Solved Nozzle digging into bed intermittently

Ok, I've had this happen 4 times so far and melt my polycarbonate plastic and BuildTak, and I have to find a solution. I'm thinking it's a glitch somewhere, because it only happens about 5% of the time. I use my LCD screen on my Robo3D R1+ to navigate to the SD card and print something. As soon as I do that, the nozzle starts to make its way downward (like it normally does), but instead of stopping about 5mm from the bed and starting the homing sequence (G28), it keeps going until it goes through the bed and some major grinding happens until I shut the machine off. Once I start it back up again, I use Pronterface to raise the nozzle back up 10mm, then I try again through the LCD screen, and it works fine. It could work fine 20 more times, then out of the blue the problem would reoccur.

My start G-code:

G1 Z5 F5000 ; lift nozzle
G21 ;metric values
G90 ;absolute positioning
M82 ;set extruder to absolute mode
M107 ;start with the fan off
G28 ;move X/Y to min endstops
G29
G1 Z5.0 F9000 ;move the platform down 5mm
M104 S[first_layer_temperature] ;set extruder temp
M140 S[first_layer_bed_temperature] ;set bed temp
M190 S[first_layer_bed_temperature] ;wait for bed temp
M109 S[first_layer_temperature] ;wait for extruder temp
G92 E0 ;zero the extruded length
M117 Printing... ;Put printing message on LCD screen

 

No, the problem is, before it goes through the homing process, before it moves anywhere along the X or Y axis, it goes straight down into the bed and beyond. When I begin a print, the normal behavior is for it to home X, then Y, then it goes to 0, 0, then it touches the build plate. And this is what it does 95% of the time. The other 5%, it digs into the bed and keeps on digging before it goes through the homing sequence.

 

The only indication I think it's a glitch is that it's intermittent. Completely random. I'm using Marlin 1.1.0 RC8 MESH Leveling. But no problem, I will persevere. I've been putting a metal plate directly under the nozzle just before initiating any print just in case it decides to do it again, then I hit the emergency button to shut it down as soon as I detect that it has gone to far.
Thanks!

 

ok this is easy to fix just set a Z Offset using M565 in your start up gcode, place it between the G28 and G29 and start with M565 Z-1.0 then if it's too far make it less negative, too close make it more negative.

 

If my Z offset was the problem, this would happen every single time. But it only happens 5% of the time. The other 95% it is the perfect distance from the bed. My Z offset is set perfectly, but thanks for the response!

Edit: I regularly use G29 S4 Zn.nn to configure my offset.

 

so is this fixed or do you still need help?

 

No, unfortunately the problem still occurs. But again, only occasionally, as if it intermittently skips the first few lines of code in the start G-code. For example, the very first line says:
G1 Z5 F5000 ; lift nozzle

Yet the very first thing it does instead is go straight down and digs into the bed. But when it works as expected (which is 95% of the time), it goes up 5mm, then proceeds with the homing sequence.

 

try the code i gave you, i personally think its easier for the computer to understand than go to another code and then say life nozzle, the code i gave you does all that

 

M565 is no longer supported in Marlin v1.1.0 RC8 with mesh bed leveling. You use G29 S4 Zn.nn for that now.

In my humble opinion, based on what I'm seeing, it seems exactly like it's related to software for the same reason I mentioned, which is that if the printer was following the g-code exactly, the very first thing it should do is raise the nozzle up 5mm, right? But it doesn't do that. Something is telling the Z motors to go downward instead. But again, this only happens intermittently.

And strangely, this just happened: I just sat down, turned the printer on, and using the LCD menu I went to the bed leveling option and started the mesh bed leveling process. Normally it would do a homing sequence, then start the process of leveling (I've done it like 20 times already since I started using mesh bed leveling). But instead, it went downward and started drilling through the bed! Luckily I stopped it before it did any real damage. The problem is, it seems to be skipping the homing sequence for whatever reason. So it's not just when it's reading g-code from an STL. It's also when initiating other things, like bed leveling.

 

Hey, thanks for chiming in!
The problem has been ongoing for a couple weeks, probably since I updated to the new Marlin RC8. I've shut off the printer (including unplugging the USB) at least 50 times since then. In addition, every time the problem occurs, I push the emergency switch button, and this forces me to reboot the printer. This happens about 5-10 times a day, and I must say, my emergency button-pushing reflexes have never been better!

 

I don't know, I can't see anything in my g-code that would cause issues. I've pasted it below, I don't know if you can see anything. But also, I would print the same STL from the SD card, and sometimes it would work, other times it would just drive the nozzle into the bed, so this makes me think it's a bug in the firmware.

I set the homing sequence before the heating so that when the nozzle touches the bed it doesn't melt my polycarbonate or acrylic or whatever I happen to be using.

G1 Z5 F5000 ; lift nozzle
G21 ;metric values
G90 ;absolute positioning
M82 ;set extruder to absolute mode
M107 ;start with the fan off
G28 ;move X/Y to min endstops
G29
G1 Z5.0 F9000 ;move the platform down 5mm
M104 S[first_layer_temperature] ;set extruder temp
M140 S[first_layer_bed_temperature] ;set bed temp
M190 S[first_layer_bed_temperature] ;wait for bed temp
M109 S[first_layer_temperature] ;wait for extruder temp
G92 E0 ;zero the extruded length
M117 Printing... ;Put printing message on LCD screen

 

Hmm you do send a "G1 Z5 F5000 ;" i.e. an absolute metric position before setting units to metric and enabling absolute positioning... maybe, maybe not.

 

Hmm... That's a good catch. I'll change that. But I still don't know why the same thing would happen when I try to level the bed, as that has nothing to with the g-code.

 

Ok so here's what I did... I removed the following lines from the start G-code:

G1 Z5 F5000 ; lift nozzle
...and...
G1 Z5.0 F9000 ;move the platform down 5mm

...and now the nozzle neither goes up nor down right before a print, but goes right to the homing sequence. Which is totally fine by me, since the nozzle is always well above the bed before I start any print and there's no chance of it scraping against the bed while travelling in the x or y direction. It's consistently working fine now without digging into the bed. Before I did this, it had gotten to a point where it was doing it every other time.
It's as though for whatever reason instead of moving the nozzle upwards like the g-code instructs it to do in the G1 code, it negates the 5 and travels downward instead. So when the nozzle is less than 5mm above the bed, it would go down 5mm and dig into the bed. But then again, at other times it would work fine and go upwards, so I don't know what's going on or why it would do that. Just speculation that's probably way off. My guess is that it's a bug in the firmware. Or it could be hardware like you say, Geoff, but I wouldn't know where to start troubleshooting.
I appreciate all your responses.