Going past the z limit for tool change

I am retrofitting a mill that has a atc that works a bit special (the machine is an Emco VMC100). It works as follows:

The spindle goes up past the z limit switch until it reaches another limit switch, doing this first the tool holder clamp is mechanically released and then the tool holder pulled out of the spindle. Once the second limit switch is reached the turret is free to rotate.

Can linuxcnc go past the z limit or do I need a relay that deconnects the z limit switch while changing tools? I think I've seen it done this way with a retrofit using Mach3.

Kind regards
Martin

If you have a hal-pin that represents if a toolchange is active you could use simple logic components in HAL (AND, OR, NOT) to disable the signal from your physical limit switch being connected to the z-pos or z-neg pin.

Deleted, double post.

Disabling a hard limit switch would be easy, The trickier part is how to make the soft limits happy. I know some ideas on this subject have been discussed on the forum in the past. I do not know if the person(s) originally asking the question ever found a solution they were completely happy with. Some of the proposed work arounds involved adding a move with an alternate axis (such as W) or using an offset component to extend the Z's travel. I am fairly confident I could make either option "work" but I've never done it my self. (Heck, I don't even have a machine with a tool changer running Linuxcnc. Maybe someday I will finally get annoyed enough at it's original control to get around to retrofitting our old SCM Routomat.)

Thank you both for your replies! I think I haven't really understood how linuxcnc handles soft limits yet, I didn't even take the soft limit problem into account before.

As far as I have read the ideal case would be to have at least two switches per axis, a soft limit one and a hard limit one, however this is not the concept of this machine, it has only one switch per axis and they are all inductive sensors. That's all I have and there's not really a good possibility to put on additional ones.

In addition I know the concept of homing only from portal mills, but the bed mills I know do not home to a specific location the way that they have to move there to meet a switch once you turn them on. Basically they are happy as long as no end switch is hit. I do however not know how this specific machine handled it with the old control, not having end switches on both sides of each axis it probably did home but I don't know.

You don't really need end switches at both ends for soft limits to work. But for them to work you must home the machine if the soft limits are set up correctly after you have home you should never run into a switch or the other end.

The axis will do a controlled slow down just before the soft limits..

linuxcnc.org/docs/devel/html/config/ini-homing.html

What about using external offsets?
Does that allow for going past limits?

linuxcnc.org/docs/2.8/html/motion/extern...ts.html#_soft_limits