So, I have a second hand Lulzbot mini and I am still trying to get familiar with the printer. First, I do want to ask opinions on calibration/what to check since it’s second hand. Secondly, I am puzzled by a recent experiment I performed. I printed 12 20mm test cubes and recorded their actual dimensions with calipers in excel. My goal was to find an average dimensional error so I could determine my printer’s accuracy (eg: my printer can print to +/- 0.15mm). Instead, I found that my printer was pretty consistent but not accurate. My X axis was biased too large while my Z is biased too small. It might be easier to take a look at the screenshot I attached to see what’s going on. I’m curious if this is normal, just my printer, all lulzbot minis, or if there is something I can tweak physically or in software.
For the X and Y, I would start with an extruder calibration. Over extrusion will produce a part larger that the design size. Make sure you are actually extruding 100mm when you command a 100mm extrusion. Adjust your ESTEPS value to correct over or under extrusion. If the extruder is moving the correct amount of filament, you can tweak the flow rate for a given filament in the slicer. Here is an info page about adjusting steps per mm from terminal commands if you don’t have an LCD display on the Mini. http://marlinfw.org/docs/gcode/M092.html
Your Z measurements look pretty good. For my purposes, I wouldn’t worry about it. If you want to try to dial that better than what it is, I would suggest getting X and Y corrected and see what happens to Z. Z steps per mm can also be tweaked with M092 but you might find yourself chasing the mechanical limits of the machine.