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SoliForum - 3D Printing Community → Electronics & Hardware → New bed system questions
So I am envisioning a new heated bed with PCB and a better system to hold glass onto said bed. The question is.
How come when the SD3 homes it moves the tip of the hotend below the bed surface, and how do we fix/stop that?
It shouldn't ever be able to do that. Are you sure it goes below the bed surface and not just to 0.00mm?
yes the head is below the surface of the glass. and if you execute a kill job at that point the head will hit the bed edge.
with a correct Z-tab set, my SD3, with glass or even a much thicker ceramic tile, never does that. Wooden or aluminium platform. it homes to 0 mm for the anchor, and thats it. never below, unless my bed is unlevel or z-tab off.
Check your start gcode if you are 100% your z-tab is set correctly - the whole point is it hits the limit switch at 0.00mm and stops ! It can only now move the bed higher, and thus bring the nozzle BELOW the bed, if it is physically pushing the screw upwards at that point! 
Something is not right otherwise for it to be able to physically move the bed above the nozzle...
Mine doesn't go below the bed on my sd3.
I highly recommend the qu-bd silicon bed. 4 minutes to above 100C without any sort of insulation to aid! On my sd3.
I am using the kisslicer with copied code from the slic3r gcode section.
And this behavior was also seen with stock slic3r generated gcode.
Can you explain how physically it's even possible without just breaking off your z tab if its set correctly... It stops when it hits that. To move above the nozzle it has to move that up somehow, which is not possible without breaking.
The only way to end up with the bed Above the bottom of the nozzle is if you have an off z tab. You cannot move the platform with ANY g-code of ANY sort above the limit switch - thats its job!!
So.. you have either set your z-tab wrong or your table is not level, or are not seeing what you think you are seeing. It is not possible - plain and simple.
Post a video, including in frame the Z limit switches, because seriously, you are adamantly arguing something thats not possible by mechanical design is happening....
Seen the z-tab here, the thing with 2 gold standoffs attached at the back.. its pressing down on the limit switch and the bed is aligned.
For you to have the bed move *any* higher, two things must occur. Either the Z-Tab gets moved up, allowing further movement, or it doesnt, and the bed rips the z-tab off the printer
It has nothing to do with G-Code at this point....
Yes I understand. The bed is level, the printers are awesome. Sticking is not a problem. It is almost as if my gcode there is a bed offset being applied. So Home is actually with the extruder tip below the bed surface.
M107
G21; set mm units
G28 ;home all axis
G90; set absolute coordinates
G92 E0; reset extruder distance
M104 S220; set extruder temp and start heating
G1 Z5 F300 ;move platform down 5mm
G1 X195 Y195 F4500 ; move to back right corner
M109 S220; wait for extruder temp to be reached
G1 Z.3000 F200 ;move platform close to nozzle
;G1 E7; extrude anchor
G1 F3000; center print head
G92 E0; reset extrusion distance
G90 ; use absolute coordinates
G21 ; set units to millimeters
G92 E0
M82 ; use absolute distances for extrusion
G1 F4500.000 E-1.00000
G92 E0
;G1 Z0.300 F3600.000The reason I know this is the case, is because if I do a kill job command while homing is going on at a job start the extruder crashes into the edge of the bed. Since its glass it moves.
And for some additional proof, so you all know I am not crazy, video.
Please correct me if I got this wrong and I hope I did.
You did. Hugely.
F is simply a Feedrate setting, not a movement .
http://reprap.org/wiki/G-code#G1:_Controlled_move
This is why its important to provide advice to others on subjects with which you are certain... Otherwise you're just spreading bad info across the forum for other unsuspecting people to stumble upon.
I empathise with your enthusiasm... But its important to help others with accurate advice
Thankyou, I will try to do better next time.
Downeasta: I'm not trying to be an ass; honestly... its just bad info has already been issues in the past on here and many other forums leading to people struggling to find the *correct* answer. No point spreading more of it around eh? anyway, you will probably find http://wiki.solidoodle.com/gcode-guide very interesting (it describes the start.gcode in great detail as to whats happening). I'm not saying I don't give the incorrect information on many occasions - but I usually reserve giving it to areas I'm pretty confident about through practice and I try to always correct any false statements I have made in threads (like handing out wrong bearing sizes - which i had also stated with 'don't quote me on it - ....' .
Nlancaster: Nothing in your g-code would exhibit the behaviour you describe. I still maintain its mechanically impossible as long as your Z-Offset is set correctly, and your bed is level and not warped.
For you to have the head move below the bed surface, the z-axis must move above it at 0. If you have set 0 with the extruder resting on the bed.. then thats it - it is at its minimum z travel. For it to have the bed move ABOVE the nozzle, it now has entered -Z territory...
Can you please post a video of the phenomena... because I'm extremly eager to see how this is possible. The only consideration I have for it is that your bed is warped - thats the only way that a Corner can be Higher than the Nozzle - but that also is not possible as the issue because that doesn't explain how your printer maintains then resets its Z height.
But theres nothing to be seen in your Start G-Code.... (and compare it against http://www.soliforum.com/topic/231/impr … end-gcode/ as well perhaps if you want)
Different thought.
Can you post a picture of your z stop?
one other thought is maybe one or both of your x or y rods is bent vertically near the end allowing the printhead to drop below where the z was zeroed...
What method do you use to level the bed? at each of the bed adjustment screws or at each corner?
I level the bed first at each adjustment screw then check the surface in about 9 spots. its really pretty flat with the glass on. Very little if any warp. I then print a square test pattern of 6 loops and level the bed exactly so those loops are all perfect. I do that about 1-2 time a week it does not seem to drift much, unless the temperature really swings day to day.
