All you want to know about
QUALITY in OEE
FAQ’s around the definition of ‘Quality’
1. What if the 'Quality' can not be determined immediately?
Q: Sometimes our process is being interrupted; i.e. the oven is stopped at a different moment than planned. At that moment it is not clear whether the product will be OK or not. We will block the product and take samples to determine the quality. This could take a couple of days. Wen everything is OK the product is released, otherwise it is being scrapped. This would have a retrospective effect on the OEE I guess? A correct quality rate -and thus OEE- for the day is no longer possible in this way. What number should I now use?
Arno Koch • I would approach as follows: The product was not first-time-right. The process is not in control. I would like to see this expressed in the OEE via the quality rate.
I would define two categories of Rejects:
- Product OK after oven-problem
- Product rejected after oven problem
Both would decrease the OEE, yet you would still be able to see the correct amounts of good- and rejected products.
2. What is First Time Through (FTT) (or First Time Right (FTR))
Daniel Högfeldt • Quality rate could also be called first time through. It is important to get a correct FTT number in the OEE calculation.
Arno Koch • I used Daniels example in a casting environment for a more generic description here.
First Time Through (=FTR – First Time Right)
In the quality Rate of OEE the minimal information that is needed to be collected is
- Good parts
- Rejected parts
Rejected parts can be categorized as:
- Scrap: Has to be destroyed
- Rework: Can be correct in order to still meet the specifications
- Sub-Spec: Does not meet the spec, but can be sold in a different market, b-product etc.
FTT is all the output that met the spec immediately. In terms of OEE: Good Product.
How is FFT Rate calculated?
The ratio of good parts to total parts will give the FTT rate.
On many machines, to detect this is not really an issue. Often it is straightforward to count the amount of scrap and total parts the machine made during the shift.
Other names for FTT
There are a lot of abbreviations; all reflecting to the same KPI:
- FPY = First Pass Yield
- FTR = First Time Right
- RFT = Right First Time
- DRIFT = Do it Right First Time
- TFT = Through First Time
3. How to handle FTT (Quality Rate) when rejects were not being detected?
Where it becomes a problem is when the operator can not immediately detect the rejects. If scrap parts are not noticed bij the operator, such parts will go on to next step (or even to the customer).
This might show a too high FTT but would lower the FTT at the next step (assuming it causes a reject there).
A clear feedback loop should be established to make sure the problem is being solved where it arises.
Where to calculate FTT?
FTT should be calculated at the machine where the specification of the intende conversion has not been met.
So what when it was noticed in the next step?
One approach could be to send all scrap found in the next step back to the previous step, where then
- it would need to be registered in retrospect and bring the FTT to the correct level.
- a root cause analysis should be startet to prevent this from ever happening again.
We see people resist this approach; it takes an effort, it needs to be registered etc.
A quality defect always has to be noticed in the quality part of the OEE: this is the most expensive part of the loss-cascade.
When a machine loses time because of incoming scrap, this has to be registered as an availability loss.
After the actual scrap is sent back to the place where it was made, there it has to be registered as a quality loss.
4. What to do in FTT with parts placed on hold?
The quality rate (= FTT) in OEE also separates parts that are on hold or may be condemned. They were not First Time Through!
To visualize this and still have a correct material balance, you may define a rework category: ‘Released after second inspection’
5. What is the difference between FTT and OTIF?
OTIF means: On Time In Full and references to the DELIVERY of GOOD product ON TIME.
OTIF does not only references to the right Quality but also to the right moment where this quality is being delivered
6. How to consider Spillage?
Let’s say I’m producing 90 pieces instead of an expected 100. Then my OEE should be 90/100 = 90%. But now consider the conveyor is faulty because of which 10 pieces got spilled (without stopping the line). These 10 pieces spillage are waste as those can’t be use as finished goods. So 90-10 = 80 is my actual output now. In this case, my OEE should be 80/100 = 80% right?
Arno Koch • Basically you are right. Now the question raises: Where in the OEE will you see this spillage as a loss? If the lost items are somehow being detected and registered as waste, they can pop-up in the quality-rate. That would be the most correct way to visualize the loss.
If they are not being detected, they will pop-up as minor stops in the performance rate.
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