A process can drift for hours while every reading passes. Read the check sheet for direction, not pass or fail: readings rising several checks in a row, readings past halfway to a limit, or a cluster near one limit that was not there at the start of the shift. Write a call rule on the sheet so the operator stops the machine while the parts are still good.
It's noon on a Thursday. The operator on Lathe 3 measures the hourly check piece of shaft SH-3306 and writes 24.010 on the in-process inspection sheet. The outside diameter is 24.000 plus or minus 0.020. Every reading on the sheet is in tolerance.
The operator looks at the column anyway. Seven checks since 6:00 read 24.002, 24.006, 24.003, 24.006, 24.002, 24.008 and 24.010. Nothing is wrong by the drawing. But the last three go up, and the last one is halfway from nominal to the upper limit.
A drift passes every check
A part can drift for hours while every reading passes. Pass or fail against the print will not show it, because the print only tells you when you have already lost. What shows it is the direction of the numbers.
Three things to look for on any check sheet:
- Readings moving one way for several checks in a row.
- Readings using up more than half of the tolerance on one side.
- A cluster near one limit that was not there at the start of the shift.
This sheet shows all three. Nobody is out of tolerance yet, and that is the point. The time to act is while the parts are still good.
Decide what "creeping" means before you need it
Operators hesitate to stop a machine that is making good parts. Take the argument away by writing the call rule on the sheet: three readings in a row rising, or any reading past halfway to a limit, means stop and call the lead. The operator here made the call because the numbers said to, not because of a feeling.
The mistake to avoid
Waiting for the first out-of-tolerance part. By then you are sorting, not preventing, and you have no idea how long it has been going on.
- Pull this week's in-process check sheets for one machine.
- Look for readings moving one way across three checks in a row.
- Add a written call rule to any sheet that has none.
Open the printable Monday Board
Related on ShopFloorOps
The part, the machine, the shift and every number in this book (shaft SH-3306 on Lathe 3) are a made-up example. The tool sidebars show ShopFloorOps tools, which I make and sell. Every chapter teaches the step itself, so you can use your own forms. New to a term? See the quality glossary.