Sleepless Nights – Gauge Business
Sleepless Nights – Gauge Business
Sleepless Nights – Gauge Business – It is one of those stories that sends shivers down the back that causes sleepless nights and maybe even the gain of a few pounds for us old timers in the dimensional gauging business. We ship a gauge that we know is looking great. It designed to be use on the shop floor, passed all the calibration and repeat testing that specifies for the application. A little while after it gets into the user’s facility and then the call comes.
The rejected gauge, which is not working well, does not seem to repeat on the parts and the operators have lost confidence in the system. After some support calls, complaints about the perils of shipping, and numerous emails sent back and forth with rising panic from the manufacturer that they need to inspect parts, the decision is made. Ok, let us bring it back. Send it with the masters and the parts measured and we will make it right. It is an apparent black eye.
At times like this, I pull out that little scrap of paper that I keep in my wallet. Seems I pull it out more often than I want. Pretty wrinkled now and the corners are shredded. Written on it is one word: “SWIPE.”
That is our acronym for defining the measuring process: “Standard,” “Workpiece,” “Instrument,” “People” and “Environment.”
The two things that the gauge manufacturer can control are initially be check out: the Standard and the Instrument. The master is clean, no rust or nicks and the certification is original. Then the gauge is set up and inspected. Looks like it survived the travel OK. Things properly aligned; everything is tight. It is clean and there is no damage to the contacts or reference anvils.
So on goes the master and the gauge zeroed. After making a quick repeat study on the master by measuring it 30 times, things are looking good. The repeat on the master is less than 4% of tolerance, which indicates we should easily pass a GR&R test. I am starting to feel a little better.
Two of the five parts of the SWIPE process are working well.
From experience, the Environment seems to be one of the largest sources of errors in the measuring process, dirt and oil being the major contributors. However, this customer’s work area reports to be clean and the parts are dry and clean when received. Maybe dirt is not the problem. Temperature is a big factor with the environment. Nevertheless, usually it will show up as short- or long-term drift errors. This is because it takes time for the gauge to change with any sudden or slow environmental change. This is not the customer’s issue. The parts maybe warm coming off a machine and the gauge is actually detecting the size change as the part cools down. Again, the customer is measuring many parts that are far from the point of manufacture. Apparently, Environment crosses off.
There are People: the ones actually operating the gauge. With comparative gauges like the one in question a lot can be done to ensure that parts go into the gauge the same way every time. Guides, backstops and even spring loading employees to ensure hands-off measuring, eliminating variation by the operator as much as possible. However, all these standard gauge design criteria once employed with this gauge and with multiple operators inserting the master into the gauge. It was repeating well. It does not appear that the operator can influence the part measuring largely.
Then there are the Work-pieces. A quick look at these and the lights start to come on. The parts are in need of some good finishing. The surfaces appear roughly machined, not nearly what one would expect for this type of part. So, before the first part goes onto the gauge for testing, the whole lot of parts goes to the inspection room to get measured for diameters, roundness, square and surface finish — all the dimensions being checked by the gauge.
The results are not too pretty. The parts show significant roundness variation (over a short span), significant out-of-square conditions, and some large surface and profile conditions. The results are clear that the gauge has the capability of reading these variations. Is the gauge just reporting what it is “seeing?” Even without placing a part in the gauge, it is obvious that there is going to be significant variation within each part. The gauge is doing its job.
In addition, that is exactly what appears to be the issue. The gauge is measuring and reporting the variation in the part. There is one way to try to eliminate the part variation during the testing process of the gauge. That is to place an orientation mark on each part. With this, the gauge will always measure at the same point on the part and report the same relative value every time. However, with some parts even this has its limits. If the roundness is so significant or the profile so bad, even small positioning errors will show the variation and appear to show bad gauge performance. Nevertheless, the parts are still causing the issue.
So A few more phone calls, an exchange of measurements and some tips about part alignment and the gauge owner is reluctantly convinced. They were not thrilled about learning the condition of their parts, but that is another story. I am sure there is some machine tool builder or insert company getting a phone call — and losing some sleep over these same parts. However, the weight been lifted.
Custom gauges designed to position the part repeatable in the gauging station to eliminate operator influence.
George Schuetz, Mahr Federal Inc.