On most of the Da Vinci printers the bed is warped to one extent or the other. The glass is very thin is warps easily. It is either high or low in the middle compared to the edges.
I would suggest going to your local glass shop and have them cut you one or two pieces of 1/4 or 3mm thick tempered glass to the same size as your current bed. Then mount it on top of the existing glass. The screws and clips have enough thread to still hold three more mm securely. Then just add 3mm to your Z offset.
The other issue is that some of the printers have a warped Y axis rod. Yo can test this by moving the head over the bed to where is is very close on the right side and then on the left side. Once it is over the bed, turn the the axis rod by hand and watch the nozzle to be clearance if you see in changing then you will know your rod is warped. Do the same on the left side. I found mine had nearly a 1 mm bend in it. Luckily one of the users here had scrapped one of his and had to bars that where still straight.
Printing since 2009 and still love it!
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