Offset positioning

  • Hi All,

    Possibly going a bit mad with this, trying to get offsets all configured etc.

    Coming from a RAMPS/Marlin build, so its been a fun build!

    Printer in question is a newbuild Cartesian, all fresh hardware etc (p3steel build), titan aero, bltouch probe, 200x300 bed. Duet 1.04, 2.01 FW.

    Probe is setup to be used for Z-homing, and that looks all ok (in that it homes and doesn't dive into the bed), actual z-offset needs a bit more work/tuning as it seems to drift.

    So, my query is...X & Y offsets to start:
    -Assuming that X endstop is triggered by the back of the extruder motor, that position there is X=0, the actual nozzle is for the sake of argument 50mm away from that point, so X=50.
    -The probe is from X=0, 25mm away so X=25
    -->This offset here, is that from the X=0 position, or the nozzle position (ie is it 25 or -25)

    Similar sort of question for the Y offset, is it relative to the nozzle or Y=0 position.

    As I am typing this its starting to get a bit clearer possibly, but just need a sanity check please! I suspect I have been sat at my desk just that bit too long before getting around to this and its really not as difficult as I'm making it.


  • Moderator

    You're kind of talking about 2 different things.

    First, for the probe offset

    Measure your XY Offset. To get accurate probe results, the Duet needs to know how far the BLTouch is from the nozzle in X and Y. To measure this, place a piece of paper on the bed and use tape to hold it in place. Move the nozzle down until the it touches the paper and leaves a small imprint. Make note of the current X and Y coordinate. Use a marker to make the spot clearly visible. Now jog the print head in X and Y until the BLTouch pin is on the exact spot you marked. Make note of the new X and Y coordinates. The difference between the two will be your X Y offset in the G31 command.

    Second, the homed position. 0 is where the nozzle is. if that's still on the printable surface, you're good to go. If the nozzle is off the bed, you can change the axis minimum to be a negative value so that it knows the bed surface starts some positive value away from the endstop. If that makes sense?

  • Cheers for the clarification, I'll give that a go once this current print has finished, I did something similar but ended up with some weird results that didn't look quite right.

    I think reading the G31 docs here is whats thrown me it off a bit:
    "X and Y offsets of the Z probe relative to the print head (i.e. the position when the empty tool is selected) can be specified. This allows you to calculate your probe coordinates based on the geometry of the bed, without having to correct them for Z probe X and Y offset."

    The print head to me at least means the "unit as a whole" not the nozzle etc (possibly taking it a bit literal), so 0,0 would be the front left of the motor assembly (what would be X=0,Y=0 as it were)

    Its currently running at:
    G31 P25 X-26 Y48.7 Z3 (so looking at it from the front, 26mm left of the nozzle, and 48.7mm towards the back of the printer behind the nozzle), so as long as the +/- looks right there that's pretty close.

    Its predominantly causing some issues with the bed levelling, in terms of "probe cant reach X,Y" but that's I think another question.

  • Moderator

    @datasquirrel said in Offset positioning:

    "X and Y offsets of the Z probe relative to the print head (i.e. the position when the empty tool is selected) can be specified. This allows you to calculate your probe coordinates based on the geometry of the bed, without having to correct them for Z probe X and Y offset."

    I think what this means is that you generally always use the nozzle position with the duet. The offset of the probe is stored in G31 and applied automatically, so you don't have to take it into account every time you make a move.

    I'm not aware of anywhere in the firmware that takes the clearance of the full print head into account. It's all nozzle tip relative.

  • administrators

    Tool and Z probe offsets at specified relative to the Head Reference Point (HRP). You can choose the HRP to be wherever you like on the print carriage. On a single nozzle printer, the tip of the nozzle is always used as the HRP, which makes the tool offset zero on all axes. On a dual nozzle printer, either one of the nozzle tips or the XY mid point of the two nozzles and the Z coordinate of the lower nozzle is typically chosen.

  • Thanks both, went back to the drawing board with it and started that section of config again, was pretty close I think but after adjusting the probe homing parameters and ruling everything else out, I discovered 2 things:

    • The probe I had "borrowed" off the other printer had a slightly marginal pin (that was sticking, now cleaned and re-calibrated), it would not give consistent readings between probes.
    • My original measurements were near enough spot on, solved by fixing #1 (and just replacing the probe with the new v2 that arrived with its plastic pin), the other v1 works fine on the other printer but its not as sensitive as the new build.

    Just goes to show check your hardware first and assume nothing, and that each build has its quirks.

    Managed to get the bed levelling working a treat after that, so all good.

    If I get a few minutes I'll put up on the docs a diagram detailing the offsets and whatnot if thats helpful @dc42? The point about HRP is a good one, perhaps its just the weird way I interpreted the docs.

    I also noticed a potential docs/site indexing issue (if not intentional etc), if I stick into google "duet bltouch troubleshooting" the guide linked by @Phaedrux doesnt show up, other docs from dozuki come up on other searches etc.

    Thanks again

  • Moderator

    @datasquirrel thanks for pointing that out. I probably didn't create it properly.

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