How do I rest z axis

[JAZZCNC]

At every powering on the machine should be homed. Period.
Suppose you have micro stepping drives and when your machine is stopped at the end of your day, motors are at "some" micro step position.
When you switch power off, motors will spring to the nearest FULL step position, defined by the magnets in the motor.
As drivers are not intelligent enough (read: dumb), they will at power on be at the full step position, and the result is, you may guess :) : unreferrenced machine.

Lazynes for clicking at one button after power up?

The problem is his machine doesn't have any HOME or Limit switches so he can't send HOME.

When setting the machine limits, the only way I know to reset the z axis to zero is to press 'ref all home' which sets everything to zero.
The problem is that when I switch the machine off the z axis drops a bit so the next time I switch on I dont want to have to reference everything again. So how do I only set the y axis for machine limits (not workpiece offset)?

Grant, without HOME switches you cannot get around this problem. Like ZASto mentions if the machine had HOME switches fitted you would and should HOME each axis when starting up or after every E-stop for the reasons ZASto stated above.

When you are pushing REF ALL HOME now what you are actually doing is setting MACHINE coordinates to ZERO, because you don't have any switches on the machine the DRO's just change to ZERO's but if you had switches fit then when push REF ALL HOME each axis would move towards the Switch on each axis then ZERO the DRO's.

Also, with HOME switches you will always be able to accurately find WORK ZERO again if something goes wrong or you have to E-STOP.

At the moment you have issues you don't even know you have and HOME switches will partly help with this.?

Because the Y-axis uses 2 ball screws you have no way of knowing if the gantry is square, so HOME switches will allow you to set the gantry square.

However, worse than this and highly likely to happen on your machine is if one motor stalls and the other keeps moving the gantry will twist up, this can very easily bend the ball screws if it happens at higher feeds, which is often the case because torque is lowest at higher rpm's.

Now, HOME switches won't help with this but they will allow you to straighten out any twist in the gantry after it happens.

To deal with handling the stalling motor requires extra wiring in the control panel and feeding signals back to Mach3. If the machine was built properly this should have been done but as you know it's not so you'll have to do this and I FULLY recommend you do it sooner rather than later because it will happen at some point without a shadow of a doubt.