Multiple Machine Operation—Evaluating Specific Applications

To determine whether a given application consistently falls within or exceeds the operator-utilization cutoff point, I’ve developed a spreadsheet that you can download at Columns From: 4/2/2008 Modern Machine Shop,

To determine whether a given application consistently falls within or exceeds the operator-utilization cutoff point, I’ve developed a spreadsheet that you can download at http://www.cncci.com/resources/tips/machineutil.xls

Let’s begin evaluating a specific application by listing a typical sequence of events involved with having one operator run two machines (see chart).

 

Sequence Of Events
  Machine A  Machine B 
1. Setup of machine A Machine B is idle
2. Setup of machine A is complete Setup of machine B
3. Start running production Continue setting up
  During machine B setup, interference for machine B is:
Time to load and check parts on machine A
Tool maintenance on machine A
Other tasks on machine A (tool breakage, blowing chips and others)
  During machine B setup, interference for machine A is:
Time to break away from machine B to load parts (the walk)
Time operator is gathering for machine B setup
3. Continue running production Machine B setup complete
4. Running production Running production
  During production run, interference for machine A is:
Part load interference to load machine B
Tool maintenance interference (change machine B tools)
  During production run, interference for machine B is:
Part load interference to load machine A
Tool maintenance interference (change machine A tools)
  During production run, interference for both machines is:
Internal tasks (deburring), if they take longer than the longest run time
5. Machine A finishes, Machine B must continue
6. Setup next job on machine A Continue running machine B
  During setup of machine A, interference on machine B is:
Break away to load machine B
  During setup of machine A, interference on machine A is:
Load and check parts from machine B
Tool maintenance on machine B
7. Machine B finishes while machine A continues

I’m assuming that the operator is responsible for everything that happens on the two machines, and that the operator begins with both machines down and waiting for setups. The smaller the lots and the shorter the cycle times, the more often both machines will concurrently require setups.

For this example, there are two similar jobs, each with 100 parts to run and total setup times of 1 hour each. Run time is 3 minutes, and load time is 15 seconds. Machine cost is $30 per hour and operator cost is $20 per hour. With only 100 parts to run, tools will last for the entire job (no tool maintenance will be required during the production run).

As soon as the first machine is set up and the first workpiece passes inspection, it will start running production. This means that while the operator is setting up the second machine, he or she will have to break away every 3 minutes to load the first machine. The operator will also have to do deburring, inspections and SPC reporting on each part, adding to the time it takes to make the second setup.

When both machines are in production, they will sometimes complete their cycles at the same time. We’ll say this occurs two times per hour. Internal operations for each part include only deburring and spot checking, which the operator comfortably can do within the 3-minute run time.

One machine will finish first. In our case, it will be the first machine to be set up. The operator will begin the next setup on this machine while completing the production run on the other. There will be the same kind of interference that occurred when the initial setup was made for the second machine.

Here are the totals for these jobs (from the spreadsheet):

Elapsed time to completion if a separate operator runs both machines: 6.418 hours
Elapsed time to completion if one operator runs both machines: 8.538 hours
Total interference time: 2.12 hours
Cost with two operators: $641.83
Cost with one operator: $619.28
Savings: $23.55 (3.67 percent)

While there is a modest savings, notice that a machine will sit idle for a total of 2.12 hours. The cost savings is not substantial enough to outweigh this lost production time.

For another example, say we come up with a way to keep setup time from interfering when both machines require setup. Maybe we dedicate a person to making setups for those times when both machines happen to go down at the same time. This new criterion eliminates interference during setups, and here are the results:

Elapsed time to completion if a separate operator runs both machines: 6.418 hours
Elapsed time to completion if one operator runs both machines: 7.288 hours
Total interference time: 0.87 hours
Cost with two operators: $641.83
Cost with one operator: $527.84
Savings: $113.99 (17.78 percent)

Now, the lost production time is only 0.87 hours. This also shows the effect that eliminating interference can have on costs.

Machine cost can also dramatically impact expected savings. We’ll set the same criteria as with the first example, where the operator does everything, but this time we’ll use very expensive machines that each cost $100 per hour. Now the results render:

Elapsed time to completion if a separate operator runs both machines: 6.418 hours
Elapsed time to completion if one operator runs both machines: 8.538 hours
Total interference time: 2.12 hours
Cost with two operators: $1,540.40
Cost with one operator: $1,700.28
Loss: $159.88 (-9.4 percent)

This time the results render a 9.4-percent loss. In this situation, of course, having one operator run both machines would cost more than having a separate operator run each machine.

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