Quality Rate

Quality rate is the OEE factor most people skip when they say "the machine ran fine today." A machine that ran most of its scheduled time and ran at full speed can still produce a quarter of its output as scrap, and OEE will reflect that. The quality factor isolates equipment driven defects from operator errors, material issues, or inspection mistakes upstream. It is the cleanest measure of whether the equipment is currently capable of producing what was asked of it.

"Running the machine all day at full speed doesn't matter if the parts don't pass inspection."

How quality rate works

The calculation uses two counts: parts produced and parts good. Parts produced is everything that came out of the machine during the run. Parts good is the subset of parts produced that met specification on first inspection, without any rework, regrinding, or adjustment after the fact. Dividing the second by the first gives the quality rate. If a press produced 1,000 parts in a shift and 970 passed inspection on first look, quality rate is 97 percent.

The "first pass" qualifier is the part most shops get wrong. A part that was off spec but got reworked back into spec is not a first pass success; it counts as a defect for quality rate purposes. The thinking behind this is that the equipment produced something it should not have, and the rework hides that loss. OEE wants to surface the loss; counting rework as success defeats the purpose. The same logic applies to inspection variation. The count of good parts uses the first inspection after the machine, not the final inspection at the end of the value stream.

Quality rate captures two of the six big losses: process defects (parts wrong because the machine produced them wrong) and startup losses (parts scrapped during ramp up after a stop or changeover). The fix for each is different. Process defects are usually traceable to a specific equipment condition, often through quality maintenance work. Startup losses come down through better changeover procedures and pre setup runs.

Where quality rate fits on the shop floor

Picture a 15 person plastics injection shop running consumer goods parts for a couple of brands. Availability on the lead press is 92 percent and performance is 88 percent. The owner has been told OEE looks great. Quality rate measurement reveals a different story: 82 percent. The press is producing roughly one part in six that has cosmetic defects, mostly sink marks and short shots, that get caught at inspection and scrapped before the customer sees them. The shop has been absorbing the scrap cost as normal.

The investigation finds two root causes: a worn nozzle that has been allowing inconsistent shot weight, and a chiller that has been running slightly warm because the pump is failing. Both are equipment condition issues that quality maintenance work would catch. After a half day of maintenance and a chiller rebuild, quality rate climbs to 96 percent. OEE moves from 66 percent to 78 percent. The shop did not just save the scrap material cost; it recovered roughly 14 points of effective capacity it had been throwing away.

Common mistakes with quality rate

• Counting reworked parts as good. Rework hides equipment loss. Quality rate is first pass only.
• Inconsistent treatment of jams and ejected parts. Decide whether jammed parts count as produced and stay consistent.
• Inspecting too far downstream. Quality rate uses the first inspection after the machine. Inspecting three stations later mixes equipment loss with other process losses.
• Tracking quality rate without acting on it. A low quality rate is a pointer to equipment condition. Without follow up it is just a complaint metric.
• Confusing quality rate with overall quality. Quality rate is equipment focused. Overall quality includes operator error, material issues, and design issues that quality rate is not designed to catch.

Quality rate and related Lean tools

Quality rate is one of three factors in overall equipment effectiveness, multiplied against availability and performance rate. It is closely related to first pass yield, the broader process focused version of the same idea. The TPM pillar that exists to drive quality rate higher is quality maintenance, which keeps equipment in a condition that cannot produce defects.