This coupler clamps directly to the lead screw, while also clamping to the motor via a rubber tube slipped over the shaft. This gives some tolerance to misalignment or bent screws without causing Z position errors like a springy helical coupler. It's just a tweak of Triffid Hunter's great iteration; in previous generations of the part, the clamping screws conflicted with the rubber tubing if the lead screw was smaller than the tubing O.D. This just locates the clamp screws relative to the larger of the tubing OD or the lead screw diameter, thus working with narrower screws (such as my 6.35mm ones) or thicker-walled tubing.
Update: there was a missing semicolon in the original SCAD. It's fixed now.
Mine worked sweetly just as printed from the STL. If you have .250" (6.35mm) lead screws, chances are your local hardware store has inch size tubing, like mine. I used 1/4" O.D. x 0.170" I.D. vinyl tube which measured almost exactly 7 mm when snugly stretched the motor shaft.
Triffid Hunter suggests printing with 90% infill. I used PLA.
This really improved my bottom layers (and therefore prints) by making my Z zeroing repeatable. My cheap, easily bent lead screws are now a lot more tolerable.