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

 

[Jen Kirley]

Re: Functional Gages give different results

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]

Re: Functional Gages give different results

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]

Re: Functional Gages give different results

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]

Re: Functional Gages give different results

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]

Re: Functional Gages give different results

...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]

Re: Functional Gages give different results

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]

… 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.

 

[JRKH]

Re: Functional Gages give different results

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.

This has been my experience too.
Assuming that the application is commercial, how much real difference in measuremnet is there between the two threads? Assuming, as pointed out by the quote in Wayne's post, both thread gages are in tolerance, we are talking about a very subtle difference in thread size where one gage accepts and another rejects.

One would need to look at this aspect and assess the "Risk" involved in accepting defective threads vs rejecting acceptable threads.

Peace
James

 

[Wayne]

… Our suppliers are using (multiple) calibrated gages and the parts pass and then we receive them and the parts fail with our calibrated gages.

I have found that plug and thread gages disagree with each other from wear and even minute handling damage.

Jennifer is totally correct. Be careful when declaring a screw thread gage calibrated.

First; it is hard to believe that a piece of tool steel hardened to Rc60/62 could become deformed, but it happens. Any deformation to the screw thread gage will cause a functional reading error. An experienced calibration technician will do a visual check of the tool.

Second; the Pitch Diameter of the screw thread gage may be worn. The Pitch Diameter is the most looked at attribute of the screw thread gage, and every calibration technician knows that this is what is checked to do a screw thread gage calibration. On a calibrated screw thread gage, unless worn by use, the Pitch Diameter is most likely accurate.

Third, the GO Screw Thread Gage Crest Diameter may be worn. This feature is more important than the Pitch Diameter to be checked because it is the first feature of the GO screw thread gage to wear in most cases; regardless; some calibration technicians skip this critical measurement during calibration. The GO Screw Thread Gage Crest Diameter wears first because the part of the cutting tool that wears first is that tiny tip which cuts the root of the thread. When the screw thread root is not cut deep enough the GO screw thread gage crest rubs on the product thread root. Because of the small contact area the force is easy to overcome by the gage operator, which means he does not have to shut down and replace his cutting tool. The gage operator is lapping the GO Gage Crest Diameter with every pass of the gage into the part; once in and once out. The nice thing for the informed person is that the GO Gage Crest Diameter is easily checked by micrometer for the plug gage, and by pin for the ring gage.

Fourth, gage calibration is not a time thing, it is a usage thing. Once a year is not a good determining factor for the gage calibration cycle. Just because a gage has not hit the ‘recalibration date’ does not mean that it is calibrated.