The curve for a Thread feature must represent a single groove of the thread. The images below are examples for OD thread curves.
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This curve is replicated along the curve. The curve length must be greater than or equal to the thread pitch. If the thread length is less than the pitch, a linear segment is inserted between the grooves, for example.
When creating a Thread feature from a curve, the distinction between an OD and ID feature is determined from the curve. For OD features, you should model an upward facing groove. ID curves should be downward facing grooves. As an example, if you are creating a rope thread, the basic shape of the groove is a sine wave.
If you want to create this thread on the OD you should use the red curve shown below.
To create an ID thread, you should use the downward facing curve shown in red in the following image.
The taper angle of the Thread is determined by the X angle between the first and last points of the curve. For straight tapers, the two points should have the same X coordinate. For tapered threads, the angle between the endpoints will be used as the thread taper. In the image below, the thread has a 5 degree taper.
