Why is the center of my glass print bed lower than the corners?
I have a new Creality Ender 3.
I suspect that I have not adjusted the eccentric nuts correctly, on the X-axis head carriage mounts.
Even after a glass bed upgrade, using the Level Corners routine of the TH3D firmware, I can get the head to scrape a sheet of paper all 4 corners but that same sheet of paper does not then scrape the head in the center, unless I fold that paper in half.
I could understand this if the rail on which the hot end travels is very slightly higher at the side opposite the extruder. I have tried turning the eccentric nut on that side until the head does trap the paper, but when I then repeat the Level Corners routine, the gap at the center has come right back.
I already adjusted the eccentric nut on the extruder end because the wheels on the hot end carriage were showing an accumulation of brown dust in a ring around each wheel, which I heard was likely a sign that the carriage was "too tight."
So basically, I am messing around with the eccentric nuts at both ends of the X-axis rail, chasing two problems at the same time, but I don't really know what I am doing.
Is the glass flat? Put a metal ruler on its side to check. There are known slates of glass that are not flat. A BLTouch sensor could help out in such cases.
Could be a bent x axis rail? Are the gaps at center x independent of y?
Please  your question to tell if the glass is flat or not. Else you need to look at the X carriage linear support rail!
This seems to be a common problem with ender-3 and cr-10 printers from Creality. Mine is the same way but not enough to keep prints from adhering.
Typically the aluminum bed is not perfectly flat. If it’s not the glass may be able to flex enough that it can make a difference. There are a few ways to try to fix it.
Shim the low spots in the aluminum bed with aluminum foil or another thin material. Then the glass will sit on a flatter more well supported surface.
Bend the aluminum bed until it is flat.
- Add a 5th leveling point under the middle of the bed. You could either make it adjustable which would be tricky to get to or create a support piece accurately or get one close and shim it.
The shimming process is probably the easiest and the one I may end up doing. But whether you shim it or do the other trick you need to measure for points that are out of flat.
If you get a decent straightedge and some feeler gauges and/or or shine a bright light from the other side and look for spots of light between the straightedge and the aluminum bed you can see where you need to shim or otherwise adjust it. You’ll need to move it at various angles through the center and other spots on the bed. That way you can see whether it is just the middle or if the edges are an issue too.
Also, once you check the aluminum bed, check the glass as it may not be flat either.
There was a YouTube video on the cr-10 I think that showed part of the shimming process and checking for level. I’ll try to see if I can find it to add a link here.
I still didn't find the video I was looking for, but here is a useful related one that talks about tramming the bed (what 3d printing people call leveling)
And here's a reddit thread about the issue. Someone incorrectly says the glass plate can't flex that much but it certainly can. We're talking small tolerances. Even granite slabs flex small amounts.