One side has a course pitch right hand thread while the other has a fine pitch left and thread
Actually, don't they need to be the same hand? The one pitch just needs to be coarser than the other so that it gains a slight amount over the trailing fine pitch. The differential being the difference in lead. I used M16x1.5 and M16x1.25 because taps were readily available for an effective lead of 0.25mm*rev. Some experimentation was required to figure out how many thread 'head start" the fine pitch was given before engaging the coarse so that when it nipped up tight the ends of the differential bolt (or stud, actually) ended up flush with the adjacent surface as desired.
If the threads were opposite hands, wouldn't the pitches be additive (in the previous example, 2.75mm*rev)?
Re- The PH Horn carbide insert interface. Another disadvantage is the lack of surface area carrying load. just six flank surfaces. The rest of the face does not contribute and indeed cannot touch the opposing surface least it prevent the joint from clamping up tight. Not unless you were able to maintain very precise tolerances like the combined short taper/face joint of a D1/A1/A2 lathe spindle nose. As said, vee-serrations are probably
easier.
-Doug