> Date: Thu, 14 Oct 1999 05:24:52 EDT
> From: [email protected]
> To: [email protected]
> Cc: [email protected]
> Subject: Accuracy
>
> To group,
> I have a client who has been using pin gages to measure reamed holes. The
> tolerance of the hole is 20 mil of an inch. The pin gages have been verified
> to an accuracy of 20 mil of an inch. Because the reaming process does not
> guarantee this tolerance, they use the pins as go, no go gages to sort them
> according to size and group the parts. Do the pins need to be verified to a
> tighter tolerance than 20 mils. If so they will have to make a huge
> investment in more accurate pins. Also the holes are reamed into parts of a
> mold cavity which is later verified through measurement on the product
> produced. This is for an ISO 9001 system not guide 25. I am stumped on this
> one and would appreciate any advice.
> Mike Gospodinsky

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Date: Fri, 15 Oct 1999 15:23:45 -0700
From: Bruce Mayfield
To: Greg Gogates
Subject: Re: Accuracy

This is an interesting question that I was involved with not long ago. A manufacture who's products go into space satellites had a seminar problem. They used a caliper to check the diameter of a pin gauge. The pin gauge verified a hole that was drilled into am assembly. Into the hole fit an explosive bolt. The bolt was used to separate the stages of a launched booster rocket.

With all of this information, the caliper was not calibrated nor were the pin gauges. An ISO finding was not issued because the hole was tapped at the next stage to fit the bolt. The hole was tested with 15 bolts prior to acceptance. The bolt it self became the means in testing the specification. So there was no purpose in calibrating the caliper or the pin because they were not used to inspect the drilled hole.

I asked why they checked it at all, they stated that the hole had to be slightly under size to properly accept the tap. The check was to determine that the hole was about the right depth and not oversize more than the pin. The pin used was selected to be undersize of the tap requirement. The caliper was to make sure the ware on the pin did not affect the quick check. A log was kept of the caliper and pin verification. All of this was documented.