Stabilizing a CNC Machining Process

Process stability problems often can be attributed to the appropriateness of workholding devices and cutting tools.

Columns From: 3/20/2012 Modern Machine Shop,

Editor's Commentary

From the monthly column: CNC Tech Talk
The stability of any process directly contributes to how easy it is for setup people and operators to complete production runs. Assuming these people know how to set up and operate machines, process-stability problems often can be attributed to the tooling.
 
For example, an adequate workholding device ensures that the workpiece doesn’t move or flex during machining operations. It can optimize cutting conditions and minimize cycle-completion time. However, if workholding is inadequate, machining workpieces to required specifications might not be possible.
 
The same is true for cutting tools. An assembled cutting tool must be rigid enough to optimize cutting conditions. Anything less causes inefficient machining operations. In the worst-case scenario, it is not possible to machine without chatter.
 
Use the following three steps to ensure CNC machining process stability:
 
1. Make good workpieces. Unfortunately, it can be difficult to tell if a given tooling problem is workholding or cutting tool related—or both. Inadequate workholding that causes the workpiece to flex has the same effect as a flexing cutting tool. Either case results in chatter or the inability to consistently machine surfaces within tolerance bands. On the other hand, a workholding device that grasps the workpiece too tightly can deform it. When released at the end of the cycle, it will spring back to its original shape, causing machined surfaces to move out of their tolerance bands.
 
2. Ensure that CNC operators can perform required tasks at the same rate as the machine. The operator must be able to keep up with the machine for the process to be stable. For example, operators must be able to quickly and easily replace and secure workpieces. They should also be able to perform any tasks related to workpiece cleaning and deburring, measurement, and sizing adjustments during the CNC cycle.
 
3. Ensure that an acceptable period of unattended operation can be achieved. By unattended operation, I’m not necessarily talking about eliminating all human responsibilities. While there are situations that require long periods of completely unattended operation, most CNC processes require operator involvement at least once per cycle to load and unload workpieces. 
 
Instead, I’m talking about being able to machine at an acceptable level without too much operator involvement. The single, largest problem in this regard is selecting optimum cutting conditions for reasonable cutting tool life. Admittedly, this is rather subjective. A common mistake is to select overly aggressive cutting conditions to machine workpieces faster. This often causes cutting tools to wear more quickly than they should as well as excessive tool maintenance.
 
Production-run volumes dictate what to do in this regard. With small lots, all cutting tools likely will last the entire production run without dulling. No compromise in cutting conditions may be necessary unless the same cutting tools are used from job to job. In this case, compromises can minimize excessive operator intervention. As lot sizes grow, it becomes more likely that tool maintenance will be required during the production run. 
 
If the machine must be down during tool maintenance, it is important to find a good compromise between cutting conditions and tool life, especially for long production runs. However, some tool-life management systems enable tool maintenance to be completed while the machine is in production. This is accomplished by using multiple, identical cutting tools and ensuring that operators can remove and replace dull tools while the machine is in cycle. With this kind of system, compromising cutting conditions is not as important because tool maintenance is done during production.
 
One last point that I’ll make about tooling-related process stabilization has to do with engineering involvement. In many companies I visit, CNC setup people and operators are left completely on their own to get jobs up and running and keep them that way. While they get the job done, they might be using questionable methods to do so. 
 

The compromises setup people and operators make between cutting conditions and tool maintenance tend to err on the side of making tools last as long as possible. With more appropriate cutting conditions, workpieces can be made more efficiently, without excessive tool maintenance. As another example, operators often change/index all cutting components (inserts) at the beginning of every shift. This wastes time and tooling. Unless engineering/management get involved, these wasteful methods will go unnoticed.   

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