I have been having problems with what I believe to be a slightly warped print bed on my Simple Metal, so I decided to try adding a glass plate, with copper foil patches placed on top of the glass to trigger the Z-axis probe.
The copper foil patches seem to trigger the Z-axis probe properly, so I thought I was home free. However, when I tried some test prints I found I had to significantly change the Z-axis offset (M212 commandI) to get decent first-layer adhesion, depending on where the print was located on the bed. If the print location was near the X/Y home position, then a Z-axis offset of -0.7 seemed to work great, but printing that same item in the far right corner required a Z offset of -2.0 - MUCH bigger than I had ever had to use with the original print bed.
Also, If I left the Z-axis offset at -2.0 (for a print in the far right corner) and then tried to print the same item near the X/Y home point, the extrusion head started cutting into the painters tape - badly!
Based on the above results, it seems the glass plate setup is tilted DOWNWARD for increasing X & Y, but I'm not sure why the 'auto-levelling' (actually more like 'software tilt correction') isn't compensating properly for this.
Moreover, when I moved the bed/print-head around manually, I got exactly the opposite results. When I moved the bed to the left (i.e. increasing X, I think) and/or the Y-arm toward the front, the print head started to score the painters tape. This implies that the print bed is sloped UPWARD for increasing X & Y.
I haven't been tracking these numbers, so I don't know if the difference between 0.88 at (10,143) vs 1.98 at (10,10) and (143,10) is significant or not
So basically, I'm completely baffled by what's going on here. I wrote all this up in a post with photos on my "Paynters Palace" blog (apparently as a new poster I can't post the URL, so please search for 'glass-print-bed-for-printrbot-simple-metal'). If anyone has a clue as to what is happening, I'd love to hear it. In the meantime, I guess I'll go back to printing on my original print bed, warps and all
Added Note: I went back and looked at the probe results from the last print attempt (the one that ran the extruder tip into the painter's tape).
14:38:46.107 : Bed x: 10 y: 143.00 z: 0.88
14:38:49.927 : Bed x: 10 y: 10 z: 1.91
14:38:53.989 : Bed x: 143.00 y: 10 z: 1.98