Hi Dickybird, I've an Orac and use an opto sensor like the one MC showed one the page before. On the Orac X slide at the rear on the right side there is a hole with grubscrew that had a rod which travelled with the axis and pressed on a switch located just above the stepper motor. I replaced the rod with a flat sided bar with a slot cut in it and replaced the switch with the opto sensor. My X axis rapid is 2000mm/sec and I home the axis at 80% and it is accurate to less that 0.01mm.

John 

Hi John, nice to see a fellow ORAC user here; I may have to pick your brain later!;D

Yep, I'm familiar with the switch actuator rod you're referring to.  Mine didn't come with one and I figured if I had to make something I might as well use a manual switch on that rod and add the indexing disc/opto sensor to the stepper to get even more accuracy.  I have double shaft motors so it shouldn't be too hard to do.

Ahh, OK thanks.  I figured I'd have to play around with it to get it to work.  Hopefully the sensor electronics don't do anything weird on power up but come to think about I could leave it powered up and just switch the signal?

It's not very clear from the ebay listing as to how the signal functions in regard to the sensor being blocked/clear, but your suggestion is doable.

If you arrange things so once the switch closes it connects the sensor signal to the BOB then it will work. You'd just to need to make sure the BOB input is set to pull-up/down to match whatever the blocked (or unblocked if using an index tap instead of an index cut-out) state of the sensor is.
For example, if using an index cut-out and the blocked sensor signal is 5V, then set the BOB to pull-up.
That way with the index switch untriggered/open circuit, the BOB input sees 5V. Then when the switch is triggered, the BOB input still sees 5V, until the sensor becomes unblocked and switchess the BOB input to ground.

Thanks mc!  You explain things so clearly even I should be able to get it working without letting any smoke out!

Hi ya’ll, is nice Mr. mc still on board??  I’m trying to make sure I don’t do something really stoopid with this pull-up/pull-down thing.  My latest grandson isn’t quite ready for pull-ups yet so I better get busy.  The other 3 figured it out for themselves so I reckon they’ll all be electronics whizzes.

I now have the hardware mounted solidly with a NO micro-switch adjusted to close about 3 turns of the stepper motor before the axis bottoms out and I mounted the opto on the motor with a half-circle disc on the motor shaft.  The opto is powered up with 5V and gnd from the C11 B.O.B.  The micro-switch is connected in series with the SIG wire from the opto and I stand ready to connect it to the B.O.B., pin #12.

I verified that the opto operates as shown in the spec sheet ie: sensor blocked .16v, sensor open 4.9v which is backward from the example given above by mc.

What’s the preferred method of hookup re: sensor/disc block/unblock, pull up/pull down.  I know, I know it’s real simple but my almost 65 yr. old brain confuses itself very easily.

Whether you use pull-up or pull-down won't make that much of a difference in terms of operation, the main thing is syncing the microswitch to ensure it switches well before the opto switches. Personally I'd aim to have the microswitch switch when the opto is about quarter (1/8th of a turn) of the way into it's untriggered half of the disc. That way the switch should be firmly activated by the time the opto triggers.

If you use pull-down on the BOB, you need to ensure the microswitch closes when the opto is blocked (at near 0V), so that when the opto is unblocked, it pulls the BOB pin up to near 5V.
Or if you use pull-up on the BOB, you need to ensure the microswitch closes when the opto is unblocked (at 5V), so the opto can pull the BOB pin down when it becomes blocked.

Either way will work, it's just that the microswitch trigger point/adjustment may make one way easier than the other.

Thanks for the clarification mc, much appreciated!

No problem.
I'm glad it clarified things, as the only thing it done was confuse me when typing it!

Maybe I am missing something but is there a reason using this method would be more accurate than just using an opto alone?
Index homing from an encoder and captured in hardware I can see being more accurate but doing it this way I cant.
Hood

Maybe I am missing something but is there a reason using this method would be more accurate than just using an opto alone?

I hope it does Hood.  Lotta wasted effort if not.  A "normal" homing opto reads the axis movement directly and would be pretty accurate depending on how good it is I suppose but I want to try it at the stepper to take advantage of the the reduction.

The way I'm looking at it, a lathe's X axis homing should be as perfect and repeatable as possible.  I figured with the ballscrew pitch, the belt reduction and motor/controller's half stepping the resolution should be very good.  The microswitch is adjusted to switch the opto in just before the end of the axis' travel and the opto will switch the B.O.B. then.  

Admittedly, I haven't set up the Mach homing config on any machine yet and there may be a land mine in the software that prevents it but I was thinking this should work OK, yes?