Behaviour of G92

Nandox7

Hi all,

Just checking on the seen behaviour of G92 command.

As it may make some difference here, my machine uses a Z probe and all triggers are done at their minimum position, so 0.

Yesterday I was trying to resume a print job and because I could' run the Z homing the idea was to home X and Y and then manually set the Z position.

So after moving the Z axis to the position I wanted I issued command G92 Z50 with the idea that the system would assume the current position as 50. But as soon as I tried to move it afterwards the firmware would output Z as -100 or similar and start moving non stop.

So my question is, shouldn't the G92 make the firmware determine the current postion as the one sent in this case 50 for Z, so that if Issue after G1 Z55 it will move 5 up (in this printer case it would be up)

Cheers

Repetier

G92 shifts coordinate system for G0/G1 commands and I think M114 also returns this shifted coordinate system. Host and lcd will show absolute position from homing position (in your case z where enabled is z=0) If you started at the top and did go down and said )2 Z50 that would still show same negative coordinates. Sende G1 Z55 should in deed move 5mm up then.

Nandox7

Ok thanks, I'll review the Gcode available again.

I need to be able to "force" the system to assume a given position as absolute.

Specially when using the Z probe in case a print fails and I need to emergency stop, I need to be able to tell

the printer where it is without doing the homing process or it would crash agaisnt the part.

Repetier

That is why I always advice a z max endstop, It resolves these problems without any tricks and makes bed leveling also much easier.

R_Klauco

One comment here - although the firmware seem to handle the G92 command properly, the LCD display is not refreshed after issuing it. So when I do
G28 Z
G1 Z20
G92 Z10
I would expect the LCD to display 10mm as Z position - but it displays 20.
M114 reports Z on 10mm.
When I issue G91 Z15, the printer goes 5mm the correct way.
Perhaps a display refresh should be called after G92?

R_Klauco

I did a bit more investigation - the display still only shows the "real" coordinates - what the printer considers to be OK.
Even after the move above to Z15, the display shows 25mm - which (in reality) is the valid distance, but I would expect to display the 15 instead.

Repetier

Exactly, we show real coordinates. Dev version now shows G92 modified coordinates in cnc/laser mode.

johnrpm

I tried the dev version to get the modified coordinates in cnc mode, although homing is off it will not move in both directions in all axis, also question marks flash on the display, indicating its expecting to be homed, the stable version does not do this.

Repetier

Check M119 if a endstop shows triggered preventing moves. Also make sure you allow moving without having homed for xyz motors if you do that.

Indicating missing homing is contained for a while. Maybe not so important in cnc/laser mode to make it blink as you define coordinate system there often manuall depending on object position.

johnrpm

thanks for the fast response, homing is off as I have not switches, its a small seig x1 mill conversion, headless to save space,

with ramps and lcd, sd, I have two files on sd, g92 x0y0 to reference xy and g92 z0 to set z, I can stick with stable until

this works, just wanted to elliminate any confusion with display out of sync with position.

thanks for this firmware and all your hard work, I looked for a grbl for ramps with lcd and sd but could not find,