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.
That's funny tekochip - I had the opposite problem. When I was a test engineer and parts started failing, everyone always wanted to point to the test set. I always kept calibrated "golden" units around so that I could verify tester operation. I made sure my golden units included passing units at both ends and the middle of the spec as well as rejects. Most of the time that would satisfy all involved that we needed to look at the parts themselves...
The author did a good job isolating the problem. I think perhaps the other folks were not so experienced in troubleshooting. I'll never forget as a young tech making an assumption that wound up delaying a resolution to a problem we were having. I don't remember what the problem was but I sure remember my angry boss pulling me into his office and writing ASS U ME in large letters on his white board. He then asked me if I knew what happens when I assume - If you look closely, I am sure you can figure out the rest of the story...
That happened in 1990 but is a lesson I carry with me to this day!
The 'When you assume...' phrase is common, but useless. Everybody makes assumptions. Assuming the rejected parts are actually bad, or assuming the rejected parts are really good is a beginning of troubleshooting. The real trick is to know and realize what your assumptions are, and be prepared to revisit them when troubleshooting doesn't agree. During a light-hearted conversation I was asked of I knew what happens when you assume. I replied 'Did you assume that I was listening to you ?. Of course you can't say that to your manager.
In one of the more famous (TV) episodes of the ODD COUPLE, Felix Unger told the judge a simple fact ....... When you ASSUME something, you make an ASS of U and ME! Very sage words, indeed!!!!
The Odd Couple - Felix and Oscar - haven't thought about them in years! I probably saw that episode, I can certainly picture Felix doing exactly that, LOL! Thanks for the memories, OLD_CURMUDGEON!
Most any Quality Manager worth their salt will run Gage R&R (gauge repeatability and reproducibility) on their test and verification people and equipment on a regular basis. When you don't do this, you haven't a clue as to what you're producing. You need to understand or get get full control of all variables in your inspection processes.
Our people are amazed when we report the source of variables and variation in our most basic inspections (like using a mic or caliper). It's a real eye-opener for most people outside the Quality field. Managing Type I and Type II inspection errors is a fundamental problem in most every company.
I read that the position which failed held the batteries in a "canted" position, but what was the precise reason for the test failing? Did the canted batteries fail to make contact at all? Was there poor contact resistance, or what? The observation and deduction is smart, but I feel I am left hanging.
I appreciate that you want to be fair, but step # 1 in ANY endeavor (Engineering or other) is to identify the task or requirement. In this case, to identify what kind of failure you are investigating (luckily there was a quar. set of failed batteries to test).
If you manually test your failed bin of parts and the failure rate matches the expected (hopefully low) failure rate, then NDF (no defect found). Of course, you want to cut a few "good" ones apart to make sure there are no latent or intermittent problems before you announce to your boss NDF. Task identified: find out why good batteries tested bad.
Which is what the author did, which is what makes this one a good story. Kudo's.
Apologies for keeping you hanging...the canted condition caused bad or high resistance contact and the test registered as a failure. The observation that the test position was canted jogged my memory about other tests I had done varying the terminal contact which led to a wide variance in test results.
When part & component designers experience fit problems with their designed parts, they often point a finger at the tooling & mold engineers; "must be out of spec ; making bad parts".
When industrial automation engineers experience testing and analysis problems, they often point a finger at the component engineers; "must be a design flaw; back to the drawing board".
Then tooling & mold engineers experience trouble getting mold cavities to spec, they often point a finger at faulty heavy equipment manufacturers; "the CNC has slop in the indexing plate".
Truth be told to all, problems and anomalies can exist in every aspect, and looking at yourself first can save a lot of time and embarrassment.
We used to have a mission statement: "Fix the Process and not the Part".It tought a lot out looking at issues from a system level
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