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So bed scratching seems to be a real thing, and not a single post I've come across (I've looked at more than 30 by now) actually answers the question on exactly how to stop this.

Now, I have an Ender 3 and this is becoming enough of a headache that I want to just give it away. I thought that the old bed was just wrecked so spend £30 on a new glass bed .. Guess what .. Wrecked that too.

Please don't suggest I level the bed - it is levelled, not gravitationally, but mechanically. I have tightened everything up. I have tried altering the XYZ axis on the actual printer. I have tried adding some numbers to the Z axis in Cura, but it just snaps back down to zero. I have played with base layer thickness, with no difference. I have run nine different test prints, with every single one coming out perfectly. I have ABS at 250 °C and bed at 90 °C. Sticking to the bed is not a problem.

Why .. Why, why why, is it scratching the bed when I then go to print something? Nothing has changed!! I even zero'd the test prints in Cura the same way I did my parts prints.

Would seriously appreciate some help before I just toss this thing.

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    $\begingroup$ Hmm, can you take a picture of your test prints? Also, are your test prints just pure GCODE, or are they also sliced by Cura the same way you slice your normal prints? I'm not sure exactly what you mean by "tightened up", but I'm assuming you mean the heated bed screws. In that case, if your heated bed is tightened down all the way and the hotend is still scratching the bed, you can try physically moving the Z endstop up a little bit. Sorry if I asked something unnecessary, I may have misinterpreted some parts of your post. $\endgroup$ – Kiro Jul 9 '20 at 19:56
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    $\begingroup$ This is a complete shot in the dark from me, but another thing you can try is updating the firmware on your Ender 3 (if it is not already on the latest version), and then using mesh bed leveling, also make sure to save after leveling. $\endgroup$ – Kiro Jul 9 '20 at 19:58
  • $\begingroup$ An issue that I have with my unit (which only auto-bed leveling fixed) is that the aluminum build plate (which your glass/synthetic bed sits on) does not heat evenly and expands and contracts unevenly. Which in my case created consistent scratching as well. If you have the money try using the auto-bed leveling kit from Creality or TH3D: bit.ly/2ZY9Qr4. $\endgroup$ – Dr. Mantis Tobbogan Jul 9 '20 at 20:25
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    $\begingroup$ You almost surely have mechanical problems in at least one axis affecting positioning reproducibility. Maybe gantry not square with Z axis, or X or Z rollers with flat spots or insufficiently tightened eccentric nuts. $\endgroup$ – R.. GitHub STOP HELPING ICE Jul 9 '20 at 21:27
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Most likely your print surface is uneven. You can try rebuilding your firmware with mesh bed levelling that allows you to manually probe and store n*n grid bed mesh setting the height offsets at each point manually (however, on stock setup you might need to disable certain features for the compiled firmware to fit). Other option would be to use some kind of ABL as mentioned above. You can also try installing Z offset plugin on Cura and raise it up a bit.

The last thing to check would be comparing the test gcode to your slicer generated ones (via any text editor) looking for differences in G0/G1 Z movement in the beginning of the gcode. If there are ant differences, that would give you a hint where to progress further.

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