Adjustable home and limit switch targets for the CNC Bantam

Another little job to do for the CNC Bantam. I'm using inductive proximity switches for the home and limit switches on the Z axis. These are the Omron TL-Q sensors. These use an open ended pot core to detect the presence of a conductive body within the magnetic field generated by an AC driving voltage, due to its effect on the q factor of the circuit. The target can be conductive but does not necessarily need to be magnetic - and in fact a higher conductivity body (and most likely non magnetic) gives the smallest detection distance and hence arguably the most accurate homing distance. 

Currently, for the Z axis home target, I have simply stuck a piece of aluminium angle iron onto the leaner encoder scale extrusion using a piece of double sided tape - yes, another temporary measure taken while I proved out the function. Furthermore, I haven't fitted a target for the Z limit switch, which is the further sensor in the photo. The notion was that I would be able to move that to suit the work in hand. So if I am using a big chuck with a lot of stickout, I might move the limit switch target further from the headstock. 

The alternative would be to change the software limits in the .INI file, which is hardly user friendly - or not have anything to prevent the tool flying into the headstock/work/chuck if I have left The Stupid Fat Bloke in charge of operations.

So I need a couple of (plastic) clamps that will attach to the linear scale housing and be easily adjustable for position by sliding along the housing. 

This was the initial concept. This is the reverse side. I need to be able to assemble the thing without dismantling the scale or trying to spring the clip. This will be 100% fill ABS, so shouldn't be capable of springing that much. So there's a screw holding the rear part of the clip in place:

The target will be an M8 grub screw on the underside of the clip:

Looks OK from here, although the magnet needs to be moved out so it is above the centre of the sensor. There's a grub screw that allows the clip to be locked in position. I'll either use a plastic screw or (more likely) a steel one with a plastic pad to avoid damaging the scale housing.

With some further measurement and thought, here we are. The target grub screw is now placed in the middle of the sensor, hence the joggled end. And the limit switch is simply a mirror image of the home switch, with the same clamp piece.

If this works out OK, I will need to print the mirror image part for the other limit switch ie the max travel limit. However, it takes 6.5h, so we will have to wait and see what emerges finally. Meanwhile, I've just taken delivery of another 2kg of black ABS. The Creality CR-ABS seems to work really well and is nice and solid when printed at 100% fill density.

An interesting / useful point to note is that those M8 threads are modelled in Fusion and the printer does a reasonable job of reproducing them, to the extent that you can simply screw an M8 screw into them without needing to clear the threads with a tap. Certainly good enough for the application in this case. For more demanding applications it may be necessary to fit a captive nut or threaded bar into the body, as I did on the pipe tube cutting fixture.