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15. Calculating Quality

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15. Calculating Quality

Explanation

Calculating quality rates at multiple production runs and specifically with multiple output easily leads to mistakes!

Imagine a machine, producing in one minute 100 pcs product A and simultaneously 1 product B.

  • What is the effect on the quality rate when there is 1 scrap product A?
  • And what if there is 1 scrap product B?
  • Of course a scrap of B is far more severe! How to solve this?
  • When losing  1 piece product A, 1 minute  on this side of the machine is lost.
  • When losing  1 piece product B, 0,001 minute  on that side of the machine is lost.

This is the basis for calculating a weighted average in the quality rate.

In this example

  • Shift time:                                       500 min
  • Production Run A:                         500 min
    •   Output 1                             100 Good (Set Speed 1000 pcs/min)
    •                                                100 reject
    •   Output 2                              500 (Set Speed 1 pcs/min)

Result

  1. Availability 100%
  2. Performance: 50,02%
  3. Quality: 99,98% (Weighted average!!)
  4. OEE = 100% x 50,02% x 99,98% = 50,01 %

Calculation of Performance

  1. The performance of each product in the run is calculated:
  • (100 good + 100 Reject) / 500.000 max = 0,04%
  • (500 good)/500 max = 100%

2. The average performance for the run is  calculated:

  • (0,04+100)/2= 50,02%

50,02%

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
  • 100 good + 100 rework at 0,001 min/pcs = 0,2 min total output

Product B:

  • 1pcs/min means 1 min/pcs
  • 500 good at 1 min/pcs = 500 min total output

Total time of output

0,2 + 500 = 500,2 min output

2. Time for total good output is calculated:

Product A:

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

Product B:

  • 500 good at 1 min/pcs = 500 min good output

Total time of good output

0,1 + 500 = 500,1 min output

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

  • 500,1 min good / 500,2 min total = 99,98% quality rate

Warning: The quality rate is not only a weighted average when there is multiple output, it is also weighted based on the theoretical amount of output per product!