Z-Axis zero/home position is higher than the actual print starting position
I'm having an issue with my very first printer, a Monoprice Select Mini V2. After doing about 20 successful prints on it, the Z axis is suddenly acting very odd.
When I 'home' the Z axis and move the nozzle on the X and Y, I can do the paper test on all four corners of the bed without issue - the leveling is near-perfect. But when I start a print job, the nozzle is much lower - by at least 1-2 mm.
This causes the print head to grind against the print bed, which I unfortunately need to replace as it's pretty much destroyed. The nozzle is so much lower at the start of a print job than it is at the home position that it ground a permanent line on the bed. It's trying to go so low, there's enough pressure on the nozzle to not let any filament escape - leaving a bad gouge like I dragged a screwdriver across the print surface. Not good.
It's not the Z-axis limiter switch. I confirmed that is both working and secured tightly to the printer body. When homing the Z axis, I can hear the switch click and the printer stops at that position correctly. It's only when I start a print job that it ends up lower, almost as if it's ignoring the switch.
I also eliminated my slicer software from the equation by printing something I had printed successfully just a couple hours prior - without reslicing or modifying the GCODE file at all. I'm at the point now where I can't print anything that I could before, without having this problem. My heat and speed settings remained untouched.
How can I solve this? The issue popped up just after doing a successful print. What gives? I've heard of the opposite problem (Z-Axis 'too high'), especially after changing nozzles, but not 'too low', and I've never seen it where the print job actually starts lower than the true zero position. Help!
Check the Z-drive for any component looseness. Look at belts, gears, anything with screws. Wiggle things mercilessly checking for play.
At the start of a normal print run, the the print head typically rises up very high as the head warms up (depends on software) and then lowers to print. If your homing paper test happened with the head near the limit switch, then the difference between those two use cases is the Z-distance traveled to get to the same place. You have eliminated software and electrical considerations, which leaves mechanical considerations. A slipping gear or belt might contribute to this odd behavior you describe.
Sorry if this is a noob question - but how much tension should the belt have? Is there a best practice there or just eyeball it?
I pluck belts and listen for a musical note. Too high is too tight. No musical note is too floppy. You can also compare with other belts on your machine. Similarly, check frame for loose screws. This is very strange behavior you're seeing. Not normal and I've not seen it in years of printing.
This answer is ultimately a better one as it would apply to any scenario where the slicer software can be ruled out. In my case I thought I had eliminated that variable, but I hadn't actually, so I'll mark your answer as correct for those who might find this question and actually do have mechanical problems with the Z axis.