OpenPnP with LinuxCNC via linuxcncrsh?

So many times you see people refer to zero as home. Home is where the home switch is. Z zero is the face of the spindle flat on the table, and we never want to go there.

plhsystems wrote:

Right, I see, you consider home in Z to be fully extended, I think opposite, so I see why you think homing Z first would be wrong.
 

No, I do not consider Z home to be extended. I consider Z home to be center when controlling 2 heads.

If you have no absolute positioning for Z then your concern is not knowing whether it is up or down and having it crash into something  while homing X and Y. Well if the machine does not know whether Z is up or down, how does it know which direction home is?

That answer is absolute position via potentiometer to either disable X and Y homing until Z is in home range (retracted) via potentiometer feedback. Or I just realized after I mentioned the potentiometer that the pot actually makes it so you can home Z first without worries since the direction is known. The suggestion to home Z last was if you only have limit switches.

tommylight wrote:

-Why would anyone assume the Z axis home to be down? It is up, always and on every machine i came in contact with.
-Even if using pneumatic Z axis, it should have homing disabled if no air pressure
Or am i missing something?
I have yet to see a machine, any machine that does not home Z first.
I am sure there might be some strange machine that does that, but i can not think of one.
 

Yeah you are missing something. We are talking about homing 1 motor that controls 2 heads opposing each other. On a Mill it's pretty much guaranteed that the direction Z will move during homing is the safe direction. That is not the case here.

 image missing https://www.betztechnik.ca/uploads/2/5/6/7/25674051/s148937386964792398_p34_i67_w640.jpeg

The pnuematic thing does not equate. Think of a see-saw where home is center Advancing positive retracts 1 person and advances the other person. Homing negative does the opposite.

If you have switches at the extremes of both sides then one head is fully extended when the other is fully retracted. If XY is in some willy nilly position how do you home Z and guarantee 1 nozzle does not hit something on the table?

If your switch is in the center, how does Z  take the shortest path to center if the machine does not know where center is because it has not been homed yet? If a home switch is in the center how does it find it without hitting limits first and going back to my last point?

In the above cases, I think it's safer to home X and Y to a neutral position first, maybe that Z center switch dissallows XY homing unless it is tripped, that is another safe option but you would still want to bring X and Y to a neutral corner first. The only time I think it is best to home Z on this type of head first is if you do have absolute position feedback on it.

So I mentioned the Potentiometer for absolute position feedback. In that case it actually might be best to home Z first because you can know exactly which direction it takes the shortest path to center, there's no posibillity of extending one head during Z homing unless the pot broke. If the pot breaking or misreading is a worry, than you can get into all kinds of complicated scenarios with both a pot and a center switch but I'm sure you get the point.

I think I would still home it to the top and then use your work offsets to select which extruder was in use. Any height difference could be accounted for then 
And yes home Z first.

rodw wrote:

I think I would still home it to the top and then use your work offsets to select which extruder was in use. Any height difference could be accounted for then 
And yes home Z first.
 

Extruder? I think you completely missed the point.

It's like an echo chamber in here. Understand the issue first, then offer suggestions

LinuxCNC would know which direction to home dependent on whether the home switch was activated or not when homing commences.