Not sure if this is what you mean, and what version of NX your using
Use Tube milling.
1st turn your unclosed sheet body into a solid.
Create a curve thru the centre.
Use Tube milling operation.
See NX 11 part
As paehv shows Tube milling in NX11.0.1 would be the easiest and best option. If you don't have tube milling because of previous versions of NX then I have attached an old way of achieveing this. It's not refined as far as step over etc, it's a sample that you can improve on to get the stepover, cut percentages from each side etc. to the way you need.
Hope it also helps.
Sorry, can't really help you out. I only got NX 11
Create an offset surface on the inner face (small offset -1mm/.04" will be enough)
create a variable contour operation.
Select part surface only the inner face
Select Drive > Surface area > Select the offset surface > Cut pattern Helical
Select Projection vector > Normal to drive > Backoff 50%
Select Tool Axis > Toward point and select/create an appropriate point
Use a signature in your profile so it's clear witch version you're using
That was really use full thanks for take the time. Could you use a T-shape cutter or is it better to use the spherical shape cutter for this type of operation?
I would expect some projection issues with sharp corner t-cutters I made one that has top and bottom radius sizes that match the flute length.please check the attached sample. The other issue I could see is based on the tool axis you could possibly be plunging the t-cutter into material with the non cutting portion of the t-cutter so careful roughing would be more critical.
Hope this helps.