FIXED - First Layer starts *below* Z-endstop height

I've just recently unpacked my printer after moving across the country, and I'm in the process of re-calibrating it. It's a Prusa Mendel built from a MakerGear kit from two years ago.

For the life of me, I cannot figure this one out: The prints now start *below* the level of the Z endstop, crashing the head, and blocking the filament feed until the feeder gear strips a gouge in the filament.

Any suggestions where I might look for a wrong setting would be most helpful!

I'm running Marlin v1.0.5 firmare on the printer, and Repetier-Host Mac 0.56 and Slic3r 0.9.9 to control the print.

One unique thing I should mention is that I'm running with home to a MAX endstop on the X-axis (instead of a MIN), so I do have:

#define min_software_endstops false

…(or else I could never move away from my MAX endstop). However, I know of no reason that once zero'd on the MIN Z-endstop that any command should be given to make it lower than that level.

Any ideas?

Thanks!
Mark

Edited 2 time(s). Last edit at 02/24/2014 12:59AM by bobtiki.

Think I figured it out.

After attempting to fix it with a Z-offset, and trying this several times, I noticed it seemed to vary in height, every time the print head would home and then raise up to 5mm at the start of the print. In addition, the 5mm Z-move didn't seem to be smooth, but much faster than a usual Z-move, and made a terrible almost-grinding noise.

I noticed that in the Slic3r configuration, after it homed, the command was:

G1 Z5 F5000

I wondered if the high feed rate (F5000) was what was causing the issue, and removed that from the Start G-code, so it would raise at the standard Z rate.

This seems to have done it — it actually raises exactly 5mm every time, instead of some random amount, and the height is now consistent and predictable, so I guess the motors or the control board (RAMPS 1.4) weren't able to handle that high speed. Hopefully this helps someone else, if they run into this!

MB

Edited 1 time(s). Last edit at 02/24/2014 12:59AM by bobtiki.

HA!

Really fixed this time, I swear.

F5000 is perfectly reasonable. All I had to do is turn up the pot a tiny bit on my Z stepper driver. Perhaps it got jostled during the move.