15. Calculating Quality
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)
- Availability 100%
- Performance: 50,02%
- Quality: 99,98% (Weighted average!!)
- OEE = 100% x 50,02% x 99,98% = 50,01 %
Calculation of Performance
- 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%
Warning: Just adding all actual outputs and dividing this by all maximum outputs does not give a correct performance!
Calculation of Quality
- Time for total output is calculated:
- 1000pcs/min means 0,001 min/pcs
- 100 good + 100 rework at 0,001 min/pcs = 0,2 min total output
- 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:
- 100 good at 0,001 min/pcs = 0,1 min good output
- 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!