Cutting API Tapered Threads with CNC Lathe

Mr. Cowan,

It sounds like you are machining a part with a female thread, a.k.a "box connection". Also I'm assuming your statement about over gaging means that the plug gage has screwed in too far instead of not far enough.

I can't send you the charts out of API Std 5B due to copyright laws, but perhaps these general guidelines will help:

API 8RD Tubing & Casing, or 8V Line Pipe: Face-off .125" for each "turn of the gage that is too deep".

API 10RD Tubing: Face off .100" for each "turn of the gage that is too deep".

API 11-1/2 V LP: Face off 0.087" for each "turn of the gage that is too deep".

Note that if the box thread has an API Counterbore you will have to recut the start of the 25 or 35 degree "stabbing bevel" to conform to API Std 5B. Otherwise a 3rd Party Inspector may reject the finished part.

If I've misinterpreted your question, feel free to send specifics as to nominal diameter, threads/inch, thread style, and whether it is a box or pin thread.

Best Regards

Excellent information thank you very much

So does this formula also work when the hard gauge has under gauged? Eg, if i am cutting a 7 5/8reg with boreback,gauge stand off should be .250 but actual size is .375 doi increase the "X" wear offset .125? or half this amount.

Sorry for sounding like such a novice but its because i am, again many thanks

P.S. I imagine the same methodology is in practice for the pin connection aswel?