ShopFloorOps
Start here

Field Book · Chapter 6 of 8 · Cause on three legs

Find What Changed

Check the usual five changes against the records and the time the chart says the change began, then run the cause on three legs: why it drifted, why it was not caught, why the system allowed it.

By Chris Martin, quality manager. 2 minute read. Free, no signup.

Take the time the chart says the change began and check tool, material, setup, operator and environment against the records, one at a time. Here four are ruled out and one lines up: the coolant chiller tripped five minutes before the run began. Then run the cause on three legs: why it drifted, why it was not caught sooner, and why the system allowed it.

The chart says the change began at 9:40. Now find what happened to the process around then. Check the usual five, one at a time, against the records:

  1. Tool. The insert was changed at 5:50, before the shift. Planned life is 400 pieces, and the lathe has made 222. Not this.
  2. Material. One bar lot all shift, same certificate. No change at 9:40. Not this.
  3. Setup. First-off was approved at 5:55, and the offset history for the shift has no entries after it. Not this.
  4. Operator. The same operator since 6:00, no relief. Not this.
  5. Environment. The coolant chiller tripped a high pressure alarm at 9:35, which is five minutes before the run began. The panel is on the chiller, not at the lathe. Coolant read 21 degrees C at 6:00 and reads 29 at noon.

Four were ruled out by records. One lines up with the chart. That is the lead.

Run it on three legs

Why it drifted, why you did not catch it sooner, and why the system allowed it.

  • Occurrence. Why did it drift? The chiller tripped on high pressure at 9:35 because the condenser filter was packed with oil mist. The coolant warmed, the spindle and slide grew with it, and the diameter climbed toward the upper limit.
  • Detection. Why did we not catch it sooner? The sheet was one piece an hour, read as pass or fail. The chart had dots but no limit lines drawn on it, so the point at 11:00 above the upper limit meant nothing to anyone.
  • System. Why did the system allow it? The chiller is on no preventive maintenance list. The alarm shows only on the chiller panel. The control plan says nothing about coolant temperature, and nobody owns the condenser filter.

Each leg points at an action, and none of them is "retrain the operator." Field Book 1 goes deeper on the three legs.

Read more: how to do a 5 why and how to make a fishbone diagram.

The mistake to avoid

Stopping at the first thing that looks plausible. Check all five against the timing. If two line up, you do not have a cause yet, you have a shortlist.

ON THE FLOOR MONDAY
  • Take your last drift or shift in a process and write the time it started.
  • List what changed within an hour of that time, from records, not memory.
  • Check that your cause answers why it was not caught and why the system allowed it.

Open the printable Monday Board

Want every tool on this trail in one purchase? The Process Trail Bundle has all 13.

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.