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.
First Time Through
The only information that is needed to be collected is some combination of; total parts, good parts and scrap parts. The ratio of good parts to total parts will give the FTT rate. On casting machines this is not really an issue. It is somewhat straightforward to count the amount of scrap and total parts the machine made during the shift. Where it could be a problem is when it comes to secondary operations. If scrap parts are not noticed in the inspection after the casting operation some parts could go on to the machining operation. The question here is how to calculate FTT on the machining operations when the scrap is from the casting operation?
How to calculate FTT
One approach could be to send all casting scrap found prior to machining back to the casting operation so that it would hurt their FTT and not the machining FTT. Then it could be sent back and be counted in the casting FTT. If the part is machined and the casting scrap is found after that operation then the part is counted as machining scrap. In this case it is important to have some scrap codes at the machining operation for the casting scrap. That is important because of if the secondary machine turn out to have a poor FTT it is good to be able to see if the defects are machining defects or casting defects.
What if scrap that was found after machining were to be sent back and not noticed in the machining FTT? That would not be such a good idea. Even if it would sound unfair for the machining group to get a lower FTT because of casting defects they might not could notice, the equipment still have machined the part and therefore lost time manufacturing a scrapped part. This has to be noticed in the quality part of the OEE. If the scrap were just sent back to the origin place were it was made outside of specification that time would not be noticed. On the other hand as long as it has been noticed in the machining equipment’s FTT, then it could go back to the casting equipment as well to get an accurate FTT on that machine.
The FTT in OEE should not include parts that is on hold or may be condemned. If the part is transferred and later found to be scrap, the waste percentage should be indicative to the loss.
3. How to consider Spillage?
Q: Should spillage be considered in OEE calculation?
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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