How to mill NPT thread using a threadmill

Just FYI, I have been making some 1/4” npt brass parts for a high vacuum system…the female threads are very easy to make with a tap, I use a YG1 nptf tap and it’s very impressive. For the male fitting threads I use the Geometric Die Head with 1/4” npt chasers. I leave a stub about 0.7“ long and 0.54” diameter and the Geometric chasers cut a beautiful male thread. The tap does a similar job on the female.
 
Pipe dope really acts more like a lubricant on pipe threads to allow proper tightening. It does have some sealing component, but pipe threads are designed as a metal to metal seal. Have run thousands of black pipe connections. Mike
 
FOMOGO said:
Pipe dope really acts more like a lubricant on pipe threads to allow proper tightening. It does have some sealing component, but pipe threads are designed as a metal to metal seal. Have run thousands of black pipe connections. Mike
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I concur. Teflon tape is the same, more of a lubricant to prevent galling and get the thread to tighten up enough. Only a very small amount is used to block the leak path. See a rehash on the old hemp string/paste idea with locktite 55
 
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There are many sources of engineering data on tapered threads that indicate NPT threads do not inherently seal while NPTF threads do seal mechanically, the design is slightly different. NPT threads require sealant to avoid leakage while NPTF do not. I know I've had cases where making things tighter did not stop leaks while reassembly with sealant did.
 
You can also wrap a string around the threads then apply some sealant. The string compresses as the fitting is installed and helps seal.
 
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