Accuracy & Explosive Bolts - Using pin gages to measure reamed holes

[Marc]

From: Greg Gogates
Sent: Thursday, October 14, 1999 6:21 AM
Subject: Accuracy

> Date: Thu, 14 Oct 1999 05:24:52 EDT
> From: Mikegospo
> Cc: M69USTANG
> 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

---------------------------------

Date: Fri, 15 Oct 1999 15:23:45 -0700
From: Bruce Mayfield
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.

Bruce Mayfield
RAB-IRC ISO auditor

 

[Marc]

Date: Fri, 15 Oct 1999 06:17:28 -0400
From: "Kenneth A. Grams"
To: Greg Gogates
Subject: Re: Accuracy

Mike,

The practice of measuring a .000020 tolerance hole with a gage pin regardless of the pins accuracy is not usually considered good practice. It is very difficult to put a pin that has a diameter of .0001"-.0002" smaller into a hole. Secondly, reaming does not generate a hole with that tolerance. It usually has to be jig ground and lapped. For example, your typical XX Class ring gages are made to that .000020" tolerance and they are defininately not reamed! The normal practice is using air gages or dial bore gages with the appropriate indicator. The problem you may encounter is that the hole size may not lend itself to this method. When dealing with holes smaller that .25, an air gage is about your only choice. The pins will only get you in the "BALLPARK". Forget the pins!

Ken Grams
CMM Services, Inc.

 

[Sam]

I agree wit Ken G., sounds like they need to do a MSA.
Ref: Bruce M.
"An ISO finding was not issued ......"
IMO a finding should have been issued; IM&TE used to varify quality of the product requires calibration. The tap drill diameter is critical for maintaining proper thread geometry.
Also, if the bolts are being used to test the hole I would suggest that they too should be calibrated.

 

[Wayne]

Bolts should not be used as gages.

Bruce Mayfield, RAB-IRC ISO auditor: ... 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.

Wow. Using bolts as gages. This is very poor practice, and in a space program project? Scary.
Manufactured bolt tolerances are not the same a nut tolerances. And using 15 bolts, this would
take lots of shop time. A GO/NOGO thread plug gage would have been much faster.

...if the bolts are being used to test the hole I would suggest that they too should
be calibrated.

Sounds like this is an on going project. A special thread plug gage is relatively inexpensive and
can be obtained in about a week. That is longer than it would take to get the bolts calibrated.

 

[Claes Gefvenberg]

Wow. Using bolts as gages. This is very poor practice,

Just as a bit of trivia, I have seen worse: When I worked with initial samples (Not space industry, mind you), one of my colleagues got a funny look on his face during a phone conversation... As soon as he had hung up, he fell over his desk laughing hysterically.

He had been calling a supplier about a thread out of spec. After a while it turned out that they indeed had checked every thread... with the thread tap?! Thus the gale of laughter

/Claes

 

[Wayne]

checked every thread... with the thread tap?!

What a surprise, no parts failed.

 

[Marc]

What a surprise, no parts failed.

Can you please your reply in more detail?

 

[Wayne]

Can you please your reply in more detail?

Marc, per your request...

Using a tap to check a threaded hole has several problems:

1. What if the tap being used is not the right size for the thread requirement? Taps come in choices of size in 0.0005" increments, and none are directly matched to a specific thread class-of-fit. There are suggested sizes for specific classes-of-fit, but the tap is not made to the class-of-fit, but to a percentage of the part thread that will work for the class-of-fit.

2. Using the tap that was used to make the threads in no way assures that the tap has not worn out during production, and of course it will fit in the hole.

3. Using a new tap instead of the tap that made the parts still has problem #1. Also, if the original tap was worn, this tap will cut the thread larger with minimal effort, but that will wear on this tap. The GO gage assures that the tap has not worn out. Used during production it eliminates the re-work issue.

4. The part of the tap that wears first is the major diameter. Some people check the tap with thread wires and find the pitch diameter still good, but never check the major diameter. Then they confidently use the worn tap to check parts. Oops.

5. How do you know if the hole has been cut too big? A hole that is too large will not cause an assembly problem, but it may cause a fastener failure in the field when under stress, you know, at the critical moment. Again back to problem #1. Alternately, and at this point I will admit that I am not a tap expert, but I have been told that some tap styles, in some materials, at some speed/feed ratios may actually cut larger than expected. Using either a tap or a bolt will not detect this issue. Only the NOGO gage validates that the cut hole has not been made too big.

I hope this data answers your question.

 

[bobdoering]

After a while it turned out that they indeed had checked every thread... with the thread tap?! Thus the gale of laughter

/Claes

This is about the equivalent of using a drill bit to "measure" a hole diameter. "If it was that easy, everyone would be doing it!"