Sorry about the documentation for this feature. It needs work! Please feel free to fix any errors or missing parts that you notice in the Marlin documentation. We can always use more contributors who care.
To clarify this feature, let's look at a simple example of using the "Set Home Offsets" (M428) command:
Let's say your current home_offset is [0, 0, 0]. Assuming a cartesian machine with only min endstops: If you do a G28 command to go to the home position the current_position will be [0, 0, 0].
But, you don't like the Z position. It's too low and scraping the bed. You could adjust the endstop and do G28 again. But this might not be convenient. So, you can move Z up instead, then use "Set Home Offsets" (M428). The current Z position will become Z=0.
Let's say you moved Z up by 1.2mm. The home_offset in that case will be [0, 0, -1.2].
Now if you home with G28 your current_position will be [0, 0, -1.2]. If you then start a print, Z will first move up by 1.2mm to reach Z=0, compensating for that too-low Z endstop.
Let's say that 1.2mm wasn't enough. You've homed to [0, 0, -1.2] and you've moved Z up until Z=0, but it's still too low! So, you move it up by 0.2mm more, where Z equals 0.2mm. Now when you use "Set Home Offsets" (M428) that 0.2mm is subtracted from home_offset[Z_AXIS], giving you [0, 0, -1.4].
To clarify this feature, let's look at a simple example of using the "Set Home Offsets" (M428) command:
Let's say your current home_offset is [0, 0, 0]. Assuming a cartesian machine with only min endstops: If you do a G28 command to go to the home position the current_position will be [0, 0, 0].
But, you don't like the Z position. It's too low and scraping the bed. You could adjust the endstop and do G28 again. But this might not be convenient. So, you can move Z up instead, then use "Set Home Offsets" (M428). The current Z position will become Z=0.
Let's say you moved Z up by 1.2mm. The home_offset in that case will be [0, 0, -1.2].
Now if you home with G28 your current_position will be [0, 0, -1.2]. If you then start a print, Z will first move up by 1.2mm to reach Z=0, compensating for that too-low Z endstop.
Let's say that 1.2mm wasn't enough. You've homed to [0, 0, -1.2] and you've moved Z up until Z=0, but it's still too low! So, you move it up by 0.2mm more, where Z equals 0.2mm. Now when you use "Set Home Offsets" (M428) that 0.2mm is subtracted from home_offset[Z_AXIS], giving you [0, 0, -1.4].