APQP, Gage R&R, MSA HOW? Maintenance?

#11
Update,
The customer auditor was in yesterday. Here's his take:
A study should be done on the torque wrenches but since it is not a Key Control Characteristic but rather an insurance policy that we tighten connections he will not require it.
For our three KCCs, output voltage, output frequency and charger voltage, we must process 10 units, take 3 measurements with the meter for each characteristic (remove and replace the probe from the digital multimeter), and repeat this with 3 different technicians. My prediction is that we will have 0 deviation on the 9 readings of each of 3 characteristics taken on each unit by 3 technicians and a very slight (Cpk 5+) variation between the 10 units. So, 270 measurements to tell me what I already know. Gotta love quality! :ko:

Control Plan is the 100% test with these three measurements X-bar R charted.
Preventive Maintenance was not an issue due to the measures mentioned earlier.
 
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#12
Atul Khandekar said:
Just trying to understand...Would that really happen with the 'unit' that you manufacture or only with the reference power supply?
Completed the MSA yesterday. Learned something new. Got through it with help Atul posted in this thread
Acceptable Gage R&R about total variation vs. tolerance.
My statements above about stability of product were misleading. Our product is very stable given a set of very stable inputs. However, in a production test condition our test loads cannot provide the stability of a high-tech electronic load. Result: the output tends to fluctuate slightly. To wit:
DC charger voltage under 47.3 Amps of load can vary by up to 0.02 VDC on a 14.20 VDC reading (14.19 - 14.21) while you are taking readings. Nota bene that this difference is indeed the part, it is not repeatable with a verrrrry expensive ultrastable power supply. Part to part range of 14.20 to 14.29 for an average reading.
Equipment variation = 0.045 (due to changes in the true value of the voltage during measurement)
Appraiser variation = 0 (digital readings)
Part Variation = 0.154
Total Variation 0.161
%R&R = 28.15
Tolerance = +/- 0.3 VDC
%R&R (tol) = 7.53
A couple of things to note:
Unlike the attributes of a metal block or other mechanical measurement, electronic outputs are in motion due to a variety of influences. Reading to reading differences on the same part are common.
+/- 0.010 VDC drift on a box producing 14.2 VDC @ 47.3 Amps (670 Watts) is remarkably stable for a product that can survive being mounted in a work truck for 10 years!
Consider the current capacity and the output voltage of 14.2 VDC. 0.161 works out to +/- 0.56% total variation from the desired target voltage!!!! Wow!
MSA on electronics is an enormous pain in the Southern Hemisphere.
 

bpritts

Involved - Posts
#13
I have only a theoretical understanding of all the electronics, but let me ask
a question -- does taking the reading change the condition of the device you
are measuring? I would speculate that at some level it must, but maybe
the change caused by the DMM is microscopic compared to the other variation.
Secondarily, you indicate that the product you are testing will fluctuate due
to normal variation, so a read/ re-read is not measuring the same value.

If the change caused by the measurement process or the passage of time
is significant, then you
might justify using your original concept of doing the R & R using a reference
voltage source (or maybe two or three different reference sources!).
See MSA Chapter IV Section A. If you use different reference sources, try
to spread them out over the range of the tolerance.

We had to use some of these techniques to do R & R on torques, which you
indicated you will need. (Ours were critical safety characteristics -- bolts
on brakes!)

If you end up doing the torques I would recommend that you follow the
approach in the book Evaluating the Measurment Process, SPC Press,
by D J Wheeler and R W Lyday. I think we bought it from ASQ's catalog.

This book did an excellent job of explaining the MSA process in much
simpler terms than the MSA manual. It is my opinion that that team who
drafted the MSA 3d edition drew heavily on the Wheeler/Lyday book but
overcomplicated things. It is cited in full in the MSA bibliography -- last item
on the list.

I have used their somewhat simpler approach and obtained customer and
3d party auditor agreement.

Best of luck,

Brad
 
#14
Thanks, Brad

Wow! Good feedback. The drift is mainly due to simple physics not the measurement device: placing a heavy inductive/resistive load on a source tends to change the characteristics of both the load and source over time. Voltage=Current*Resistance, Voltage*Current = Watts = Power = Lots of Heat. Both the load and the source properties change under heat. Expensive references have internal feedback to keep the output characteristics stable, inverters meant to run power tools on bucket trucks and the load cones used to test them do not! %R&R is acceptable against tolerance, to be acceptable against total variation, I would have to use laboratory references.

I'm going to try to stay away from the torque wrenches. Unlike brakes, our hardware tightness specs are not critical, but rather a way to ensure that the tiny little mother of two and the bearlike technician both have the same definition of "tighten hardware", without hardware coming loose or pressed in studs breaking off.
 
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