O.D. thread milling

[Rick46]

I have a custom machining job coming up that requires threads on the O.D. of the part. the threads in question are NPTF threads for a fuel filter housing and the O.D. is just a little over 6 inch diameter. but require 18 pitch NPTF threads on it for a cap to screw on to. I am of course going to run a test piece on some stock aluminum to make sure I have it right before machining the threads on the actual pieces but I am new to the O.D. threads. I was wondering if there are any charts out there for machining O.D. threads like there is for I.D. threads. ie, tap drill sizes for regular screw and pipe plug threads. Also these are NPTF threads are there any things I need to be aware of when machining these like taper that is used for NPT thread milling or are these going to be straight threads. After I figure out the art of machining O.D. threads in a vertical machine I imagine the company I work for will start quoting more custom machine jobs that have O.D. threads so any charts or informational sites available for tips on O.D. threads is greatly appreciated. I'm sure I'm just thinking its going to be harder than it actually is since O.D. thread machining is new to me but just want any and all information that I can get so I can be prepared for present and future O.D. threads.. Thanks in advance..

[Guyinthedesert]

Milling o.d. threads is really no different than milling i.d. threads. Mc's threadmilling cycle lets you select i.d. or o.d.

 

You're going to need some kind of a gage. From your post, I don't get the impression that your doing aerospace work, so I would guess you could use a mating cap as your gage.

 

As far as tables, get your boss to buy you a copy of machinery's handbook.

[Rick46]

thanks for the reply @guyinthedesert.. . we have a few 25th edition handbooks floating around the shop so I can always borrow one. I kind of figured I was making a bigger deal in my head than what it was to machine O.D. threads but like you mentioned we will have a mating cap to use as a gauge to size the threads out with so I'm going to do a practice run on some stock aluminum first and get it fine tuned then take off on the real pieces then..

[Mike@Dive Lab Inc.]

if the thread table in lathe has the thread, you can click the draw thread button and get your geometry from there,

[Kasch]

I need to thread mill a 4"-8 NPT external thread.  What diameter should I be using for the toolpath?  Should it be the major diameter of 4.434" or 4.5" - the OD of the pipe?  Done plenty of thread milling but not an external NPT yet.  I have the taper angle set at 1.7833°, just not sure of the correct diameter to toolpath to get me real close without putting a huge comp value in the control.