Thing-O-Matic: Revised Homing

After adding F feed speed parameters to all the G0 commands in my start.gcode (to work around that bug), I decided to use the new-with-firmware-2.8 feature that stores the home offsets in the Thing-O-Matic’s EEPROM. That turned out to be, one might say, a thinly documented feature, so this may be a useful summary…

The Official Way to set the EEPROM, which you can find in ReplicatorG/scripts/calibration/Thing-O-Matic calibration.gcode, goes a little something like this:

  • Manually position the nozzle dead center on the build plate, just touching the surface
  • Use G92 to set all axes to 0.0
  • Home the axes to the switches
  • Use M131 X Y Z A B to store the current values in EEPROM

Having already found good values for those offsets as part of the aluminum build plate adventure, I jammed them into EEPROM using RepG’s Machine→Motherboard Onboard Preferences. The values I’m using are:

  • X = -53.0
  • Y = -59.0
  • Z = 116.0

For some unknown reason that has nothing to do with floating point representation (I mean sheesh even the 32-bit version of IEEE 754 floating point has at least 10 decimal digits of precsion), RepG modifies only the negative values sufficiently to be bothersome:

  • X = -52.964
  • Y = -58.976

Having stored the offsets, I wondered how to fetch them. That is also, of course, completely undocumented, but I eventually traced down the answer in (deep breath)


That’s not true for all the start.gcode files you might find, though, and there are many such in far more obvious places.

So, OK, I fetch the EEPROM coordinates using M132 after doing both the coarse home (they’ll be pretty close) and the fine home (they’ll be dead on, modulo the changes), then wipe the nozzle and poke the Z-minimum height switch (which is why I really really care about random changes in the stored values) to find the actual height above the aluminum build surface.

At exactly this position it would be nice to set only the Z height to the actual switch thickness, but G92 sets all un-mentioned axes to zero, so you can’t set just one axis. I have no idea how M131 and M132 behaves in that regard; none of this stuff is documented anywhere that I can find and this stopped being funny a while ago.

So, knowing the XYZ coordinates of the switch, I reset the XYZAB axes using G92.

The current working start.gcode that I devoutly hope will continue to work for a while:

(---- start.gcode begins ----)
(MakerBot Thing-O-Matic with aluminum HBP and Z-min platform switch)
(Tweaked for TOM 286 - Ruttmeister MK5 stepper extruder mod)
(Ed Nisley - KE4ZNU - July 2011)
(Hack to work around bad G0 speed)
(- set initial conditions)
G21		(set units to mm)
G90		(set positioning to absolute)
(- begin heating)
M104 S210 T0	(extruder head)
M109 S110 T0	(HBP)
(- coarse home axes)
G162 Z F1000	(home Z to get nozzle out of danger zone)
G161 Y F4000	(retract Y to get X out of front opening)
G161 X F4000	(now safe to home X)
M132 X Y Z A B	(fetch home offsets from EEPROM)
(- fine home axes)
G0 X-51 Y-55 Z114 F400	(back off switches)
G161 Y F200
G161 X F200
G162 Z F200
M132 X Y Z A B	(fetch home offsets from EEPROM)
(- manual nozzle wipe)
G0 F6000 X0 Y0 Z10	(pause at center to build confidence)
G4 P500
G0 X40 Y-57.0 Z10	(move to front, avoid wiper blade)
G0 X56				(to wipe station)
G0 Z6.0				(down to wipe level)
M6 T0				(wait for temperature settling)
G1 Y-45	F1000		(slowly wipe nozzle)
(- Make sure the XY position matches the G92    )
(- home Z downward to platform switch)
G0 F6000 X56.4 Y7.6 Z3		(get over build platform switch)
G161 Z0 F50					(home downward very slowly)
G92 X56.4 Y7.6 Z1.60		(set Z height)
G0 F6000 Z6.0				(back off switch to wipe level)
(- start extruder and re-wipe)
G0 X56 Y-45     (set up for wipe from rear)
G1 Y-57.0 F1000 (wipe to front)
M108 R2.0		(set stepper extruder speed)
M101			(Extruder on, forward)
G4 P4000  	    (take up slack, get pressure)
M103			(Extruder off)
G4 P4000  	    (Wait for filament to stop oozing)
G1 Y-45	F1000	(slowly wipe nozzle again)
G0 F6000 X0		(get away from wiper blade)
(- build some pressure)
M108 R2.0		(set stepper extruder speed)
M101			(start extruder)
G4 P100			(run for a bit)
(---- start.gcode ends ----)

For what it’s worth, I put that file in the sf_40_alterations directory, blew away the previous versions in all the profiles, and replaced them with symlinks to that single file. When the next change comes along, I can modify one file and all the profiles will pick up the change at once.

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