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Delta grid calibration seems to be useless

Posted: Wed Oct 31, 2018 10:28 am
by mihai
I purchased a TLM last year but since then I only used it to print small parts and I didn't notice that the bed had a big difference in height between various regions. Now that I need to print larger parts, the bed is so low in a region that the layers don't even stick to it.
I ran a delta grid calibration procedure and although that seem to correctly observe the differences and save them into the grid file when I print it seems like it's not using them because the nozzle is still too high in the areas where the bed is lower. Actually it seems to be just as high as it used to be without any calibration.

I'm using octoprint to control the printer.
Here is my calibration procedure:

Code: Select all

; home
;bltouch alarm release
M280 PS10.6 
; probe pin down
M280 S3

; set bed temp to 80C
M190 S80

; do delta calibration

; save trim to sdcard 

; probe the grid and enable grid compensation

; save the grid to sd card

;To set the zero for the print head
;Make sure there is no left over material on the nozzle ( heat it and wipe it ) 
;go home

;go close to Z=0
G0 Z1

; put a sheet of thin paper under the nozzle
; Then move the head until it presses on the paper sheet until you feel the friction when you pull on the paper. This is the Z=0 point
; Use this point as the homing offest
M306 Z0

;save to SD card

After this I tried a print ( a big disk with a radius of about 110 mm ) and the layers still won't adhere to the bed on one side and I could see that they almost float in air in that zone. The octoprint bed visualizer plugin generated the attached graph based on the saved grid data ( it's square when the bed is round but I guess it's still useful )

Why would the printer not use the grid to compensate for the difference in bed height?

Re: Delta grid calibration seems to be useless

Posted: Sun Nov 18, 2018 11:23 am
by Xerxes
The bed glass isn't the flattest thing in the world, but the glass generally averages out to be flat on my printer..

You didn't mention if you had the bed sticker on or not. Everyone's sticker bubbles over time and becomes impossible to put down a first layer.

If you're using the bed sticker, remove it and print directly to the glass with an adhesive (sugar water, abs juice, tape, etc) depending on your filament.

This improves it _a lot_. I can make large sized prints on my bed. The surface is a bit uneven compared to machined glass, or the flatness of borosilicate glass. To improve it further, add a sheet of borosilicate glass cut to fit on the bed, and use some thermal bonding compound or mat between them.

I will consider doing that myself in the future.. (upgrading the bed with another sheet on top, mostly just to get a smoother finish since the glass surface on the TLM is textured)

You seem to know what you're doing with the software, I'd suggest you try upgrading the firmware, you might find a better calibration routine in the newer versions of smoothieware (btw consider donating to Smoothie project.)

On an unrelated note, you may be interested once you level your bed. I've been playing with the TLM in a bowden setup and finding ghosting at outer layers over 40mm/s .. Looking into it the issue is traced to the firmware using a configured value of 6000mm/s^2 acceleration, which is insanely high. 2000mm/s^2 would be really fast, and is more than most printers can handle. For example a Prusa i3 seems to use a value around 800mm/s^2, corexy around 1500mm/s^2 .. 2000mm/s^2 is plenty. There is a lot of quality and speed to be gained by dropping the acceleration..