A supplier shipped a lot of machined housings with an out-of-tolerance bore. It passed their final inspection and receiving inspection, and was only found at the customer's assembly station. The line had roughly two shifts of stock left.
What we did first
We contained the stock before starting any analysis. Defining suspect stock by heat lot instead of ship date pulled in two lots the supplier had not flagged. Certified sorting went in at three points: the supplier, our warehouse and the customer dock.
What the data showed
The bore drifted on one of two spindles, and only in the second half of each tool life. The supplier's control chart sampled once per shift, so it could not see drift within a shift.
- Sampling moved to five pieces per tool change, plotted against tool life instead of time.
- Tool life was cut by fifteen per cent, which cost less than one hour of sorting.
- We corrected the control plan to match what the process really does.
Outcome
There were no further escapes in the following nine months. The sorting came off after six weeks of clean data.
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