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16. Calculating Quality; extreme example

What happens in extreme situations

The following example shows how deceiving it can be when products with quite different maximum speeds are produced on the same machine.

Looking at this situation you might say:

  • Product A is 50% Ok, product B is 100% ok: Thus a quality-rate of 75%.
  • Or: 3 products produced, 2 are OK: thus quality rate is 2/3=66,7%

Then how can the quality rate be 99,9% ??  

In this example

  • Shift time:                                      500 min
  • Production Run A:                         500 min
    • Output 1                                 1 Good (Set Speed 1000 pcs/min)
    •                                                  1 reject
  •   Output 2                                         1 (Set Speed 1 pcs/min)

Result

  1. Availability 100%
  2. Performance: 0,1002%
  3. Quality: 99,9% (Weighted average!!)
  4. OEE = 100% x 0,1002% x 99,9% = 0,1001%

Calculation of Performance

  1. The performance of each product in the run is calculated:
    • (1 good + 1 Reject) / 500.000 max = 0,0004%
    • (1 good)/500 max = 0,2%
  2. The average performance for the run is calculated
    • (0,0004+0,2)/2= 0,1002%

Warning: Just adding all actual outputs and dividing this by all maximum outputs does not give a correct performance!

Calculation of Quality

  1. Time for total output is calculated:

Product A:

  • 1000pcs/min means 0,001 min/pcs
  • 1 good + 1 rework at 0,001 min/pcs = 0,002 min total output

Product B:

  • 1pcs/min means 1 min/pcs
  • 1 good at 1 min/pcs = 1 min total output

Total time of output

0,002 + 1 = 1,002 min output

2. Time for total good output is calculated:

Product A:

  • 1 good at 0,001 min/pcs = 0,001 min good output

Product B:

  • 1 good at 1 min/pcs = 1 min good output

Total time of good output

0,001 + 1 = 1,001 min output

3. The ratio of good output time versus total output time is calculated

1,001 min good / 1,002 min total = 99,9% quality rate

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