OEE multiplies three ratios: availability, performance and quality. A machine that runs 90% of planned time, at 90% of rated speed, producing 90% good parts has an OEE of 73%. The arithmetic is easy. The honesty is hard.
Availability: planned time is a decision
Availability is run time divided by planned production time. The trap is in the word planned. If breaks, changeovers and planned maintenance are excluded from planned time, availability looks better and improvement opportunities disappear. Decide what counts as planned, write it down and apply it to every machine.
Capturing run time from the machine itself, through a PLC signal or a sensor, removes the argument. Capturing the reason for each stop from the operator, in two taps, is what makes the number useful.
Performance: rated speed must be real
Performance compares actual output to what the machine should produce at its ideal cycle time. If the ideal cycle time is set generously, performance is flattered. We take rated speeds from the machine specification or the best sustained run on record, not from the production target.
Quality: count rework honestly
Quality is good parts divided by total parts. Parts that are reworked and later pass are not good parts on the first attempt; count them as losses or your quality ratio hides the cost of rework.
What a credible OEE system needs
The data has to come from the machine where possible, from the operator where necessary and from the ERP for context such as the job, the part and the shift.
- Machine signals via PLC, OPC UA or edge sensors, with store-and-forward for network drops
- Operator downtime-reason capture at the machine, not at the end of the shift
- Job and shift context from the ERP so losses can be attributed
- Role-based dashboards: owners see trends, supervisors see today, operators see now
Use OEE to choose projects, not to grade people
The moment OEE becomes a scoreboard for operators, the data degrades. Used as a map of where time is lost, it tells you which changeover to shorten, which machine to maintain first and which part to re-quote.