I chose pathtrace compensation when defining a thread mill sequence for a 1" npt hole. First I needed to mill a taper ( the minor diameter ). I over cut by 1/2 the tool diameter. I seems the thread mill function always offsets the toolpath by 1/2 tool diameter.

Am I inputing bad data?

Try using controller compensation you do not have to machine a tapered hole if you use an NPT thread milling cutter.

My first attempt was not to make a tapered hole for the thread mill. I was using a Vardex TMNC 075. The code I was proving was provided by Vardex some time ago. The other mills in the shop occasionally do a 1"NPT. I think they run the code 2x's with changing offsets. Anyway the short of it is, the load on the tool proved excessive.

My compromise to get a tapered hole G3 spiraling Z- with the correct dimensions was to define the Diameter of the tool as .0001". This does give me the code I need. If I start increasing the tool diameter, over cutting happens.

I think Edgecam is doing something bad in the threadmill routine.

I have used edgecam a lot for threadmilling,
NPT too, but in most cases i use full-profile threadmills, then i don't have to bother about the taper,
I just drill the hole, and the threadmill does the taper itself,

but i allways go to full depth at once, and make several radial cuts, i do this by copying the treadmill cycle and change the major diameter, thread depth is allways set to 0,01mm to make sure the major diameter gets correct, and number of teeth for example 100, this will make the mill go to full depth at once and make only one revolution+lead in/out wich is set to "arc1" by default.
i use 3 cuts on tough materials, maybe even more on NPT.

I will try that method. Use the full profile thread mill and then create a multiple cut threadmill cycle.

invs879

What kind of material are you trying to machine? I use a full thread profiled insert from Vardex on Nodular iron castings. It's a 1.5-11.5 Npt(and NPS) I am able to do it in one pass and use the Vardex code generator for the code. I tried several times to use edge cam to get the code and gave up.