The answer is with the manufacturing method. With FDM 3D printers the extruded plastic often expands beyond the dimension of your design. Further more your CAD model was tessalated for into a mesh for the purpose of 3D printing and the shape was sliced into 2d layers with limited resolution (often .2 or .1mm thick). What you can expect is that the threading in regular 3D printing (not advanced high cost machines or 3D printing metal) will be with limited precision and outside of accepted tolerance without further modification of your model - you might have to shrink or expand the geometry of the thread and run multiple test print to get to a desired/acceptable result.
Youtube has great resources regarding tolerance in 3D printing such as CNC Kitchen and many others.
Since 3D printing is not the same manufacturing method of most pipes you'd have to translate your modeling in such way that the manufacturing result with 3D printing will match or approximate the one of normal threading operation when turning metal or plastic.
usually threads for 3D printed parts are designed with 3D printing limitations in mind (bigger, spaced, round - less overhanging areas that will need support material)
I hope this answer is helpful and relevant 🙂 🙂 🙂
