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Despx
(@despx)
Active Member
Dangerous print start...

Hi All, this is my custom start GCode:

G28 ;Home
G4 P1000
G0 Y180 Z150 F1500;Move the Y to 180mm and Z at 150mm
G4 P5000
G92 E0
G1 F200 E25 ;extrude 25mm of feed stock
M203 X500.00 Y500.00 Z10.00 E50.00 ;Setup machine max feedrate
M220 S100 ;Reset Feedrate
M221 S100 ;Reset Flowrate

So far everything is fine but when the print starts, the nozle is lowered to the height of the layer and then dangerously moved to the XY coordinates of the start of the print because if nozzle (for some rason) is not perfectly leveled with the bed can crash with it .

I analyzed the files generated by PrusaSlicer and I don't know why Z and XY movements are separated creating the dangerous movements:

G1 Z0.100 F6000.000
G1 X51.860 Y109.292

I tried to move XYZ together with excellent result and no danger zone:

G0 F6000 X51.860 Y109.292 Z0.100

How can I make this change automatic when the GCode is generated?

Best Regards
Despx

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ENDER 5, Z-Hop Disabled, All PrusaSlicer parameters standard

Posted : 24/02/2021 8:35 am
bobstro
(@bobstro)
Illustrious Member
RE: Dangerous print start...

On a Prusa printer, GO/G1 moves can't move the nozzle closer to the bed than 0.15mm. On a printer with manual leveling, your manual leveling should be set so a non-zero Z position doesn't hit the bed. If you're using a 0.1mm 1st layer height on an uneven bed, this might definitely be an issue. A 0.2mm 1st layer height would be safer.

From your description, my take is that having the nozzle positioned such that a non-zero Z allows hitting the bed is the real problem.If your layer height is 0.1mm, the nozzle does need to be at Z 0.1mm before the XY move to be provide even extrusion across the layer. 

From your description, my take is that having the nozzle positioned such that a non-zero Z allows hitting the bed is the real problem. A higher 1st layer or more refined bed leveling routine should resolve the issue.

Having to deal with this on an Artillery Sidewinder, I can sympathize.

My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 24/02/2021 9:38 am
Despx
(@despx)
Active Member
Topic starter answered:
RE: Dangerous print start...

My problem is the Z movement before XY.

If the 3 movements are simultaneous, there is no problem because when the nozzle is at 0.1 mm it is also on the bed in the print start position.
For example at the beginning of the print Cura moves the three axes together in the safe area, I don't understand why PrusaSlicer cannot do the same.

Posted : 24/02/2021 10:23 am
bobstro
(@bobstro)
Illustrious Member
RE: Dangerous print start...

There should be no Z movement during extrusion. The nozzle should move up the distance of one layer height at the start of each layer and stay there unless doing z-lift. If the Z height is changed during extrusion moves, you will get uneven extrusion. 

Why is your nozzle on the bed in the 0.1mm position? It should be at 0.1mm above the bed (approximately).

Are you saying Cura varies the Z height during an extrusion move? I'm trying to understand why you'd want the nozzle so close to the bed for your 1st layer, especially since your nozzle touches the bed at 0.1mm. Does it hit the bed if you specify a 0.2mm 1st layer height? 

When I've done manual bed leveling on my Sidewinder, the only way the nozzle might hit the bed would be to try to move Z below 0.

My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 24/02/2021 12:45 pm
Despx
(@despx)
Active Member
Topic starter answered:
RE: Dangerous print start...

@bobstro

I believe that you have not understood my problem well or that I have not explained it correctly.

The problem occurs only at the beginning of the print because I don't like the two-line purging system that PrusaSlicer performs before starting the print.

I prefer to purging the nozzle away from the bed, clean it and then drop it on the bed at the xyz coordinates of the start of printing.

Unfortunately PrusaSlicer moves first the Z and then XY, this means that the nozzle can crawl on the bed or, worse, crash in it.

It would be nice if PrusaSlice allowed to choose between separate Z and XY movements and XYZ together.

In the meantime, to eliminate the problem, I have written a small post-process program, if interested I put download links.

Regards

Despx

Posted : 25/02/2021 7:54 am
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