concave 300mm Cartesian printer bed?

Here is one for you.
I have a Cartesian bed 300mm printer with a 5mm thick glass plate on top of it that appears to be concave in shape with respect to the nozzle. Yup, all four corners are "higher" than the middle.
This is a traditional Cartesian, X and Y are on a gantry and the Z is moved up and down with ball screws.
So, the only thing that I can think of is that the gantry holding the 'X' carriage has its 8mm shafts bent UP in the center.
Or
Something is whack in the linear bearings. I am not using EEPROM settings and have no mesh offsets (that I know of) in the firmware, which is Marlin 1.1.8.

Anyone have any other ideas?
I am open before I start open "printer" surgery on this thing.

Thanks,
DLC

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Kits: Folgertech Kossel 2020 upgraded E3Dv6, Anet A8 upgraded E3Dv6, Tevo Tarantula enhanced parts and dual-head, TronXY X5SA Pro(E3DHemera).
Scratch: Large bed Cartesian, exchangeable heads, Linear slide Delta, Maker-Beam XL Micro Delta, 220x220CoreXY.

Ok, so to verify (correct me if I am wrong), when you say that your 300mm bed is warped respective to the nozzle, you either mean that the bed itself or the gantry could be warped. Several questions to help diagnose the issue:

• By how much is it warped (center compared to edges)?
• Is it warped in both the x and y direction or just one, because if warped in both directions, I find it pretty unlikely that it would be the gantry that would be messing with the height respective to the surface.
• 5mm is also a pretty thick glass plate. If it were me, I would check there also.

To help counter this, you could, as you already mentioned, create a map of the bed so the firmware knows to raise/lower the platform depending on location, or you could invest in an auto leveling probe to speed up this process.

(If you get the time, could you post some images of the situation so we could better see exactly what is going on here?)

Got a straight edge you can put across the glass to verify it doesn't have desires to become a bowl? (seems very unlikely with 5mm glass, but is easiest to check)

What kind of glass is it? Try turning it over and see if it reverses...

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Bingo!

The glass plate is trying to look like a bowl. A .2mm deep bowl. Why didn't I think of that?

I built this printer from a box of parts and a step file that gave me all the parts that I had to print. Included in the box were four THICK glass plates.
It appears that these plates are of no particular pedigree and differ quite a bit from each other. The person I got the parts from had never built any 3D printer, so there you go, I made bad assumption that the box had correct pieces in it.

I see that I can get 3mm thick 300x300mm borosilicate plates. They are a LOT more expensive than the common 200x200 plates are though!
I think that I'll get one.

Thanks all,
DLC

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Kits: Folgertech Kossel 2020 upgraded E3Dv6, Anet A8 upgraded E3Dv6, Tevo Tarantula enhanced parts and dual-head, TronXY X5SA Pro(E3DHemera).
Scratch: Large bed Cartesian, exchangeable heads, Linear slide Delta, Maker-Beam XL Micro Delta, 220x220CoreXY.