Thanks for taking the time to comment; I'm glad you like it.
The threads are actually a bit looser than nominal. I modelled them with the standard 6H/6g tolerance, and that printed nice and snug for me, but I know all printers are not the same so I then I enlarged the thread in the jaw by moving all the faces out by 0.05mm. The result was a bit loose for me but I figure it will at least work for most printers. I don't think I can make it any larger without it being too loose on some printers.
Your comment about clockwise to tighten confuses me. The threads on the screw and jaw are deliberately left-handed so that clockwise rotation of the screw tightens (closes) the jaws. Is that not the result you have? And if not, did you mange to print it reversed or mirrored in some way?