Adrian there is a video above downeastas posts that shows it hitting hte glass. This is after running a print that came out perfect.
Are you using a dump area? Is there some possible code in that, that is telling it to misbehave?
I still dont understand why your your z stop hasnt limited your printer from raising the bed so high. Is the z screw missing?
Adrian there is a video above downeastas posts that shows it hitting hte glass. This is after running a print that came out perfect.
Cool - I'll have to catch it in a few hours. Youtube/Vimeo no work from where I presently am (hence why I can't even see it without source-viewing the page )
Adrian, totally understandable.
Down, the z-scew is not missing. The printer prints perfectly fine. Just at z-home the tip of the extruder is below the level of the bed. Before I had glass, I hit the edge of the bed several times as well, even when it was printing fine.
The reason I am so concerned about what is happening is, that I wish to build a new bed/glass holding system and It may protrude into this area.
I meant to post this earlier but figured there was no chance of this being an issue.
Also Notice ;G1 E7; extrude anchor is commented out in my gcode, when this was enabled. The hotend would be rammed into the bed enough to deflect the entire wood bed holder assembly.
Nope no z-offset applied in either kisslicer or slic3r.
Also Notice ;G1 E7; extrude anchor is commented out in my gcode, when this was enabled. The hotend would be rammed into the bed enough to deflect the entire wood bed holder assembly.
I haven't watched the video yet.. but the more you speak of things, the more I have to insist that your bed isn't flat and level.
On how many sides and where do you have your clips. 3 or 4 sided ?
I use G1 E7 religously with my Glass and much much thicker (and therefore, would exacerbate the problem you describe) ceramic tile... No issues. The only time I have remotely gotten close to the scenario you describe, is when the rear of my bed is not level, and protrudes above the center of the bed where I set my Z-Stop against.
I have a 4 corner magnetic hold down, this keeps the glass firmly in contact with the aluminum bed. Hot or cold. I do get a gap under the glass in the middle of the bed when hot. This crashing problem occurs hot or cold. I have one clip in the front center between the aluminum and glass to keep it from moving side to side.
Just watched the video.
The issue is simply as has been described - that corner is not levelled or your rods are bent at the extents of travel.
Set your z-height using ONLY that corner, and your problem will disappear. Then you can start chasing the underlying issue of why your bed or glass plate is warped or if it is as Ronsii rightfully suggested, bent rods.
Seriously - I doubt theres a mystery here... if you set z-tab to 0 on that corner, voila... problem resolved. It is violating all known laws of physics otherwise, plain and simple Maybe you've got a quantum-string-theory printer 
I know you are convinced there is some other issue here - but really - there is no physically possible alternative explanation for you. It is also 100% not software or g-code related.
Set you Z-Tab to Zero on that corner and that corner only AFTER you have levelled the bed, and you'll be laughing... then you can start playing with straight edges and dial gauges....
And remember - Glass despite its appearances is not guranteed to be flat..
The only other thing is - which is pretty darn out there - perhaps you are stuck setting your z-ofset on a section of badly threaded rod. Stick something under your bed to cause you to have to set it all with a lower z-stop... this will change the section og 5/16" rod you are trying to call 'home'... perhaps its randomly getting caught with horrendous backlash at that spot.... which is why the second moves appear to be fine but the first move is off... But this still doesn't fit with the scenario of Z-Stop 0 is 0... there is no -Z1.....
The only way to end up with the bed Above the bottom of the nozzle is if you have an off z tab. You cannot move the platform with ANY g-code of ANY sort above the limit switch - thats its job!!
I agree that in this specific case the problem is with 99.9% confidence purely hardware. But just to clarify one small thing: the microswitch still allows maybe 1mm travel after being hit and before breaking, and an interesting fact to know is that in the firmware that we all use, the endstop switches are not actually treated as endstops, but just as homing switches, meaning that the firmware will still allow moves beyond them. This is because in configuration_adv.h we have this:
#define ENDSTOPS_ONLY_FOR_HOMINGRepetierHost is pretty smart and knows where you are and doesn't allow manual moves beyond the limits; but with pronterface you can see that you can still ram into the endstops if you are not careful
RepetierHost is pretty smart and knows where you are and doesn't allow manual moves beyond the limits; but with pronterface you can see that you can still ram into the endstops if you are not careful
Oh yes, totally agree. But you still can't go *past* the switch, even with pronterface. If the switch is set fully on at z-stop 0.. it will still smash into only z-stop 0. And yes, the switch lever has about 1mm of travel, but its a constant distance.. so again, 0 remains 0... So if the distance is set correctly, and the bed is properly level.. then zero remains a constant and you can't physically get to -Z1 for example...
Thats whats so interesting about this particular issue that keeps me intrigued.
Rincewind wrote:RepetierHost is pretty smart and knows where you are and doesn't allow manual moves beyond the limits; but with pronterface you can see that you can still ram into the endstops if you are not careful
Oh yes, totally agree. But you still can't go *past* the switch, even with pronterface. If the switch is set fully on at z-stop 0.. it will still smash into only z-stop 0. And yes, the switch lever has about 1mm of travel, but its a constant distance.. so again, 0 remains 0... So if the distance is set correctly, and the bed is properly level.. then zero remains a constant and you can't physically get to -Z1 for example...
What I mean is that the switch has 1mm of travel after being hit. And the firmware doesn't stop you if you want to go to Z-1. So it's not physically impossible to go there...
The fact that nlancaster has no gcode that would cause this effect makes this only a hypothetical possibility though.
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