A Path to Better Productivity

I can summarize in one sentence what it takes to increase a CNC machine’s output: There are only two general activities that take place, two task types that can be performed and three ways to improve productivity.

Two Activities
There are many ways to account for CNC machine time. Indeed, I know of people who seem as concerned with accounting for time as they are with making the best use of that time. But when it comes down to it, machines are either in setup or they are running production. There is nothing else. If a machine is down between production runs (for any reason), it is in setup. And the total time a machine is down between production runs is the setup time. 

Once the first good workpiece is machined (running the first workpiece and getting it to pass inspection), the machine will remain in production until the last workpiece is machined. If the machine sits idle for any reason while workpieces are being machined, it must be considered part of production runtime. Indeed, a realistic definition of cycle time is: the total length of time the machine is in production divided by the number of acceptable workpieces machined.

While you may not totally agree with my method of accounting for time, hopefully you do agree that anything that reduces the time a machine is down between production runs will improve productivity and should be fair game for your setup-reduction program. In like fashion, anything that reduces the time it takes to complete a production run will improve productivity and should be fair game for your cycle-time-reduction program.

Two Task Types
Any task that adds to the time it takes to complete a setup or production run is an online task (also called an internal task). The sum-total of online tasks performed during setup is the setup time. The sum-total of online tasks performed during a production run is the production runtime.

Any task that is done in preparation for an upcoming setup or production run, or done in conjunction with other setup tasks, or done internal to the CNC cycle is an offline task (also called an external task). Gathering all components for an upcoming setup and/or production run is an example of an offline task (as long as the machine is producing while the components are gathered).

Three Ways to Improve Productivity
Eliminating tasks should always be your first goal. If a task can be eliminated, of course, it will no longer have to be performed. Eliminating online tasks will result in shorter setup or production runtime. Eliminating offline tasks will free up time for people to perform other (online or offline) tasks.

One example for eliminating an online task applies to work on machining centers and is related to program zero assignment. If workholding devices can be placed on the machine table in exactly the same location time and time again (qualified workholding devices), the program zero location will be the same every time the setup is made. If G10 commands are placed in the program to enter fixture offset values, the task of program zero assignment is eliminated.

Since it can be difficult to justify eliminating certain tasks altogether, your second alternative will be moving the task offline. Find a way to keep the task from adding to the time it takes to complete a setup or production run. If cutting tools can be assembled and measured in preparation for an upcoming setup while the machine is running production, the time the machine is down between production runs will be reduced.

One common mistake related to this, however, is to load the CNC operator with a lot of tasks with the intention of having them all done offline. Consider, for example, having an operator perform secondary machining operations while the CNC machine is in cycle. If the operator cannot keep up and it takes longer for him to perform all the offline tasks than the CNC cycle, the task or tasks are no longer offline.

If you cannot justify what it takes to either eliminate tasks or move them offline, your last alternative is facilitating tasks. Make them simpler to perform, since simple tasks should be able to be performed quicker than complex tasks. As with moving tasks offline, this strategy provides benefits for both online and offline tasks. Facilitating online tasks will reduce setup and production runtime. Facilitating offline tasks will free up operators to work on other activities during setup or the production run.

For example, maybe your design engineers use three different methods of tolerancing on blueprints: plus or minus, different plus versus minus, and high/low limits. Each method requires its own calculations to determine mean value, target value, high limit and low limit. Entry level operators in your company could struggle with this, resulting in wasted time and mistakes in making sizing adjustments. If the design engineers could standardize on one method of tolerancing, the operator’s job would be much simpler. Better yet, if a process engineer could create a special drawing that includes the mean value, target value, high limit and low limit for every tolerance, the operator would not have to perform any calculations at all to determine tolerance values.