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Print not in the centre of bed

Posted by speighty1 
Print not in the centre of bed
June 27, 2017 07:23AM

I know this might not be the correct forum but im just wondering if anyone could help me.

I am currently using a k8200 running the latest marlin firmware on an arduino, the other day one of my A4988 driver boards failed and I replaced it with a DVR8825 driver, I re calibrated my steps per mm and the 20 x 20 x 20 mm cube I printed was within the correct margin.

I started a large print this morning that uses the majority of the y axis, the issue is it didn't start the print in the centre of the bed, the right hand side of the print is nearly touching the right hand side of the y axis, thus leaving around a 20mm space on the left hand side.

Now prior to changing the driver board the prints were pretty much in the centre, are there some settings I can change or am I better trying to calibrate the y axis again maybe with a larger print. something like a 100 x 100mm cube.

Here are some images to show you what I mean.


Re: Print not in the centre of bed
June 27, 2017 07:45AM
The slicer drops prints at the center of the bed based on the bed dimensions you told the slicer. The bed dimesions that you tell the slicer should be the extruder accessible dimensions of the bed, which aren't necessarily the same as the physical dimensions of the bed plate. If the extruder can't reach part of the bed, don't include that part of the bed in the dimensions you tell the slicer.

The printer's origin is either the left front or right rear corner of the limits of extruder motion. The bed plate's origin, which is what the slicer cares about, is the nearest corner of the bed plate to the printer's origin. If the extruder nozzle is off the bed when you home the printer, you usually enter negative offsets to tell the slicer how far the nozzle is from the bed.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Print not in the centre of bed
June 27, 2017 07:51AM
I currently use astroprint using slic3r as my slicer.

When I home the printer the nozzle is at the top left of the bed and is not off the bed.

Just seems like im missing something as like I said before I had to change the driver the prints were pretty much central, now its off to the right side of the y axis.

Edited 1 time(s). Last edit at 06/27/2017 07:51AM by speighty1.
Re: Print not in the centre of bed
June 27, 2017 09:02AM
If you changed something and suddenly things are behaving differently, then, most likely, the thing you changed caused the different behavior. You changed the driver board which means you may have changed the printer origin and dimensions, and by changing the steps/mm in the axes you may have caused the shift, especially if you had to change steps/mm a lot.

A 20 mm cube is too small to calibrate anything. The plastic never has a chance to cool before the nozzle lays down another layer and the result is some sloppiness in the print. If you calibrate to that, when you print larger objects the absolute error in your calibration gets multiplied. Calibrate using a larger object- try a 100 mm cube. The plastic will have a chance to cool and solidify before the nozzle lays down another layer, so the print will more accurately reflect the positioning accuracy of the machine, and any error in the measurement will be divided when you print a smaller object. If you have a really big caliper, print as large a square as you can measure accurately. The 100 mm square will have diagonals about 141 mm long that you can measure with a common 6" caliper. Comparing the diagonal measures is a check of the squareness of the X and Y axes. If they are square, the diagonals will be the same length.

DRV8825 drivers are not very good for 3D printing as they tend to skip usteps at low speeds. That creates visible moire patterns in print surfaces. See: [www.engineerination.com]#! The author's printer was a delta machine, the the same effect occurs in Cartesian printers.

Now you're wondering why, if they suck, do so many companies make driver modules using DRV8825 chips that don't include the diodes? The linked article came out in February 2015. The problem is well known. There are companies making add-on boards with the diodes that you plug in between the driver board and the motor. By now shouldn't the people who make the driver modules have made changes? Welcome to the world of dirt cheap 3D printing!

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Print not in the centre of bed
June 27, 2017 10:38AM
I know its the DVR8825 thats made the change, I will try calibrating using a 100 x 100mm cube and see how I get on, hopefully that will sort the ben centre issue.

Could you take a look at my start gcode, I currently use this,

G28 ; home all axes
G90 ; use absolute coordinates
G21 ; set units to millimeters
G92 E0
M82 ; use absolute distances for extrusion
M109 S[first_layer_temperature] ; wait for temperature to be reached
G1 E5 F400 ; prime the nozzle

But I have ben advised that this start code may be better, im not sure,

;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}
M140 S{print_bed_temperature} ; pre-heat bed
G91 ; relative positioning
G1 Z5 ; move Z up 5mm before homing x/y
G90 ; absolute positioning
M117 Homing... ; Put message on LCD screen
G28 ;Autohome to X0, Y0, then Z0
G21 ; metric values
M107 ; start with the fan off
G1 Z10.0 F{travel_speed} ;move the nozzle up 10mm
G0 X0 Y0 ;Start at 0/0 instead of where G29 left us
M117 Heating Bed... ; Put message on LCD screen
M190 S{print_bed_temperature} ;heat bed and wait
M117 Heating Extuder... ; Put message on LCD screen
M109 S{print_temperature} ;pre-heat extruder and wait
G92 E0 ;zero the extruded length
G1 F200 E4 ; extrude 4mm of feed stock
G92 E0 ; zero the extruded length again
G1 F{travel_speed}
; Put printing message on LCD screen
M117 Printing...
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