Compensate for thermal expansion?



  • Is there a way to probe bed before print? How to compensate for thermal expansion? First layer thickness varies with with room temperature, I get very thin first layer when room is cool and thick when warm. If machine is really cool nozzle will actually contact bed restricting extrusion. Or is there a way to home Z to bed for first layer?



  • @edkirk said in Compensate for thermal expansion?:

    Is there a way to probe bed before print?

    Do you mean bed mesh probing? I don't do it before every print but my bed mesh probing macro heats the bed before starting probing.

    As for doing it before every print, I considered it once but most of my prints are much smaller than the bed and I didn't want to wait for probing the entire bed. Would be nice if Duet would support per print probing, e.g. it probes only the area of the print and use the mesh results only for the current print.



  • @edkirk I'd be very surprised if it's thermal expansion of the bed as it would need to be very thick indeed, but more likely it would be thermal expansion of the frame (if indeed the problem is due to thermal expansion). You haven't said what you are using as a Z min sensor. I'm assuming that it must be some sort of switch which is mounted on the frame, rather than a probe. Because if it is a probe situated near the nozzle, then that will automatically take care of any thermal expansion. That is to say, the offset between the probe trigger point and the nozzle will remain almost constant regardless of any thermal expansion of the bed plate or the frame. Even better is to use the nozzle as a probe by using something like a piezo sensor. So with any probe situated near the nozzle, all you have to do is home Z before a print, preferably with the bed hot, and that will automatically compensate for any thermal expansion of the the frame due to ambient temperature variations.

    I suspect your problem is something other than thermal expansion though.



  • @deckingman said in Compensate for thermal expansion?:

    if it is a probe situated near the nozzle, then that will automatically take care of any thermal expansion.

    Yes, but only at the point of Z homing, right? Can't the thermal expansion deform the bed in other areas (that is, relative to the Z home probing point)?



  • This is a Delta with Smart Effector, I believe the expansion is in the frame. I can get the first layer I want with M665 Hxxx setting running mesh grid compensation. But when I start a new print at a later time and my shop temperature is 15 or 20 degrees different the first layer is off enough to ruin the first layer. My core xy has the optical sensor which is what I use for the z probe/end stop, the delta uses the end stop switches which are opposite the bed, the core xy is perfect regardless of room temperature. How to sense and adjust bed on first print after homing using end stops?



  • @zapta said in Compensate for thermal expansion?:

    @deckingman said in Compensate for thermal expansion?:

    if it is a probe situated near the nozzle, then that will automatically take care of any thermal expansion.

    Yes, but only at the point of Z homing, right? Can't the thermal expansion deform the bed in other areas (that is, relative to the Z home probing point)?

    Personally I'd have thought that as ambient temperature is evenly spread over the entire bed (or frame) then any thermal expansion would be uniform and it would be most unlikely that any distortion would occur of one section in relation to another. It's not impossible, because physical constraints will play a part but highly unlikely IMO.



  • @edkirk Then if it is a delta with a smart effector, the nozzle itself is the probe isn't it? Sorry, I don't know much about Deltas but I thought the smart effector utilised a Peizo sensor. If that is the case, then surely all you need to do is a single probe using G30 before each print?



  • Hey Guys, thanks for the replies. Most of my experience has been with my Core XY's, https://hackaday.io/project/12618/gallery#dcfc875d8e7f5cd9b5e055366f7bde6b.
    I have two that I designed and built and have awesome results. My experience with Deltas is fairly new in relation to my experience with Core XY which I home to the bed with the optical sensor and that is it, always the same. But I am impressed with the print quality of my small Delta, I built it to experiment with the Smart Effector which I found very interesting. I am sure there is something to explain how to home the Smart Effector to the bed but I haven't found it. I run bed compensation to see where the nozzle is in relationship to the bed.


  • administrators

    @edkirk said in Compensate for thermal expansion?:

    Is there a way to probe bed before print? How to compensate for thermal expansion? First layer thickness varies with with room temperature, I get very thin first layer when room is cool and thick when warm. If machine is really cool nozzle will actually contact bed restricting extrusion. Or is there a way to home Z to bed for first layer?

    If you have a Z probe, it's simple: just before printing, move the nozzle to about 5mm over the centre of the bed and run G30. If you home Z using the Z probe, you probably do this already to home Z. If instead you home Z using an endstop, do this immediately before printing (after heating the bed and extruder) and don't re-home between doing this and starting the print. So if your slicer start GCode includes G28, add the move to bed centre and G30 after that.

    On a delta printer you can alternatively run G32 just before printing.



  • Thank you, I put G30 in the starting script, adjusted Z in G31. Works great, will check in the morning when it is cooler. Ed



  • Another question, when I run the G30 in the starting script it is very slow, how to adjust the speed?



  • @edkirk said in Compensate for thermal expansion?:

    Another question, when I run the G30 in the starting script it is very slow, how to adjust the speed?

    You could use something like this:

    https://forum.duet3d.com/topic/11018/bl-touch-fast-homing/2



  • Thanks, I wrote it in the starting script, experimenting with speeds and number of probes, I think first probe fast followed by 3 slow probes. Fun little dance.


  • administrators

    @edkirk said in Compensate for thermal expansion?:

    Thanks, I wrote it in the starting script, experimenting with speeds and number of probes, I think first probe fast followed by 3 slow probes. Fun little dance.

    If you have already homed Z, you can use a G1 command to position the nozzle about 5mm above the centre of the bed before you do the G30. Then it only has to probe 5mm.


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