HEY, STOP! DO NOT ADD THIS THING. READ THE NEXT PARAGRAPH AND USE THAT THING INSTEAD!
I've fixed a number of things in this design, so please review http://www.thingiverse.com/thing:520580 for a better version. I wouldn't be using THIS version myself. Use the one linked above; it has many improvments!
This is an effector assembly for a Delta printer, provided in a highly configurable OpenSCAD format (also STL). It's a bit of a mashup and original design - see comments in SCAD file. It uses force sensitive resistors FSR402 in the effector itself to allow for z-probing via an interesting spring-mount of the print head. I'm still working on this so it's more an idea than a proven design. I'm pretty sure it will work fine though some sizes might need adjusting here and there.
The effector used when I started this work is available at http://www.thingiverse.com/thing:273232 (author: Nathan Matthews).
The idea of placing FSR on the printer assembly (as opposed to under the printer plate) was originally implemented at http://www.thingiverse.com/thing:386146 (author: Ed Lep) however I have modified the concept somewhat by placing the FSRs on the effector, and restricting rotational and lateral movement of the 'plunger'
This is my first foray into OpenSCAD. I have not assembled this yet, and only printed preliminary prototypes. I'm happy to answer questions and make fixes.
*** In the exploded view, the bottom part is upside down! I'll fix this later but the part is perfectly fine.
Update: Looks like I don't quite have the pieces positioned on the ground in the "PRINT THIS" part; instead, print the components individually; effector, plunger, and clamps.
The plunger is mated to the effector by placing M3 40mm screws through the outer holes. The fit of the screw should be 'sloppy' as we want the plunger to move up and down the screw shaft with little impediment. Above the plunger, on the screw shaft, sits a 22mm spring, or thereabouts. The screws are connected with embedded nuts on the base of the effector, and may be tightened/loosened to provide less downward pressure.
The plunger has three small 'buttons' underneath the main head (on the circular outcrops) which push down onto force-sensitive-resistors (FSR402)x3 which are mounted on the top of the effector in the small circular wells provided for that purpose. The tails of the FSR extend outwards and up, where they should be joined in parallel, so there is one reading for the FSR. In normal orientation, the FSR are "pressed" by the plunger and that amount of press is of course adjustable via the spring screw.
Underneath the plunger is a mounting system for the Jhead. There's an inlet into which the top of the jhead goes; it should be set to exactly the right size to take the top of the T shape of the head. Mounting under that go the two halves of the clamps, which likewise are set to the correct shape for the jhead. The clamps, the plunger are connected by long 2.5mm screws and nuts, which go right through to the top of the plunger and hold the jhead firmly in place. The idea is, first put the clamps around the j-head, then the long-screws with heads-down threaded up through the holes. Push on the plunger and do up the screws with the nuts at the top. I could put little nut indents on the top but haven't got to that yet :)
Next, thread the 5mm hole at top for the pushfit connector, and screw that in. That should do it.
How it's supposed to work:
As the print head touches a surface, there is upward force on the whole assembly. The plunger (to which the jhead is connected) moves upwards, against the resistance of the spring. By moving upwards, it reduces the downward pressure on the FSRs, and thus we can detect when the head is on a surface. Meanwhile, the movement of the jhead is fairly restricted in rotational, lateral movement by the hexagonal shaft of the plunger and tight fit to the hole in the effector. Vertical motion is limited by the extent of the screw and spring combination. It's an elegant design; kudos to Ed Lep for the basic concept.