Product Inspection Systems - Basic Question

[BobLfoot 301]

I have what I think is a basic question, but for some reason the last three Process Engineers I discussed it with had to have it explained to them.

OK, here is the Setup.

Conveying System A delivers a part to the Inspection System for PASS/FAIL determination.

Leaving the Inspection System are two Conveying Systems.

Conveying System B carries only bad parts to the trash.

Conveying System G carries only good parts to shipping for packing and shipment to the customer.

Now in reality Conveying System A, B and G are one system and the signals PASS/FAIL from the inspection System determine how the part is handled.

Now my question, Must a part be inspected PASS to be GOOD, or is it ok assume the parts are GOOD and only reject those that FAIL?

[mikeexplorer 12]

In my experience, the inspection system had to give an explicit flag to tell the part was good. I used to work in a CD/DVD manufacturing facility and every disc was inspected. Sometimes the handling arm was out of alignment so when it placed a disc on the inspection spindle, it would use vacuum to hold it and rotate it to inspect it. If the disc bounced on the spindle it wouldn't get a vacuum confirmation and the inspection cycle did not run. There would be no outputs of results, so the handling system assumed it bad.

The inspection system had discrete outputs to flag a disc good, and several outputs for bad discs depending on what kind of defect it saw. This way a graph can be shown on screen tracking what kind of defects it is seeing so adjustments to the process can be made to increase the yield.

 

Mike

 

[Joe E. 259]

In general, parts should be assumed bad until positively verified to be good. I've gone 'round and 'round with manufacturing engineers (newer ones) to explain that it's not enough to have the inspection device just hold its "good" status high until it sees a bad part. What if, for example, it misses the trigger and doesn't inspect at all? The result flag either has to be momentary or there has to be a momentary "inspection done" flag to let the PLC know the result flag is valid. Otherwise you're a failed trigger signal away from allowing bad parts through. Without knowing it...

[gclshortt 51]

+1 Joe E.

Parts are unknown (default bad) until inspected to be good. 

Regards,
Garry