Next time I get back to my printer, I will get to again go through the pleasure of bed leveling. My printer started life with a 6mm plywood print bed, quickly adding to that a heater PCB and I think 5mm glass clipped directly to the bed. I was having such issues with getting the bed level I decided that the plywood base must not have been stiff enough and was bending, so changed that to 12mm. I was still having issues so I mounted my heater PCB up off the bed and clip it and the glass together.
My problem remains that no matter which way I level the bed (this is a Sells Mendel with printed bed springs for adjustment), I always end up with high and low spots, as if the glass is dished. It’s always enough to screw up the first layer if I print on about 30% of the surface. Any thoughts on likely causes? Glass came warped or became warped with heating (standard replacement glass from a hardware store)? Smooth rods are bent or are flexing causing the X and Y carriages to slightly change height throughout their travel? Are there any slicing tricks to try to deal with this, like adding rafts in certain areas at -Z levels?