How to calculate OEE with mixed output?

Q: Our machine can run mixed output, so several different products, each shift. Till now we just recorded the time the machine was running and the output we had. Our software now requires that we register activities and quantities to account for every minute in a shift. But we are not tracking the number of minutes it ran each product.

Arno Koch •    NOT your software requires this! It is required to get an accurate OEE! And thus that is how it has to be done in the software…

Let me explain this;
If you run mixed output, so different products each shift, you might face up with products having different maximum speeds within that shift. So to know how much product you could have made (the target for a production run), you need to know how many minutes the machine ran at what maximum speed. By calculating the target output for a run, and comparing it to the actual output for that run, you are calculating a performance rate for this particular run. If you do this for every run in a shift, the performance rate is a weighted average of multiple runs (no matter the different maximum speeds per product!).

If you would just add all output and all production time, you lose the weight-factor! It LOOKES as if you get a correct OEE, but it isn’t. Many of the home build systems I saw had this algorithm error.

Example

Lets take an example: a moulding-press. It is being operated with two different dies: one die for a large part, making one part each stroke and one die for a small part making 4 similar parts each stroke.

The press can make two strokes a minute. So the standard for the large part is 2 parts per minute and for the small part it is 8 parts per minute (2 strokes x 4 parts) Just for the example we assume changeover is timeless and everything else is perfect.

Let me show you two different days;

First in the way as you describe:
‘… we just recorded the time it was running and the output we had.’

 

Day 1 (480 min)

Day 2 (480 min)

Time Running

480 min

480 min

Output

2340

1260

Obviously day 1 was your best day.
Now, can you tell me what your OEE was on day 1 and on day 2?

Lets see how you can solve this (and how good software should do…)

 

Day 1

Day 2

Minutes Part Large

120 min

360 min

Max. Output Part Large

(120×2=)
240 parts

(360×2=)
720 parts

Actual Output Part Large

180 parts

540 parts

Minutes Part Small

360 min

120 min

Max. Output part Small

(360×8=)
2880 parts

(120×8=)
960 parts

Actual Output Part Small

2160 parts

720 parts

What was your best day?

 

Day 1

Day 2

Availability

100%

100%

Performance

75%

75%

Quality

100%

100%

OEE

75%

75%

The point is: To calculate the performance rate, it is necessary to know what the EXPECTED output was versus the ACTUAL output. In a mixed production this means you’ll have to know how long you ran what product since that is what determines the expected output.

Lets make it a little more difficult

Some machines, like presses, can produce different products in one stroke.
Imagine a press that can have two different dies in one stroke (so you could have a changeover of half a press…)
So in one shift the press can first produce 1 A and 4 B parts, after that 2 C and 3 D parts
and next run 2 C and 8 E parts each stroke. Dazzling???

Your software should be able to handle this situation seamlessly due to the mechanism as described above!

Q: We have a similar machining environment and what we do is the following:
If we have 4 variants a-d running at ideal cycle times x1-x4, planned for quantities a1-a4, and in relaity what got produced was b1-b4, then performance is calculated in terms of total time as

(b1x1 + b2x2 ….) / (a1x1 + a2x2 …)

Thus it kind of gives a weighted indication of what the performance is, without skewing it like an average.
Awaiting your opinion on this, and whether there is a more accurate method of doing it.

Arno Koch •    Indeed, the way to calculate a correct performance rate in a mixed output situation is to calculate a weighted average expressed in time.

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