The following are important time-related definitions presented in past columns.
Setup time: The time it takes to go from making the last workpiece in the most recent production run to efficiently making the first good workpiece in the next production run. This is the total time the machine is down between production runs.
Cycle time: The time it takes to complete a production run divided by how many acceptable workpieces have been produced.
Program execution time: The interval that passes from the time that the cycle activation button is pressed until the control reaches the end of the program (cycle completion light comes on).
Workpiece load/unload time: The time the machine is down while the previous workpiece is removed and the next is loaded.
Button-to-button time: Program execution time plus workpiece load/unload time.
There are some exceptions to these definitions (we’re assuming one workpiece per cycle, for example), but you should agree that productivity of your CNC machines is closely related to them. Let’s add one more; indeed, it is the most important!
Throughput time: The total time it takes to complete a job divided by how many acceptable workpieces have been produced.
Think of throughput time as the “bottom line” in your CNC environment. Many companies embark on improvement programs to try to reduce throughput time, but people often get bogged down in the details and lose sight of the true goal. It is possible to employ techniques that reduce setup time, program execution time or workpiece loading time and actually increase throughput time.
One of the most deceiving areas has to do with the tasks you expect CNC operators to perform. Managers tend to pile on tasks, yet they mistakenly believe these tasks are being completed during the CNC program’s execution. If an operator cannot keep up with the CNC cycle, cycle time (as we’ve defined it) will increase, as will throughput time.
Consider the most rudimentary tasks CNC operators must perform. These include workpiece load/unload, program activation and sizing adjustments resulting from tool wear. Even these basic tasks take time. Operators may have trouble keeping up. Any additional tasks will add to throughput time if operators cannot keep up. This can be a costly mistake when you compare the CNC operator’s wages to the shop rate of the CNC machine tool.
Another mistake can be made when attempting to reduce program execution time. Many managers want CNC operators to increase cutting conditions for more aggressive machining. Indeed, program execution time will be shortened—for a given workpiece. But it’s likely that the more aggressive cutting conditions will require additional tool maintenance. Saved program execution time may be lost if the machine is down during the increased maintenance. If maintenance time is excessive, throughput time may actually increase.
Lot sizes will impact how much emphasis is placed on cycle time reduction. For small lots, improvements to cycle time will have only a small impact on throughput time. The majority of throughput time may be spent during setup, so it may be better to concentrate on setup reduction. On the other hand, with large lots, the majority of throughput time will be spent in the production run (cycle time). Improvements made to setup time may have only a small impact on throughput time.
Remember that throughput time is a dynamic value—it changes from one time a job is run to the next. The more manual intervention there is in the job, the greater the potential for variation. Because throughput time cannot be determined until after a job is completed, it can be difficult to determine how much room there is for improvement.
To evaluate potential for improvement for a given job, come up with the total of setup time, program execution time and workpiece load/unload time. Compare this to throughput time. The greater the difference, the more potential there is for improvement (especially cycle time-related improvements). This will also expose whether or not CNC operators are keeping up with the CNC cycle. If they’re not, throughput time may be much greater than the total of setup, program execution and workpiece load/unload times.
When considering an improvement in your CNC environment, consider its potential impact on throughput time. You should use throughput time as the gage for justifying any improvement. You may have other goals, but any productivity improvement should begin with the question, “How will this affect throughput time?” If the “improvement” won’t reduce throughput time, why consider it?