CNC cycle time is defined as either elapsed time from a given event in one cycle to the same event in the next cycle or a job’s total production run-time divided by the number of good parts machined.
With the first definition, the “event” is commonly the pressing of the cycle-start button. For example, someone activates a timer at the same instant the cycle-start button is pressed. The program executes, and machining begins. When the program finishes, the workpiece is removed and another is loaded. The operator then presses the cycle-start button again, and the timer is stopped.
While button-to-button time is important, it is not a very inclusive definition of cycle time. It does not take into account those things that do not happen in every cycle. Workpiece inspections (if they must occur while the machine is idle), tool maintenance and secondary machining operations are among the many tasks that are not included in the button-to-button time definition.
The second definition, which results in what I call throughput time, is much more realistic, even though it requires completing the entire production run in order to determine cycle time. This method provides the actual time it takes to produce a workpiece.
When it comes to reducing cycle time, people often concentrate too much on button-to-button time—and more specifically, program-execution time. However, in some cases, reducing button-to-button time will actually increase throughput time.
For example, consider a CNC operator who is struggling to keep up with a machine before button-to-button time is reduced. It may be that button-to-button time is very short—less than two minutes. Yet this operator may be expected to perform many other tasks, including cleaning and deburring workpieces, inspecting workpiece attributes and maintaining cutting tools. This person may even be expected to run two or more CNC machines. In this case, reducing button-to-button time will only make it harder for operators to keep up.
Before you implement any technique for the purpose of reducing button-to-button time, be sure that doing so will also reduce throughput time. One way to do so is to compare button-to-button time with throughput time. If throughput time is within ten to fifteen percent of button-to-button time, it is very probable that reducing button-to-button time will also reduce throughput time as well.
I’ve been in many companies where the throughput time for a given job is much more than double the button-to-button time. These companies tend to be utilizing their CNC people very well—but not their CNC machine tools. Again, CNC operators may be expected to run two or more machines, perform secondary operations on workpieces, inspect workpieces, and maintain the tools. In short, operators cannot keep up with the machines they run. So machines will sit idle, waiting for someone to do something. In these cases, we must find a way to keep machines running for a greater percentage of time—this will improve throughput time.
Begin your cycle time reduction program by comparing button-to-button time to throughput time. If you find that throughput time is much longer (again, by more than about 15 percent), your single largest cycle-time gain will come from drawing throughput time down to button-to-button time.
Move tool maintenance tasks off-line. There are companies that have completely eliminated the effect of tool maintenance time on cycle time. That is, they have found ways to make tool-maintenance time internal to program execution time. Tool life management systems help accomplish this. Even if moving all tool maintenance tasks off-line is not feasible, it may still be possible to move some tasks. At the very least, make sure that machines don’t sit idle while CNC operators search the shop for perishable tools, like inserts. (Ed. note: See also the article A Practical Guide To Presetters)
Get organized. You’ll likely find that at least some of the difference between button-to-button time and throughput time is the result of disorganization. Hand tools are not readily available and conveniently placed. Tool boxes and drawers are crammed full. Inserts are not stored near the CNC machines. Work areas are cluttered and messy.
Get operators the help they need. If you see your expensive CNC machines sitting idle while operators are doing other things, it should be taken as a signal that they need help. If machine utilization is the priority (over personnel utilization), you should be more willing to have people waiting on machines than machines waiting on people.