[positiverake]

How do you program a npt thread on a lathe? I think I got it right,Im using full form inserts and program a angle of 1.7834 deg, I was turning the angle before threading but found it a waste of time seeing the full form controls the whole thread,what Im wondering about is the length of thread and length of vanishing thread, do you need the whole L4 length? the npt ring gage tells me no,the reason I ask is some prints dont give me the room for a full L4 length,like a 1/4-18 npt, the machinist hand book says I need .781 or somthing like that,and the print wants .750 to a shoulder? I see that a lot, the parts work fine, but Im just wondering about the length thing.

[HuFlungDung]

I think the vanishing thread length was specified to accomodate the typical results of threading with dies. On a lathe, you can usually withdraw the tool in quite a bit shorter distance. I don't have my Machinist's handbook here with me, but you really need to allow enough for the handtight engagement plus enough for a couple of turns with a wrench. After that comes the tool retract zone.

[positiverake]

sounds good to me, thanks for the info.