Plastic Threads - Quality Requirements

[TimBow123]

We are currently creating a body and lid that have trapezoidal threads TR95(mm). The function of the lid is to screw down and with the aid of an o-ring create a waterproof enclosure. The fit between the lid and body is not to much of a concern other than we don't want the lid to skip off the body.

How do other companies control the threads from an incoming inspection point of view? We are considering thread gauges, but is this overkill? We are also not sure that the thread gauge tolerance would be too tight and over constrain the thread for something that doesn't necessarily need it.

Would anybody else have any experience in how to measure the pitch diameter of an internal thread, when the thread is in a blind hole (i.e inside an enclosed lid).

Thanks in Advanced.

 

[hendersdc]

Since it sounds like you don't care much about the dimensions, just the function, could you machine some mating parts or define some of the plastic mating parts as "golden" to screw into your incoming parts as go/no-go gauges?

Maybe you could specify torque values, one being something like "threads must turn under XX ft-lbs" if they don't then the threads may have too much interference or are out of round. Another one could be "threads must be able to withstand XX ft-lbs without striping or skipping", if it passes this one then you know it can take the force acting on it compressing the o-ring.

 

[TimBow123]

Thanks for the information. We have though about this but our quality department is pushing for a thread gauge. I guess I'm looking for what is industry standard?