again, the actual quality is irrelevant. The point is that without the control system functioning properly, you cannot be assured that the change in the part quality was a result of a change in process parameters, and without that, you are playing darts in the dark.
As mentioned, the job of a control system is to maintain process parameters. That is the key. Not necessarily the key to quality, but it is the key to consistency. If the process parameters are not maintained, then the process output will be unpredictable. Why is it that the number one fast food restaurant is so successful? Certainly not because of the great food. It’s because you can get the same consistent food regardless of where and when you visit them. A sign of a great process control system is one that provides the same consistent product all the time. People may argue about the overall quality of the product, but they would generally agree that they are getting the same product. That is what process control brings to the table; consistency, not quality. If you provide a consistent, usable product, the customer will return.
So it always must begin with the control system. Once that’s in place, you can make changes to the process parameters and predict how those changes will affect the process output or part quality.