The fact

On the Rapman, the Z end switch is placed on the lower bed (back left corner) and is facing up. When the bed is raised up during homing, it insures that it will stop before it's too late (crashed into the hot-end or raised above the threading rods length).
But this Z end switch is only usefull when the Rapman is homing and in combination with the toolhead's M5 70mm Z stop bolt (see below).
Z stop M5 bolt
Without it Z homing won't work (crash situation).

The idea

If we could just remove the bed's Z end switch and mount it as a toolhead instead (with the switch facing down), it could automatically stop our Z move whatever the toolhead's position is, and not only in the homing position.
But I don't want to remove my Z end switch ! So the idea is to have a second one that could be used as a toolhead and that would take over the regular one when needed.
I have the idea, now let's see what I can find in my gar(b)age...

The making of

I plan to use my 3D printed multi-tool holder head. It will have to hold the new touch-probe tool.
I have found another end switch that is very similar to the original Rapman's one (I guess they all look similar).
And I also came back with a pair of crocodile clip.

Now how am I going to fix the switch to my toolhead ?
I'm gonna use the same system as I used for holding my pen (it's freely moving into an aluminum tube). But instead of having a rubber band at the top, that's where I want to fix my switch.
Raising the bed up will push the pen into the tube and will press the switch, therefore stopping Z move. It's the principle of a touch probe.

The only thing I'm gonna change is that I will use an old radio antenna instead of a pen.
That way I will be able to make it the exact length I need very easily ! 

I just need to modelize and print the part that will plug onto the tube and will hold my switch.
Here it is after some SketchUping (designed to be printed without support - 2nd version) :

Touch probe switch holder

And here is my complete finished touch probe :

Touch-probe (parts)
Touch-probe

Touch-probe (finished)

Let's try it out !
After the toolhead is mounted with the touch probe, I need to connect it. All I have to do is to connect my 2 crocodile clips to the original Z switch's wires (mind that they don't touch each other).
Now I want to make sure that only my new switch will control the Z end move. All I have to do is to place a piece of duck tape onto the original Z switch to stick it down.

Touch-probe (connected)

Make a very simple test first

while doing some +Z manual move, try your new switch. It should stop the move. If not, check that your touch probe is correctly connected and that the original Z switch is duck taped down. If it works fine, you are now ready to level your bed.

Initialize your touch probe

  • make a "Home tool head" but ESC after the Y end switch stopped
  • X move to the first leveling bolt (the one near the home position)
  • +Z move until the touch probe stops
  • turn the Rapman off then on again

Your touch probe should now be in Z=0.00

Level your bed

  • Z move down a bit (the touch probe's tip musn't touch the bed)
  • go to the second leveling bolt
  • +Z move until the touch probe stops
  • read the Z value displayed on the PCB
  • adjust the leveling bolt if needed
  • measure/adjust until you read Z=0.00 (you can accept a small difference)
  • do the same with the third leveling bolt

Your bed is now perfectly leveled !
But is it perfectly flat ? You can take measures anywhere on the bed. Take a measure at it's center. If you allways 3D print at the center like I do (and like most people do) you will probably notice that your bed is concave. Mine shows a 0.5mm concavity ! After just 6 months of not so intensive use...

You can now remove the touch-probe (and the duck tape which is on the original switch) and start some 3D printing, pen-plotting or milling.

Touch-probe (mounted)