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UM1 4 point Z homing for automatic print levelling

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This is a follow up from the post related to Ultrasound Bed Levelling in "Modification & Hacks" (http://umforum.ultimaker.com/index.php?/topic/5959-ultrasound-bed-levelling-on-um1/ ), but I've posted it here to take a firmware approach to what could be a potential solution to stop worrying about bed levelling between prints.

I was thinking of upgrading the UM1 bed with an ALU bed, solder a wire to it and solder another wire to the print head.

Have those two wires connected either to GND and one of the Arduino's I/O OR connect them directly to the Z limit switch port (not sure this could create side effects tho).

We could use this system to change the UM homing behaviour, to go from:

-Home X&Y&Z


-Home X&Y in point 1

-Home Z in point 1

-Lower Z

-Move X&Y to point 2

-Home Z in point 2

-Move X&Y to point 3

-Home Z in point 3

-Move X&Y to point 4

-Home Z in point 4

Where point 1,2,3,4 are the ones used within Cura during the bed levelling wizard.

During each Z homing we would save the Z registered height and save it temporarily.

This would define the height of the print plane and the inclination it has.

We could then use this with Marlin's autolevelling feature as shown in this video:


So the Z axis would constantly compensate during the printing process for imperfections in the bed levelling.

I would like to know Daid's opinion on this and if this could be a feasible solution to resolve bed levelling once and for all :)


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Hi Daid,

Yep, replied on the other topic too. I was suspecting the residue was the source of the issue :)

Did cleaning the nozzle and bed help mitigate this problem?

A solution would be to drill 4 holes in the UM1 acrylic bed, get 4 screws and some bolts and assembly them in the following way (top to bottom).

Screw Head







Screw End

When homing the head we could use the Screw End touching on the ALU block used for nozzle heating rather than on the nozzle. The 3 bolts should be enough for this (otherwise just increase the number of bolts) and should constitute a "known" height.

Am I making sense?



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Just FYI, you only need 3 points and the Marlin firmware supports the probing and planar calculation and transformation with (only) the three points. I believe you can set these points by recompiling the firmware.

I suspected that the problems Daid encountered would prevent using the bed as a electrical contact for this.

In Marlin, if you use a switch mounted on the head, you also need to tell Marlin the offset between the switch and the nozzle. In your solution this would be 0,0,0. Alas, because of those problems already mentioned, this won't work.

The other issue with your acrylic bed solution is the lost of the print area as your holes would need to be reachable by the head and therefore in the print area. Furthermore, having the bolts in the bed means you aren't measuring the distance from the nozzle to the bed SURFACE but instead to the bold head. If the distance to the surface changes, for example because you don't use blue tape or you use kapton tape, you would need to dial in a different offset which is a little of a pain.

I think that at least those three reasons are why people use the nozzle mounted switch.



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