Stabilizing a Machining Process: the Human Factor

The process by which a workpiece is machined is of paramount importance.

A good process will ensure that workpieces are machined in an acceptable and stable manner. That is, workpieces that pass inspection will be consistently machined with relative ease. Conversely, a poor process will make it difficult to consistently machine acceptable workpieces.  
Whenever you are having trouble machining acceptable workpieces, it is a signal that something is wrong with the process. Indeed, any time a scrap workpiece is produced, it can be attributed to something that is wrong with the process.
Before you write an email to voice your disagreement with me, consider the countless factors that contribute to a working process. They include the machine tool, the sequence of machining operations, the selection of workholding and cutting tools, and the consistency of machined material, among many others. Indeed, anything that affects the workpieces being produced is part of the process. With that reasoning, CNC machine operators also must be considered part of the process because they, too, can affect the workpieces.
Taking complete control of a process and making it bulletproof can be extremely difficult, if not impossible. The more difficult it is to machine acceptable workpieces with tight tolerances and better finish requirements, the harder it is to develop a stable process. It is not uncommon for manufacturers to blame CNC setup people or operators for making scrap workpieces, when in reality it is the process that is responsible.
There are at least four factors that contribute to the stability of any process. Three of them are rather obvious considerations when process stabilization is necessary.
• Machine tool selection. You must select a machine tool that can easily machine workpieces within specification if the process is to be stable. It is a mistake to select a machine that does not provide the positioning accuracy required for tolerances specified on the workpiece drawing.
• Cutting- and workholding-tool component selection. Select cutting tools that are capable of machining surfaces within their tolerance bands. An example of this is producing a circular hole using an end mill when the roundness tolerance indicates that a boring bar would have been a better choice.
• Order of machining operations. The general rule is to rough everything before you finish anything. If a surface is finished before a powerful roughing operation is done, the workpiece may shift in its workholding device. Subsequent finishing operations will not be machined in the appropriate relationship to the surface finished first.
Again, when something is wrong with a process, we tend to look in these three areas for the cause. However, I suggest that there is at least one more area to consider:
• Personnel and how they facilitate tasks. The more complex the process is, the more important task facilitation can be. Ensure that the people you assign to a task are capable of performing it. 
This is a process area that is often overlooked. Again, some manufacturers do not consider this to be part of a process. They take for granted that the people selected to run the workpieces will have the skills needed to produce acceptable workpieces. Yet, overlooking this area can be a mistake. In processes that are stable in all other areas, people—and mistakes made during the tasks they perform—can still wreak havoc.
For example, consider an operator who makes sizing adjustment mistakes. The workpiece just produced might be a good one, but one of its machined surfaces is close to a tolerance limit. If the operator doesn’t recognize this condition or miscalculates the adjustment required to bring the surface back to its target dimension, subsequent workpieces will be scrapped.
When CNC operators are not capable of performing their tasks, you have two choices:  provide more training to increase their knowledge and skill, or lower the required skill level by simplifying assigned tasks. Often it is necessary to do both.

When developing processes, be sure to consider the people who will be involved in producing workpieces. Don’t be too quick to assume they have the necessary skills and experience required for the task. More importantly, don’t blame them for causing problems with a process when you can do something to help them.