workpiece touchoff getting lost (v2.6.4)

andypugh wrote:

I have been just trusting the system to not start the spindle when I am doing a toolchange. I agree that that isn't necessarily ideal.
Perhaps a physical interlock switch to the spindle drive is a good idea?

maybe i'm paranoid but i would not trust the computer and controller box / VFD to not take off... which is why i always press the estop before I change tools. I'm continuing to do that, which should be ok as long as no axis is moving... so i just have to make sure all axes have stopped moving, then i press the estop and then i change tools.

If your e-stop doesn't disable the axis motors, and doesn't cut power to the spindle, then it really isn't doing much at all anyway and you really aren't much safer than if you just paused Linuxcnc.

If your e-stop does disable the drives, then you really should need to rehome after using the e-stop.

If you want a safe tool change, add a manual physical disconnect switch either between the drive and motor or on the drive's power input. and change the tool with Linuxcnc paused, spindle stopped and the power to the spindle disconnected.

The only way you can have the drives disabled and be sure not to loose position is to add encoder feedback, (either switch to servos or...)

Todd wrote:

If your e-stop doesn't disable the axis motors, and doesn't cut power to the spindle, then it really isn't doing much at all anyway and you really aren't much safer than if you just paused Linuxcnc.

If your e-stop does disable the drives, then you really should need to rehome after using the e-stop.

If you want a safe tool change, add a manual physical disconnect switch either between the drive and motor or on the drive's power input. and change the tool with Linuxcnc paused, spindle stopped and the power to the spindle disconnected.

The only way you can have the drives disabled and be sure not to loose position is to add encoder feedback, (either switch to servos or...)

the estop is connected to the motherboard in the controller box which controls the VFD and drivers.

i'm not sure whether it just tells linuxcnc that the estop has been pressed, or if it internally in hardware disconnects the drivers / VFD as soon as you press the estop.

when i press the estop i see that in linuxcnc the power is cut (i.e. i have to release the estop and press f2 to power the machine again).

i could turn off the VFD... but turning it off and on all the time might not be healthy for its lifetime?

i'm trying to understand at a technical level why disabling the drives or motors would make them lose their position?

When your machine is in e-stop, can you turn the step motors by hand?
If you can then they are disabled. If you can't then they are still powered up (and your e-stop isn't doing much more than pausing Linuxcnc).
With the drives disabled you can easily shift an axis a small (or large) amount while you are changing tools. Or gravity could do it for you. At the very least the motors could shift a step as they naturally align to their un-powered magnetic poles, then when re-powered might move to the next or previous closest position.

BigJohnT wrote:

bschiett wrote:

i'm trying to understand at a technical level why disabling the drives or motors would make them lose their position?

When the axes are unpowered the Z for example may drift downward. On my Hardinge CHNC Lathe when I power the drives off as the oil film gets thinner the axes will drift a bit. It's a servo with encoder feedback so LinuxCNC keeps track of the new position. For an open loop stepper system you could lean on an axis and move it and the controller has no idea when you power up. The simple answer is to home the axes after power up every time. That is the reason LinuxCNC had a volatile home option for open loop machines and you should be using it on an open loop machine.

JT

thanks, so VOLATILE_HOME=1?