Calibrate z-home to correct bed clearance

Hello, I'm pretty new to this side of the printer, been a designer for forever. I recently got a Makerfarm Prusa i3v 12" printer. I have tried every variation I can think of but I cannot seem to calibrate the z-home position to the position when the print begins. I'll elaborate:

I have used the auto-home function on the LCD, as well as Pronterface and Repetier Host. I have leveled all four corners of the bed as well as the center of the bed. I want to be as accurate as possible so I use a feeler gauge to set the clearance.

Once I have the bed completely set (once again using the z-home function to calibrate the z-axis endstop to the clearance of the bed) I begin a print. At this point I would expect that once the print begins the nozzle would start at the correct clearance above the print bed. Not so...

Instead, the nozzle begins about 2.2mm (measured by lapping multiple feeler leaves together to add to the clearance produced) above the print bed. Also, when I hit z-home in Repetier Host sometimes the carriage will lower, hit the endstop, retract and then lower and stop. However, sometimes, it doesn't go through that whole process but rather moves up a bit and then down or a variation thereof...

Colin at Makerfarm is very adamant about using an older version of Slic3r, Pronterface, and only the firmware provided. Likewise, he said that rather than using the feeler gauge method, I should level the bed to the nozzle and then adjust the z-axis micro adjustment to set the level where the print head begins. This seems pretty un-scientific to me since that means that I have to either run a print without extrusion and pause it to get the clearance, or I just have to 'eyeball it' to get appropriate clearance. He also mentioned that this could very easily have the z-home position with the nozzle below the level of the glass in the corner, that means that if I z-home somewhere in the bed it could crack the bed glass...

So my primary question: Shouldn't the z-home function produce consistent results as to where the nozzle height begins and shouldn't that also correspond to the actual z-home location? Is my endstop sending varying information? Is there some z-offset somewhere within the firmware or g-code?

I really appreciate all of your patience with my newbie questions and technical help.

Thanks!

Send an M503 command to the printer, which will give you its current settings and should tell you the offset. You could also try explicitly setting the offset to 0 using M206 [reprap.org]

---

[3DKarma.com] - suppliers of quality, affordable 3D printer kits and filament for the UK market.

Homing the Z axis will indeed move down quickly first, then raise and lower more slowly to get a more accurate reading. The found Z position by default should be the position of the nozzle. You can however use Z offset using the display and control knob in menu control/motion to change where the nozzle is if you don't want to adjust the z-endstop. If that's set to 2.2mm then that should be the cause.

Also check if the x end and x motor mount (the whole x axis) drops down properly by holding it lightly down with your finger. If it then homes correctly before the print then the Z nut is too loose and will not allow the x axis to be lowered consistently when the z axis is lowered if you know what I mean. It hapened to my i3 too and fixed it with a tiny bit of rubber to make the z-nut fit a little tighter.

It shouldn't matter if you use a sheet of paper or a 0.1mm feeler gauge to level the bed and calibrate the z homing.

Also check if your z steps are set correctly. If there's too many steps, if your first layer height is 0.22mm and this is a factor 10 off it will start at layer 2.2.

Thanks again guys. My x axis assembly is solid, no looseness. In the LCD Control/Motion menu I see no value for offset. My settings are:
Accel: 500
Vxy jerk: 20
Vz jerk: +000.40
Ve jerk: 5
Vmaxx: 250
Vmaxy: 250
Vmaxz: 2
Vmaxe: 25
Vmin: 0
Vtravelmin: 0
Amaxx: 1000
Amaxy: 1000
Amaxz: 5
Amaxe: 1000
A-retract: 500
xsteps/mm: 80
ysteps/mm: 80
zsteps/mm: 4000
esteps/mm: 855

I just sent the M503 command to the printer and there are no home offsets on line M206. As best I can tell my steps are all correct for all the axes and the extruder. The only thing I can think is that somehow the switch is sending an incorrect signal?

Any help is appreciated.

Take a look at your g code in a text file viewer and look for the first G0 Z or G1 Z command. If it's setting the height above 2mm, the issue's in your slicer settings.

---

[3DKarma.com] - suppliers of quality, affordable 3D printer kits and filament for the UK market.