z height changes with every print

Discussion in 'Calibration, Help, and Troubleshooting' started by lance, Jul 16, 2016.

  1. lance

    lance Well-Known Member

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    the distance between the nozzle and buildplate on my bigbox seems to be changing every single time i start a print. its either too high or too low, i adjust it in the offsets and then it is perfect on the print i do right after changing the value, then on the print after its off again.

    how does the IR sensor work? could it be fucking up? would the glue i put down on the buildplate fuck the sensor up somehow? I have no idea how or why this is happening.
     
  2. Shinzou

    Shinzou Member

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    I have the same problem. Is there anyone who has experience with this?
     
  3. Bluemalu

    Bluemalu Well-Known Member

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    Do you use the heated bed? When you calibrated the bed was it hot or cold? The bed will expand / move as it heats up / cools down. Try setting the bed level with the bed preheated. I've also played around with the nozzles up to temp as they will move but the change was there but not really that great to be an issue but all these small errors add up.

    Another option is to put a piece ob blue masking tape in the area of the IR sensor pickup if you think it's a detection issue.
     
  4. lance

    lance Well-Known Member

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    well i have it homing itself at the beginning of every print, which i guess it doesnt really need to do. but it is always fully heated when it does it.

    would having glue on the buildplate mess with the sensor?
     
  5. mike01hu

    mike01hu Well-Known Member

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    My experience is that ambient temperature does affect the Z height. My printer is in a conservatory that has large changes (15-20C this week) and in the morning the homing is OK but later in the day I need to do a MBL. I have not done the maths on lead-screw expansion against acrylic expansion to see what the difference is but it won't make any difference if I did as the change will always take place and I will still have to do the MBLo_O
     
  6. R Design

    R Design Well-Known Member

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    Interesting idea @mike01hu !

    Approx (according to type...)

    Acrylic: 0.075 mm / m / K
    Steel: 0.017 mm / m / K

    So acrylic has it by a factor of 5

    Just looking at one of these to get an order of magnitude : half a metre of plastic and 10 degrees => 0.5 * 10 * 0.075 = 0.375mm shift

    In bed levelling terms that's enormous!

    Looking at the printer my first guess is that the acrylic sides move the bed downwards (whilst the lead screws move it upwards).

    However there's lots of other stuff that could be going on (including the X axis!).

    My impression is that when it's HOT then the nozzle is actually CLOSER to the bed??
     
  7. mike01hu

    mike01hu Well-Known Member

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    Yes, that is an anomaly and the wrong way around for my theory, so back to the drawing board; anyway, the auto home should take care of that too. Is it the IR sensor and firmware management I wonder?
     
  8. Dr Jeep

    Dr Jeep Well-Known Member

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    Unless it was causing something to twist then really temp expansion between zeroing home isn't going to matter is it ( well apart from if it affects the relationship between the nozzle and the IR sensor) Everything else is relative to z home which gets reset (in my case for every print because I have auto home in my start script)
     
  9. Dr Jeep

    Dr Jeep Well-Known Member

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    That said my old k8200 used to drift by a lot but that was because the z end stop reference points are nowhere near the bed or nozzle but next to the leadscrew so there was plenty of stuff to shift the relative positions of them
     
  10. richgain

    richgain Well-Known Member

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    I also have lots of problems with getting the Z-offset height correct. It seems to change significantly with the amount of glue stick I have on the bed and I wonder if the sensor reading is affected by this kind of bed coating.
     
  11. mike01hu

    mike01hu Well-Known Member

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    Yes it is affected by any coating and you should clean the bed thoroughly in the homing area.
     
  12. mike01hu

    mike01hu Well-Known Member

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    I did a bit more measuring yesterday as there was no printing taking place. At 08:00 I setup the dial gauge on the bed with the tip on the end of a X rod, the temperature was 18C. At various times throughout the day I checked the reading and by mid-afternoon the temperature was 37C and the dial gauge read +0.55mm. I left it alone and checked it again when the temperature had dropped to about the same as the morning temperature and the dial gauge had returned to near zero again. That is quite a significant height difference but so is the temperature change and indicates that I need to move the printer to a more temperature-stable environment.:(

    It is not an issue for my printing, as the printer is homed before each job, and any variation in height over the duration of the run is spread over the whole height not just one layer, a pain for engineering part accuracy but not for less critical models.
     

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