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Prusa I3: Distance between nozzle and bed higher after bed leveling.

Posted by Hatmpatn 
Prusa I3: Distance between nozzle and bed higher after bed leveling.
September 18, 2016 09:52AM
Hello!

I have a Prusa I3 with servo bed leveling(a servo that extends an arm and probes 9 points on the heatbed).

When I perform a simple homing, the distance between the nozzle of the hotend and the heatbed is perfect at Z=0.

Although, when I perform a G29(an auto bed leveling sequence) the distance between the nozzle and the heatbed is now about 2mm too far apart. And as a result when I print the nozzle just extracts plastics mid-air. Why is this??
Re: Prusa I3: Distance between nozzle and bed higher after bed leveling.
September 18, 2016 12:45PM
How do you level it with the probe if not a G29 code? I would sugest editing your start gcode before a print to have that code instead of G29, think it should be that simple.


If you need some help, or don't understand what I just said, feel free to send me a PM anytime

Printer: Prusa i3, 2 E3D v6 Hotends, Arduino + RAMPS 1.4 with a Bypassed 5V Regulator, 400w Insignia ATX PSU, Custom Designed Bowden Extruders
Re: Prusa I3: Distance between nozzle and bed higher after bed leveling.
September 18, 2016 03:56PM
It might appear as if you are correct, maybe you can take a look at the code and where to correct it?

Here are some relevant extracts from Configuration.h:

//    +-- BACK ---+
//    |           |
//  L |    (+) P  | R <-- probe (20,20)
//  E |           | I
//  F | (-) N (+) | G <-- nozzle (10,10)
//  T |           | H
//    |    (-)    | T
//    |           |
//    O-- FRONT --+
//  (0,0)
#define X_PROBE_OFFSET_FROM_EXTRUDER -4  // X offset: -left  +right  [of the nozzle]
#define Y_PROBE_OFFSET_FROM_EXTRUDER 40  // Y offset: -front +behind [the nozzle]
#define Z_PROBE_OFFSET_FROM_EXTRUDER -1   // Z offset: -below +above  [the nozzle]

//
// Probe Raise options provide clearance for the probe to deploy, stow, and travel.
//
#define Z_PROBE_DEPLOY_HEIGHT 15 // Raise to make room for the probe to deploy / stow
#define Z_PROBE_TRAVEL_HEIGHT 5  // Raise between probing points.

//
// For M851 give a range for adjusting the Z probe offset
//
#define Z_PROBE_OFFSET_RANGE_MIN -20
#define Z_PROBE_OFFSET_RANGE_MAX 20

* R/C SERVO support
* Sponsored by TrinityLabs, Reworked by codexmas
**********************************************************************/

// Number of servos
//
// If you select a configuration below, this will receive a default value and does not need to be set manually
// set it manually if you have more servos than extruders and wish to manually control some
// leaving it undefined or defining as 0 will disable the servo subsystem
// If unsure, leave commented / disabled
//
#define NUM_SERVOS 1 // Servo index starts with 0 for M280 command

// Delay (in microseconds) before the next move will start, to give the servo time to reach its target angle.
// 300ms is a good value but you can try less delay.
// If the servo can't reach the requested position, increase it.
#define SERVO_DELAY 300

// Servo deactivation
//
// With this option servos are powered only during movement, then turned off to prevent jitter.
#define DEACTIVATE_SERVOS_AFTER_MOVE

And here is the start code:

;Sliced at: {day} {date} {time}
;Basic settings: Layer height: {layer_height} Walls: {wall_thickness} Fill: {fill_density}
;Print time: {print_time}
;Filament used: {filament_amount}m {filament_weight}g
;Filament cost: {filament_cost}
;M190 S{print_bed_temperature} ;Uncomment to add your own bed temperature line
;M109 S{print_temperature} ;Uncomment to add your own temperature line
G21        ;metric values
G90        ;absolute positioning
M82        ;set extruder to absolute mode
M107 		 ;start with the fan off
G28  		 ;home all axes
G29        ;Auto Bed Level
G1 Z15.0 F{travel_speed} ;move the platform down 15mm
G92 E0                  ;zero the extruded length
G1 F200 E3              ;extrude 3mm of feed stock
G92 E0                  ;zero the extruded length again
G1 F{travel_speed}
;Put printing message on LCD screen
M117 Printing...
Re: Prusa I3: Distance between nozzle and bed higher after bed leveling.
September 18, 2016 06:09PM
G1 Z15.0 F{travel_speed} ;move the platform down 15mm

What is that doing there?
Re: Prusa I3: Distance between nozzle and bed higher after bed leveling.
September 19, 2016 03:48AM
I don't know really but removing that made no differnce. The nozzle still is too far above the platforum.
Re: Prusa I3: Distance between nozzle and bed higher after bed leveling.
September 19, 2016 08:47AM
hmm, wait. Are you sure that you have your probe configured correctly? your z offset for the probe being off by a bit would cause exactly this problem
Re: Prusa I3: Distance between nozzle and bed higher after bed leveling.
September 19, 2016 11:45AM
Why would the Z-offset being off describe this problem?

When I home all axis and drive the extruder down to Z=0 it is completely perfect. But when I do G29 and start the print it is 2mm above the perfect position. If the Z-offset didnt have the correct value I wouldn't get it perfect at regular homing either?

Edited 1 time(s). Last edit at 09/19/2016 11:45AM by Hatmpatn.
Re: Prusa I3: Distance between nozzle and bed higher after bed leveling.
September 19, 2016 02:11PM
So, to be clear, a "normal" homing routine has a perfect z=0 obtained through a G28 command? but when you use G29, the auto leveling probe, it is off by ~2mm? From there, I assumed that the G28, used your mechanical endstops (which you may not have on second thought) and therefore the ~2mm z error must come from the probe. That is, assuming that the printer thinks its first layer is on z-0 (or .2 or watever). Does the printer display say z=2 when printing the first layer?

Edited 1 time(s). Last edit at 09/19/2016 02:28PM by DaGameFace.
Re: Prusa I3: Distance between nozzle and bed higher after bed leveling.
September 19, 2016 02:37PM
Thanks for answering. During a normal G28 command, the probe is also used, as the Z-endstop.

When typing M114 during the first layer of a print, it says Z=0.25mm which is my layer height. Although, it really is on Z~2,25mm

Edited 1 time(s). Last edit at 09/19/2016 02:38PM by Hatmpatn.
Re: Prusa I3: Distance between nozzle and bed higher after bed leveling.
September 23, 2016 09:25PM
Asg, thought i hit post but aparently forgot. Think I know what you problem is, make sure you have your z steps per mm set correctly. so that when you tell it to move 1cm it goes 1cm. I bet your firmware is configured for threaded rod while you have lead screws, or expects to see more microsteping than you have
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