Print layers drifting towards rear

[cliffyk]

As we are clamping a sphere, the perimeter of the inverse conical nose of the OEM set screws will want to do one or two, or both, of two bad things; a) they will dig into the sphere and make irreparable divots that will become the new favourite seating/setting positon for the sphere and screws, or b) they will cause the sphere to rotate along the screw nose's path as the screw is tightened screwing up (no pun intended) up any positioning of the platen just accomplished.

I also put a dab of high pressure grease on the screw's new ball nose to minimise any tendency of he screw's nose to rotate the sphere.