I've recently been thinking about this same questions and it does surprise me that I cannot at least find some quantifiable values that some companies use...
We have a new threaded part that we will be shipping to Mexico and at this point do not know how they will gage them or how picky they will be and we want to be proactive.
What I found out, is that our quality engineering and inspection people all agree that with this 3/4 -14 plastic NPT threaded insert, that when gaging, using about 2-3 inch pounds seems reasonable and we can agree on that amount of force. Even though this may not be an industry agreed upon value, we will at least have an internal benchmark and know how we are testing them and how we are establishing our accept/reject criteria.
But, as I have been reading some of the comments here, it occurred to me that this could also be evaluated by not using a specified amount of force, but by doing a G R&R study. If we had a set of parts that us and our customer could agree on (some good parts, some parts too small/tight, some too big/loose) and then we do a GR&R study, then we would know that our system does have the ability to descriminate between a good part and a bad part, which is the goal. Then if we would do a correlation between us and our customer, there would not need to be an actual amount of force specified.
For us geeks, that do like data, we could then take the readings for each part and measure them with a torque gage or meter.
Meaning that if the group agrees that part #1 checks at 3.75 turns, then record the actual force it takes to turn the gage in to 3.75... this way it is actually scientifically documented.
Another reason I like this suggestion is that the actual/reasonable force you want on a plastic to plastic fitting would very much depend on the size of the thread and be much different if you were talking about a cast pipe to a brass thread-curshing NPTF thread. You may end up with less than 1 inch pound on the plastic thread and look for 10 inch pounds on the cast pipe to brass thread-crushing NPTF thread...
Just my humble opinion... but if anyone has any actual scientific studies or publications related to this, I would love to see a copy. You can e-mail me at "[email protected]".
thanks and let me know what you think.
thenson
We have a new threaded part that we will be shipping to Mexico and at this point do not know how they will gage them or how picky they will be and we want to be proactive.
What I found out, is that our quality engineering and inspection people all agree that with this 3/4 -14 plastic NPT threaded insert, that when gaging, using about 2-3 inch pounds seems reasonable and we can agree on that amount of force. Even though this may not be an industry agreed upon value, we will at least have an internal benchmark and know how we are testing them and how we are establishing our accept/reject criteria.
But, as I have been reading some of the comments here, it occurred to me that this could also be evaluated by not using a specified amount of force, but by doing a G R&R study. If we had a set of parts that us and our customer could agree on (some good parts, some parts too small/tight, some too big/loose) and then we do a GR&R study, then we would know that our system does have the ability to descriminate between a good part and a bad part, which is the goal. Then if we would do a correlation between us and our customer, there would not need to be an actual amount of force specified.
For us geeks, that do like data, we could then take the readings for each part and measure them with a torque gage or meter.
Meaning that if the group agrees that part #1 checks at 3.75 turns, then record the actual force it takes to turn the gage in to 3.75... this way it is actually scientifically documented.
Another reason I like this suggestion is that the actual/reasonable force you want on a plastic to plastic fitting would very much depend on the size of the thread and be much different if you were talking about a cast pipe to a brass thread-curshing NPTF thread. You may end up with less than 1 inch pound on the plastic thread and look for 10 inch pounds on the cast pipe to brass thread-crushing NPTF thread...
Just my humble opinion... but if anyone has any actual scientific studies or publications related to this, I would love to see a copy. You can e-mail me at "[email protected]".
thanks and let me know what you think.
thenson
).

(I assume a customer) for a copy of their gage certification and look for the convolution certification.
If they cannot prove their gage is in tolerance...
