Designed a folding post so your print area should be only minimally effected by this add-on. Moving the bed to the bottom deploys the post and after using the post to deploy and retract the probe, you can retract the post by pushing it up slightly.
UPDATE!! Changed parts so they lock more securely by moving the magnet up. I also changed my post setup so it is actually screwed into the platform, as opposed to using a binder clip. Will upload better pictures of the post (thing used to switch the probe up/down) soon. It's way more stable now, but the attachment to the print head is still not optimal.
This is a Z-probe mount for ultimaker! It uses a pushbutton switch to attach your min z endstop to your head! A simple printhead motion lowers and raises the probe to allow fully automated platform sensing and printing. Never worry about getting the perfect z-height again!
Video here http://www.youtube.com/watch?v=a-A90wP4Wus
Or maybe do because this thing doesn't work perfectly. WIP!
Use two 2mm cube-shaped neodymium magnets; they get glued into the little square spots in "swivelmount.stl".
For the bearing I used a standard little bearing that you can get out of most hard drives. 9.5mm OD, 6.35mm ID, 3.17mm thickness.
I used a momentary push button switch with a lever arm similar to what the ultimaker uses normally (you could probably just take off the z min endstop and mount it on this thing; the holes may not be correct though). A screw placed in the medium sized hole in "swingarm" acts as a target for magnet attraction. Adding some nuts on that screw also helps as they have flat faces that increase attraction.
The swivelmount attaches to the print head using the same screws as hold the printhead together; substitute them with slightly longer screws to attach the part.
Finally, route some flexible wire to connect to the switch. This part I haven't completely figured out how to do properly; the attachment of the wires does matter because the wires exert a slight force on the lever arm; you want the force helping to push the arm DOWN.
The last part which I have not yet designed is a little post that the ultimaker will use to raise/lower the sensor. I bent a piece of wire and attached that to the platform using a binder clip.
The procedure for leveling goes like this: Home the platform to the bottom of the printer. Either reprogram your marlin install to home to max or tell the bed to go to +1000 with endstops enabled. Then, raise the bed up so the switching post is at the correct level to switch the lever arm. Move the head so the lever arm switches down, then move to the middle and slowly raise the bed. Since you enabled endstops during moves (you did, right?) the platform should stop as if it has reached the max endstop when it clicks the button. This may take some fiddling as if you don't have a good metal part to hold the lever arm to the magnet with, it might pop off during this process. WATCH YOUR PRINTER LIKE A HAWK during the first few tries. Once you manage to get it repeatable it should be ok, but again this is WIP so it can probably use some re-designing.
Anyway just watch the video if this is confusing.