What type of cutting tool is used for thread milling on a gantry router/mill

There are a couple options. A single pitch tool is like a pointed keyway cutter. It needs to make as many rotations as the thread depth. These can do a range of pitches.

The other, more expensive, option is like a tap with several pitches on the same tool. These only take one revolution. However they are limited to one pitch.

I paid the big bucks for a carbide NPT thread mill. It works great for getting that just right fit.

Check out MSC or similar online supplier.

Note: There are listings for internal and external cutters and I can't figure out why. I've used the same one both ways. I have not done accurate inspection though.

www.alliedmachine.com they have really good ones and the best part is they right the program for you online just typ in what tool you have what material your cutting and thy do the rest then just cut and past it. you can also email info@alliedmachine.com