Z axis retracting too far on initial zero setting (S5P)

Brand new user here. Brand new, unused machine bought used and out of warranty/support period.
Things are basically going great so far, but having a problem when setting zero for a run. I jog the machine into position at the corner of the stock and use the paper technique to jog the cutter to near contact with the stock. I then hit the “zero all” button and the cutter moves to the bit height setter to do its thing. As it retracts there is a grinding noise at the top of the retract motion. When the cutter returns to the start position it will cut about 7.5 mm too deeply unless corrected. Going back to the zeroing procedure and repeating the process yields a correctly set zero and the job can proceed to cut as desired.
It seems that the Z axis is trying to retract farther than it can actually go on this first zeroing attempt, throwing off the zero point. (I hope the grinding isn’t doing permanent damage, so far I don’t know how to stop it)
Does this mean that the Z upper limit switch is in the wrong place, or is there a software parameter that I need to adjust?

Thanks for your help!

If you are getting a grind noise there is a mechanical problem. That causes you to loose steps and that is why you are cutting too deep. Lubricate the linear rails with Mobil Vactra #2. You should remove the Z axis to lubricate the bottom screws but just squirting some oil in the top and wiping the rails with the oil will work unless it is dry.

Fix whatever is causing the grinding and you should be good. There is coupler at the end of each stepper motor that can come loose. Check that all nuts and bolts are tight and check for square of the machine If the machine is out of square that can introduce mechanical issues.

OK, I’ll check that tomorrow-- I really should go through and check all the screws for tightness anyway. The machine seemed square after I moved it here. . It definitely seems like it’s trying to raise the Z farther than it’s intended to travel. The machine has less than an hour use on it. I’m not sure why a mechanical problem would only show up the first time I set the zero and not the second.

I had the same issue with my brand new machine, but only after I installed the gas strut. I worked with support, they sent me a new one, which resolved the issue.

Do you have a gas strut installed on your machine? If so, remove it temporarily and see if that solves your issue. Just a thought.

Do definitely check and tighten all of the screws. I ran a project yesterday and a small black screw fell out towards the end. It was one of the 3 screws holding the ‘slide’ on the Z-Axis. Then another one fell out today. It was a silver one holding the Z-Axis plate against the Z-Axis mount.

Since your second hand machine was unused, you should have a small amount of oil in one of the accessory bags along with some droppers. There is always time for lubrication.

Yes, I installed the gas strut kit that came with the 2.2kW spindle. I can try taking it off-- maybe I’ll try support on Monday, as I did buy the spindle kit new from them and it should be covered.

Got it on screw tightening! Do you apply loctite to the problematic screws?

Do not use locktite. Locktite gums up the works. If the same screw keeps loosening you could apply but only REMOVEABLE. Never Permanent. Permanent requires heat to release it and fire and Shapeoko do not mix.

Careful when testing without the gas strut. I didn’t know you had the big spindle. I think it requires the strut or the Z-Axis may drop. Better see what support says.

When your Z retracts do you see the red light come on the homing switch? Maybe you need to adjust to down slightly.

The action is smooth all the way to the top with the gas strut detached. I’m thinking the strut is the problem at this point. I have a call in to support to talk about it.