Multiple Machine Operation—Interference

I urge you to proceed with caution when studying the feasibility of having one operator run two machines. Be absolutely sure that doing so will render the desired results—and that it will not cost more than having a separate operator run each machine. One cause of interference that is often overlooked prior to the operator-utilization decision is related to the tasks you expect your operators to perform on each finished part upon its completion.

Columns From: 5/1/2008 Modern Machine Shop,

I urge you to proceed with caution when studying the feasibility of having one operator run two machines. Be absolutely sure that doing so will render the desired results—and that it will not cost more than having a separate operator run each machine.

One cause of interference that is often overlooked prior to the operator-utilization decision is related to the tasks you expect your operators to perform on each finished part upon its completion. These tasks commonly include cleaning, deburring, measuring and reporting measurement results to the SPC system. Ideally, the operator will complete these tasks while the CNC machine is in cycle. In this way, the time required for the tasks will be internal to the CNC cycle and will not add to the time it takes to complete the production run.

While an operator may be able to comfortably keep up with these tasks when running one machine, her or she may not be able to keep up when running two machines.

Consider, for example, a job with a 3-minute cycle. The tasks the operator must perform on each part take 2 minutes. In this case, an operator running only one machine can comfortably keep up with the machine, and these tasks will be internal.

Now, consider having this operator run similar jobs on two machines. The machines are running concurrently, so he or she will have 4 minutes of work to do during a 3-minute period. The operator won’t be able to keep up, which leads to 1 minute of interference for every part.

In order to be internal, tasks done on each of the parts must be less than half the longest run time for the two machines. Otherwise, the operator will not be able to keep up with the machine.

Obviously, if an operator cannot keep up with these tasks while running one machine, he or she will not be able to keep up with two machines. Deciding to have one operator run two machines in this case will have a terrible impact on interference.

For example, if an operator running one machine has a 3-minute cycle and 4 minutes of work to do on each completed part, the machine will be idle for 1 minute per part. When running two machines (with identical run times and work), this minute must be added to the 4 minutes of work to be done on the other part—totaling 5 minutes of interference per part.

Here is an example using a spreadsheet that stresses what happens when an operator can’t keep up (find the spreadsheet at http://www.cncci.com/resources/tips/machineutil.xls). Machine cost is $30 per hour each, and operator cost is $20 per hour. Two similar jobs will be run, each with 200 parts, 3 minutes of run time and 15 seconds of part load time. We’ll say that the operator must spend 1 minute per part cleaning, deburring and spot checking—and he or she can comfortably do so within the 3-minute cycle time. Here are the results from the spreadsheet:

  • Elapsed time to completion if a separate operator runs both machines: 11.836 hours.
  • Elapsed time to completion if one operator runs both machines: 14.09 hours.
  • Total interference time: 2.568 hours.
  • Cost with two operators: $1,183.67.
  • Cost with one operator: $1,062.58.
  • Savings: $121.08 (10.23 percent).

Now, let’s change the criteria, making the tasks an operator must do for each workpiece more intense. Say it now takes the operator 2 minutes per part instead of 1 minute. Again, because the machines run concurrently, the operator will have only 3 minutes per cycle to perform 4 minutes of work. One minute of interference will be experienced per part. Now the spreadsheet renders:

  • Elapsed time to completion if a separate operator runs both machines: 11.836 hours.
  • Elapsed time to completion if one operator runs both machines: 17.335 hours.
  • Total interference time: 5.766 hours.
  • Cost with two operators: $1,183.67.
  • Cost with one operator: $1,306.94.
  • Loss: -$121.27 (-10.41 percent).

Total interference time went up, which means the operator can’t keep up with the machine. This stresses just how important it is that you ensure that tasks performed on each workpiece are kept internal to the CNC cycle.

Even when you have determined that it is feasible to have one operator run two machines, and regardless of how much study and evaluation went in to your decision, I strongly recommend that you proceed slowly. Be sure to leave yourself a “back door” just in case things don’t work out as smoothly as planned.

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