"MadHatter" wrote in message
...
On Jan 13, 1:23 pm, "anorton"
wrote:
"Randy333" wrote in message

...

I need to do some thread milling, never did it before!! I need to cut
2- 1-1/4"-11.5 NPS exteranl threads, a 1-3/4"-16 external and a
1-7/8"-18 internal thread.

I found a thread mill for 11.5 NPS but in the catalog it's listed as
internal only, why? Any place I can get this at under $300 bucks that
MSC wants?

Remove 333 to reply.
Randy

I have never done anything like this either, but I could see that a tool
meant to cut right-hand internal threads would work better for left-hand
external threads than right-hand external (think about the slope direction
of an external thread versus and internal as you look at it from the
cutter's point of view.)

You would use the same tool to mill either left or right hand
threads. The only thing that changes is the direction of the helix.
------------------------------------------------------------------

I see you are right. As an amateur machinist I was under the mistaken
impression that thread mills with multiple cutting rows had a helical pich
to them like taps.

Looking into the reason for different internal and external cutters I see
that the UTS spec has different profiles for internal and external threads.
I guess you could cut both with the same cutter if you did not mind extra
open space at the root of the external thread.
http://en.wikipedia.org/wiki/File:IS...Dimensions.svg