by cvoinescu » Thu Dec 20, 2012 9:48 pm
Sorry I wasn't very clear. A threaded rod with a rigid coupler will be constrained in the XY plane at two points: the nut on the Z carriage, and the motor mount. So, those points can't move along X or along Y. If the rod is bent, like this (, the bottom will be at an angle from vertical. (If it were vertical, the bend in the rod would move the lead nut sideways; the lead nut can't move sideways, so the bottom of the rod must twist out of vertical, just like the bottom of that parenthesis.) The rigid coupler can't bend, so this twists the motor too. It's not a linear sideways push (the Z lead nut isn't nearly stiff enough to constrain the rod to vertical at that point), it's a twist around a horizontal axis perpendicular to the radius of the bend. With a motor that tries to twist to a side, the relatively flexible mount twists too, and that results in a raising and lowering of the motor.
I have this habit of being long-winded and less than clear, so let me try a different approach.
Imagine I removed the Z carriage entirely and left the threaded rods pointing up, attached only to the motors. Push the tip of one rod horizontally toward the center of the machine. What happens? The rod, rigid coupler and motor move as a unit, and flex at the point of least resistance, which is the motor mount. That is attached to the rest of the machine to one side, so the motor lifts a little. Push the rod the other way, the motor lowers a little. Use a bent rod and hold its tip fixed, and it's the same as pushing it sideways in one direction or another as it rotates.
I think the best solution, in addition to a straight(er) rod, would be to replace the rigid coupler with a flexible coupler that allows the rod to bend out of vertical, but not move up and down (e.g. a regular flexible coupler with a bearing ball in it and the tips of the shaft and rod resting against that ball; or file the tip of the rod slightly round and have it rest directly against the shaft of the motor inside the coupler). That, and much stiffer Z motor mounts. A Z nut that allows some off-axis twist would also be good.