Functional Gages give different results - Thread Plug Gage

[czielsdorff]

Hi All,

I have searched through the other threads, but I did not see anything that addressed my question directly.

We have an inspection where we check threads using a thread plug gage. These are standard thread sizes (8-32 UNC 3B, etc) and we have multiple thread gages. We have run into a situation on multiple times where one gage will pass the threads but another gage will fail the same threads.

I understand that these gages have tolerances on them and there certainly can be some overlap depending on where the part dimension actually lies, but the issue gets even more complicated when we add suppliers into the mix. Our suppliers are using (multiple) calibrated gages and the parts pass and then we receive them and the parts fail with our calibrated gages.

What have you guys / gals done in this situation? Any advice would be appreciated.

Thanks,
CZ

[Jennifer Kirley]

What is your tolerance? Is it less than that of the gages?

I have found that plug and thread gages disagree with each other from wear and even minute handling damage. This is the hardest thing to control with such gages, especially with suppliers.

[czielsdorff]

We use the standard thread tolerances (i.e. 2B, 3B, 2A, etc).

We have had several discussions on how much pressure to use and how many threads it can turn before failure etc.

Assume the same handling and technique as the same operator will try two different gages and get two different results.

Thanks,
CZ

[bobdoering]

In Reply to Parent Post by Jennifer Kirley

I have found that plug and thread gages disagree with each other from wear and even minute handling damage. This is the hardest thing to control with such gages, especially with suppliers.

Absolutely! Even the amount of torque the calibration tech uses versus the inspectors can create a variation in passing or failing a part. It is a very dicey measurement technique, especially for tight tolerances. For that, you may have to section the part and measure the threads - or use a tri-roll thread gage - to get variable data.

[Jim Wynne]

In Reply to Parent Post by czielsdorff

We use the standard thread tolerances (i.e. 2B, 3B, 2A, etc).

We have had several discussions on how much pressure to use and how many threads it can turn before failure etc.

Assume the same handling and technique as the same operator will try two different gages and get two different results.

Thanks,
CZ

I've seen these situations arise as completely theoretical exercises, and threads (male or female) that would work perfectly well in the application get rejected because of questionable gaging.

[czielsdorff]

In Reply to Parent Post by bobdoering

...or use a tri-roll thread gage - to get variable data.

Thanks for the info. I have not heard of a tri-roll thread (which is not too surprising). Can you give me more info on that? How does it provide variable data?

Thanks,
Casey

[Sturmkind]

The best explanation can be found in ASME B1.3M-1992 Screw Thread Gaging Systems for Dimensional Acceptability. The Johnson gage company www.johnsongage.com founded thread gaging and input into many of the North American and European standards. Their variable gaging is the best way to meaure threads largely because any attribute gage accounts for all thread variables and, in so doing, artificially fails product that is actually OK. If the attribute gage is even slightly worn, it will pass product that is actually Not OK.

The variable gaging (and 3 separate gages are needed) may be deemed expensive until a Level 2 containment decision is imposed by the customer. Suddenly, the variable gaging solution becomes quite a modest investment.

[Wayne]

In Reply to Parent Post by czielsdorff

… We have run into a situation on multiple times where one gage will pass the threads but another gage will fail the same threads. …

Chris;
This situation has many causes. The problem has been discussed by many over time and has been considered by the authors of the ANSI/ASME standards. Let us look at two screw thread gages, both new, directly from high quality gauge makers. There can be a situation where one new screw thread gage will pass the product and the other new screw thread gauge will fail it. This is the result of gage tolerance differences. The conclusion of the B1 committee:

ANSI/ASME B1.2: pp 2.2.1 Product threads accepted by a gage of one type may be verified by other types. It is possible, however, that the parts which are near a limit may be accepted by one type of gage and rejected by another. Also, it is possible for tow individual limit gages of the same type to be at opposite extremes of the gage tolerances permitted, and borderline product threads accepted by one gage could be rejected by another. For these reasons, a product screw thread is considered acceptable when it passes a test by nay of the permissible gages in ANSI B1.3 for the gaging system specified, provided the gages being used are within the tolerances specified in this Standard.

I do not know when this provision is invoked. Most people seem to think that their screw thread gage is the correct screw thread gauge and insist on it being the final determining tool.