Judging CNC Machine Utilization Levels

It's no secret that CNC machines are highly productive. In fact, this is one of their greatest benefits.

Columns From: 8/1/2002 Modern Machine Shop,

It's no secret that CNC machines are highly productive. In fact, this is one of their greatest benefits. But this important benefit can lead to a problem. If you are thoroughly satisfied with the productivity of your CNC machine tools, how can you tell when they are being under-utilized? I define CNC machine utilization as the effectiveness level of your CNC machine tools. You can contrast your judgment of machine utilization against many factors, making the judgment of machine utilization level very subjective.

Don't confuse utilization with application. A machine's application is simply what you are using the machine to do. A vertical machining center that is being used as little more than a glorified drill press is every bit as important to its owner as an identical machine being used to produce three-dimensional shapes in molds.

Also, don't confuse machine utilization with personnel utilization. You may think you're making the best use of your CNC machine tools when in reality, you're making better use of your CNC people. For example, you may have one person running two or more CNC machines. Or you may have a CNC operator performing secondary operations during the CNC cycle. These are common practices, but they lead to under-utilization of CNC machine tools. It is likely that CNC machines will often sit idle—waiting for a CNC person to do something.

Because machine-time cost (shop rate per hour) is usually much greater than the hourly wage of a typical CNC operator, you should be more willing to have a CNC person waiting on a CNC machine than a CNC machine waiting on a CNC person. As human beings, we can't stand to see anyone sitting idle, waiting for a machine to complete its task. If your goal is to keep people busy, assign them tasks that are unrelated to the operation their CNC machines are performing. But be sure they understand that the CNC machines are the top priority.

Things have been changing in the CNC environment that are making companies take a closer look at machine utilization. Among other things, lot sizes are getting smaller, quality expectations are higher and lead times are shorter. Sticking with current methods will lead to severe problems. Fortunately, most companies have built-in potential for improvement. Again, maybe you've been so satisfied with the performance of your CNC machines that you've never questioned utilization. It's likely that at least some of your machines have been under-utilized, allowing room for improvement.

My first suggestion for judging a CNC machine's utilization level is simple: look at the percentage of time per shift, day, week or other period that a CNC machine is in cycle. If, for example, you find that during an 8 hour shift, the machine is in cycle 7 hours (not in cycle for one hour), this machine's utilization level is 87.5 percent (seven divided by eight). Acceptable machine utilization levels will vary from company to company. A product-producing company will have higher expectations than a contract shop.

Unless your company has already implemented some kind of improvement program, there probably will be room for improvement. Fortunately, improvements in this area will be relatively easy. Just come up with ways to keep machines running. It may take rethinking your company's personnel utilization (teaming up on jobs as opposed to having one person do everything). It may require better organization (why does the operator have to go all the way to the tool crib to get more inserts?). It may require more work in preparation, getting managers and lead people more involved with scheduling.

My second suggestion for judging CNC machine utilization level is to consider bottlenecks. Is there one machine or department that's always behind? Is there a machine or department that has an unusually high scrap rate? Is there a machine or department that has unusually high turnover? Bottlenecks of any kind are an important symptom of under-utilization. Improvements will likely be more challenging, often requiring an overhaul in processing, workholding, and cutting tools, generally re-thinking how the job is done.

My last suggestion for judging CNC machine utilization level is to examine the number of mistakes (especially repeated mistakes) being made. If a mistake causes scrap, the time it takes to run the scrap workpiece cannot be counted as part of the machine utilization level calculation given above. But even if mistakes don't cause scrap, it's likely that they do cause a decrease in machine utilization level. Repeated mistakes should be a signal that you need to increase skill levels through training or decrease the skill level required to perform the mistake-causing task through task simplification.

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