BLTouch Montage an MK8 extruder

thingiverse

Initially, I had mounted the BLTouch on the object cooler. However, I was experiencing problems with the first layer. It was clearly worse than manual adjustment without auto bed leveling. After checking each measurement point using debug mode, I noticed that even with perfectly aligned beds, the BLTouch measurements over the X-axis were off by at least 0.2mm. The cause was quickly identified. The carriage on the X-axis is guided by two rods with linear bearings. These two rods should be exactly vertically aligned. Measuring with a tilt gauge showed an offset of 1.1 degrees from vertical at X=20 and 1.5 degrees at X=170. Now, if the BLTouch is not equally far from vertical as the nozzle, then it will result in a drop of the BLTouch measurement point relative to the nozzle due to the change in angle over the X-axis. In my example, the distance Y_BLTouch_Nozzle was 41mm. This results in an error of 41(sin(1.5°) - sin(1.1°)) = 0.286mm!!!!!! Therefore, auto bed leveling is unusable. The solution: Mounting the BLTouch at the same Y-distance as the nozzle will prevent the X-carriage tipping from causing any measurement errors (0(sin(1.5°) - sin(1.1°)) = 0! In the Marlin firmware, this distance is called Y_PROBE_OFFSET_FROM_EXTRUDER