Well, I'm annoyed again.
My machine has three hall-effect proximity switches for homing. These have inbuilt magnets and react to plain steel vanes on the slides, or the leadscrew nut in the case of Z. They trigger a logic pin on my second port with an arrangement that uses a 2k2 resistor in series with the sensor, and the whole lot across a 9V supply (these sensors originally had a 9v line). Across the resistor is the input side of a darlington-output optoisolator in series with a common LED to give a combined junction voltage of around 2.2 volts (plus a current limiting resistor). Effectively when the sensor is triggered its resistance rises from around 300 ohms to 13K; the voltage across the 2k2 resistor drops sharply to below 2.2 volts and the opto shuts off, allowing the logic pin on its output side to float high.
All this is very fine and well, but when I reference the axes, zero all, turn off the automatic zeroing and move all the axes -10mm, then re-home them all, they end up in slightly different places to where they started.. not usually much, 1-2 microns usually, with the exception of Y which can be much further out.
A few questions:
- Do hall proximity sensors work best if the vane passes very close by it, or does a bit of separation help?
- Can i expect day-to-day drift due to temperature and age affecting the interface circuit?
- Is it really reasonable to expect a machine to home accurately to EXACTLY the same position 24 hours after it last did it?
- if so, how?
I can find precision switches giving 1 micron repeatability on RS, but if I'm to have three of them they'll be around £500, and I won't be popular with the bean-countress.
Suggestions? I did consider that of "buy a new one from Haas" as very favourable, but... see above
Thanks in advance.