Need Help!- Preparing hole for Large NPT thread

[ttieuli]

Hello Everyone,

I have to thread mill some 2"-11-1/2 and 3"-8 NPT holes in aluminum. I've never thread milled NPT holes before to say nothing of holes this big.
Should I prepare the holes with a tapered mill before thread milling them? If so, can I use a tapered mill with a 1-1/2 degree taper per side and let the thread mill put in the 1.79 degree per side taper or do I need a 1.79 degree per side taper mill?

Thanks in advance for any help you can give!

T

[ttieuli]

I just realized that the standard taper per side for NPT threads is 1.47 degrees.
For some reason the print for this job calls for 1.79 degrees. I don't know how that will affect the thread.

[RJ40]

Thrd milling is different than tapping. The only reason you ream to a taper hole for tapping is to take it easy on the tap. The thrd mill will mill its own taper as long as your programmed right. I've done lots that big. Even 4140ht.
Straight bore ,no problem especially aluminum.

[har78233]

You were right the first time. The taper on that thread is actually 1.783 deg which is 1 deg 47 min. They sort of got mixed together.

[ttieuli]

Oh, Yeah! That makes sense!