Tips on Machining Vol. 20
Tapping trouble and solutions
Part 3 (Thread accuracy faults)
- The importance of checking with a thread gauge -

In this continuation of our series on solutions for tapping trouble, we cover thread limit plug gauge (go and no-go) used to check thread accuracy.

Thread limit plug gauges are measuring instruments that are inserted into a thread to inspect it. For the Go side, the thread passes inspection if the gauge can be screwed in by hand without difficulty over the total length of the thread for the specified fit length. For the No-Go side, the thread passes inspection if the gauge cannot be screwed in by hand without difficulty more than two turns of the gauge. When performing this inspection, a thread that does not meet standards will cause the go gauge to stop (when it should pass) or the no-go gauge to pass (when it should stop), indicating a thread accuracy fault.
Let us now consider what can cause thread accuracy faults, and how to resolve them.