I looked and looked and saw that I was not alone in determining that the print head sags under its own weight when traveling to the center of the horizontal round bars for the X axis. And I found that there was an upgrade to larger 12 mm rods. But being that I am cheap and lazy, I devised another plan. Why not sag the bed too?
I created some very simple clamps that are adjustable (slightly) and that you can just stick on the bed to achieve what you need to mostly eliminate the center sag from the rails. OK FINE, not eliminate, but mask it pretty well.
- Print the parts, attach the inserts, washers, and screws but leave the screws loose.
- Clip the clamp over the edge of the build plate where I have mine and verify they will not hit the ends of the Y axis mounts. The clmaps will slip between the rod mount and the curved bracket.
- Level the bed so it’s JUST touching the clamps but not pressing against them. They should feel loose and fall off easily.
- Check the nozzle clearance at the X axis left and right limits at the max and min of the Y axis (i.e. the corners) and then at about 50% of the Y axis travel (aka the middle). These should all be the same.
- Now carefully check the nozzle clearance at the front (min Y) and back (max Y) of the build plate at 50% X axis, but make sure it doesn’t scrape. It might.
- Start tightening the compensator clamps slowly until you achieve approximately the same nozzle clearance at the min/max Y and 50% X positions. You may need to iterate the bed leveling and clamp tightness until the nozzle clearance is consistent everywhere.
- Once you nail it, do some test prints. I’d try some of the one or two layer discs in various places of the machine and check for consistency.
They slip around a bit so some fixture tape (thin double stick tape) might help hold them in place. If they move, they might crash at the Y rail brackets.
You will also be building parts that are slightly convex on the bottom in the X direction. Since my goal wasn’t to make large flat parts (except, I guess, the one printing above!), but rather to make several small parts that were consistent Z heights, it isn’t a big problem for me.
You could use a nut on your screw instead of an insert but it’s just harder to tighten. I suppose you could melt it in place or change the bottom hole to a hexagon.
For you non-barbarian types, a 30 mm long M2.5 x 0.45 screw and McMaster 94180A321 insert would work.