Our plant manager dropped several files on my desk and told me that previous colleagues had not found a solution to a problem where C-cells were being sporadically rejected at final test. Now it was my turn to find the answer.
I spent some time reading through the files and noted that my predecessors had worked back from raw materials, through detail parts and subassemblies. The attempt to find the source of the problem was terminated at various times, which indicated that the problem had either disappeared or my predecessors had moved on to other jobs. Their thoroughness showed they were not able to identify anything unusual in their investigations, so I decided to work backwards from cell testing to the place they had stopped their investigations.
When the employees in the testing area went on their breaks, I examined the test machine. It consisted of a multi-station carousel somewhat like the chamber of a revolver into which cells were fed. The chamber carried the cells to terminals, which carried out a loaded volts test. If the cell passed, it was carried on to a station that rolled the cell down a plastic channel and a worker stored the cell in an insulated tray. If it failed, a gate opened and the cell dropped into a plastic bucket, and the worker would stack it in another tray as a rejection.
On impulse, I started up the machine and fed some rejected cells through the tester. Most of them passed. When I hand-tested them, they were verified as OK. I continued doing this until I had accumulated about a dozen “rejects.” During the exercise, I marked one location which had intermittently led to a rejected cell, and when I passed the dozen rejected cells through the machine, they all passed -- except the ones at the marked location.
I then took a batch of passed cells and manually cycled the tester until the cell at the marked location registered with the test terminals. Sometimes they passed, but most of the time they failed. I noticed that the failed condition had the cell sitting canted in the chamber. By manually reorienting the cell, it would pass the test.
By this time, the tool-room foreman had turned up because he noticed the machine was running during break time. I showed him what was happening. He disappeared and returned shortly with another carousel and had one of the mechanics replace the chamber. When the test workers returned from break and resumed testing, the reject level dropped dramatically.
Over time, a large number of rejected cells had ended up sitting in the warehouse. A quality audit revealed that many of them were OK when hand-tested but showed intermittent failures when machine-tested. Because the confidence level was low, the cells remained in quarantine. After the testing machine defect was identified and verified, we issued a directive to recycle all rejected C-cells in the warehouse for re-evaluation.
This entry was submitted by John Mitchell and edited by Rob Spiegel.
John Mitchell was self-employed through Mitchell Research. He worked mostly in aerospace design/liaison engineering with excursions into product/quality engineering on batteries, forensic engineering analysis, and Hovercraft. He is now retired and working on vertical axis wind turbine systems and small electric vehicles.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.