5point versus mesh bed map

  • If I use 5 point bed map from automatic bed levelling button I can level bed and then if I run bed mesh levelling with same corner points I get a tilted bed which differs from 5 point map. What is happening and am I missing something in config file or other settings? Also with glass bed backed by black paper I get a map like the Himalayas (mesh measured at 20mm separation). but if I use black card with no glass I get a flat mesh.
    Comments and help appreciated
    Rob H

  • What sensor are you using?

  • Sorry should have said- I use DC42s excellent IR sensor

  • If you use black card and its flat but with glass it is not then the sensor is not accurately reading the heights through the glass. The IR is excellent but it is limited by the IR reflectivity of the surface under it. Any unevenness and when you are talking about very precise measurements then slight variations in reflectivity count, it struggles to read well.

    All probes have their limitations.

    There may be a trick to make it work with your setup Ill leave that to DC.

  • administrators

    Do you have coating (e.g. hairspray, PVA glue etc.) on top of the glass? If so, then unfortunately the IR sensor is less than excellent with that surface. The variability in the coating affects the transparency, which unfortunately varies the contribution of the reflection from the back of the glass. Using black paper under the glass reduces the reflection from underneath the glass, but unfortunately there is still a significant reflection from the underside of the glass itself.

    Aside from using a different type of Z probe (for example, the Smart Effector if your printer is a delta, or DjDemonD's piezo sensor), there are a couple of potential solutions:

    1. Don't use a coating on the glass. If you are printing good quality PLA, you can get excellent adhesion to clean uncoated glass if you prepare it correctly. See my blog for details. This is how I do all the prints on my Cartesian printer.

    2. Use tinted (preferably black) glass so that there is no reflection from the back.

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