Quote Originally Posted by ixlr8 View Post
2mm is enough to show up if you put a known straight edge on the platform to inspect gaps, like a perfectly straight tool (even ruler side of a caliper) to see if the platform is warped.

Have I done this? No. What did I do? I attached a dial indicator on my extruders and ran them in the X and Y direction to test a theory. It turns out that the extruders are as much as 0.005" (0.127mm) closer to the build platform at X center. It's not 2mm by any means, but it's enough to make you want to slightly change your leveling process, like making sure the paper you are using as a clearance guide has some friction between the extruder and the build plate at the platform center, but is more free-sliding near the platform edges.

The next question is: What is warped? The build plate or the X guide rods? Is the simple weight of the extruders enough to cause a slight deflection of the X-rods when the extruder is near the center of the platform? (At an atomic level that answer will always be "yes", the question is...how much?) Are both plate and rod warped?

I haven't taken a straight edge to my build platform yet, but I doubt I would see much if any gaps with only 0.005" warpage (a sheet of copy paper is about 0.003"). Your situation may be different.
I know this is an old post but I am having a similar issue that ixlr8 describes. No matter how carefully I level under the level screws, the center is always high. I thought my build plate was warped so I installed a glass plate and it is still high in the center with new glass. So it seems that the extruder guide rods must be sagging in the center. I have tried slightly rotating the rods in case they were bent (theory being if the rods are bent down then rotating would show high in the center after rotating) but no change after rotating rods.
Has anyone come up with a fix for this?