First layer height change at will?

I have a 8" Makerfarm i3 and I love the printer but the one thing that drives me nuts is that I can print great. Turn it off and back on and I have to recalibrate the first layer height.
Im not talking about .1-.3 off. Im talking about 1mm off or so.

Ive taken timelapse videos of the Zswitch and screw and neither move during prints so its not walking out.
I just did 6 prints the other day and didnt have to mess with it once because I left the printer on. Last night I went to print again and the first layer was about .9mm off and I have to re home the machine to fix the layer height.

I have the latest Marlin and everything prints wonderfully when I get the first layer planted.

Any ideas?

Do you have a glass bed or just PCB bending with the heat and.. ? Bed screws / spring problems? I once had the extruder /hot end bolts loose and I thought what the he** is happening with my prints.!

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I think its firmware related or something. Nothing on the bed or z switch moves or backs out.
Yes I have a glass bed on the PCB heater.

Does this happen with an off/on of the switch? How about off - 30 seconds - on? Or does it take hours for this to happen?

I'd be looking at the Z mechanism to see if there's any play.

I can power up and run for as many prints as I want back to back.
If I turn off the power to the printer and turn it RIGHT BACK ON its off. Sometimes I get lucky but 9/10 times I have to rehome again.

Ok. Last night I had a perfect print. Maybe 20 minutes to finish. Then right after I started a 1 hour print and the nozzle was about 1mm too low and it was crashing into the bed.
Nothing changed but somehow it homed that far off.

Just want to understand something: you mention having to re-home after turning back on the printer but having no problems with 9 straight prints as long as the printer stays on, doesn't your printer home all axis at the start of every print? Do you have a G28 command in your start G code? With re-homing are you referring to bed calibration or actual homing function? What you really mean is that you don't have to recalibrate your bed as long as the printer stays on, but once off and turn back on you need to recalibrate?

Yes it rehomes for every print. But I seem to get a range of Z home readings when the actual screw and switch dont move.
This is honestly one of the last things about this printer that I cannot stand. After this and replacing a LM8UU bearing im in love.

Perhaps its the switch?
I have another switch im going to try out and see how it holds a tolerance.

Better.

Does your switch have the lever still on or you took it off? If still on it might be the cause of the problems.

One method I'm using is two bolts each one with a wire acting as switch, only the top has a spring to maintain pressure but act on contact. Regular switches make contact only after you compress the spring, this way you make contact first and the spring is a safety feature.

Can you take a photo?
By default the makerfarm unit uses a ziptie to attach the z switch.

Tonight I drilled two very small holes and then screwed the switch onto the frame. It will take a few prints and days to see if it works.

I don't know the MakerFarm... I have a PrintrBot.

After many inconsistencies in prints, I went round and squirted hot-glue around every zip-tied thing (bearings & rod ends mainly) to stop the thing from moving. That tightened up my printer and prints considerably.

I hard mounted my switch last night and had the same results...
One print perfect and the next crashed into the bed by quite a bit. After a manual adjust I restarted the print and all was ok.