Tool Length deviation after shutdown on ShopSabre 4896 ATC

I am having an issue with my router which is only 2 years old with part-time use. I now realize this issue was there from day one.
I would often being cutting too deep or too high by up to .040", and thought it was not Z zeroing after restart.
Then I discovered but checking my tool measurement values that I got skewed values if I re-measured after the machine had been turned off.
If I re-measure during operation the values repeat accurately within a few ten thousands and often exactly the same 5 times over.
If I shut down or E-stop the values will be off by up to .040" in either direction, all tools not just one specific one.
Tech support said it might be,
#1 bad surface on tool measure-it's perfect.
#2 dirty power coming in-checked in diagnostics not that either
#3 Not shutting down programs before turning PC off- I always do,even E-stop the machine before shut-down.

Any insights appreciated. I will follow up with ShopSabre but they were moving when I contacted them so they were a busy bunch no doubt.

I'm not familiar with ShopSabres. But to me it sounds like inconsistent referencing on the Z axis. How is it handled on that machine. Does it home to a limit switch when you turn it on? Are the axis motors servos or steppers? If they are servos, does it use absolute encoders or incremental? Is it homeing to an encoder index?

It has servo motors and homes to a limit switch. I will have to find out about the last 2 questions.
Thanks for the reply Todd.

Do you perform a G28 (home all axes) on every startup? Why do you Estop prior to shut down?

Yes I always home it on start-up. The machine won't do anything else. E-stop is just a habit I guess.

IS this an ATC machine?
Does it use the WinCNC tool measure switch?
Is there a conductive device to set the Z, either fixed or movable plate?
Are you using and G92 offsets on the Z? Do they toggle off when you home the machine?
Does hitting the estop dump servo power?

Other than turning off the power can make this repeatable?
Do you verify tool length by moving Z axis to Spoilboard?

Yes it is an ATC, 8 position rack.
It uses WINCNC and has a tool measure pad. I checked it for loose wires and such but looks fine inside.
To set Z levels I touch off on the spoilboard or work surface, no devices.
I use "Z" zero button on the control screen, I'll check tomorrow what the offsets do when I home.
If I E-stop the head stays up even if I turn the air supply off. It has air assist on the head. Again I'll check tomorrow.
Yes I often check tool length to work surface or spoilboard.
Wasn't sure what you were asking about repeatable but only way I get the deviation is to shut it off.

Sorry, no answers, just more questions.

How often do you "calibrate switch", and to what surface?

Have you ever brought that bit back to that location, commanded the bit to Z=0, and verified that it is actually there?

I went by the instructions and calibrated to the MDF bolted to the machine. I just repeated the instructions again this morning.
If I hit E-stop power stays on to the servo's, verified that by trying to turn the pulley on my x axis.
Yes I frequently do" Z 0 " to my surface to check height.
Even if I leave power on and turn the air off to the machine ( which then requires homing again ) I am still getting the deviation.
As follows machine on, measure tool= 7.2832 ,again = 7.2836
Turn off air, rehome measure tool = 7.2498 .0338" difference

For clarification...

When you say: "Yes I frequently do" Z 0 " to my surface to check height." Can you list the steps to doing that process?

I home the router
Tool measure
Touch off on spoilboard and press Z-Zero button on control screen
Move bit up and away and come back to same spot
Type in Z0 in input bar ,press enter and bit touches spoilboard within 2-3 thou . Easily visual error.

A couple things, and please understand that SS's macros may be different than the WinCNC versions but here is what I think:

If all your tools are measured accurately you do not need to re-measure any of them unless they have been replaced in the holder.

If your tools are measured accurately you do not need to touch off on the spoilboard, the tool should be set there already.

As long as there is not a user offset (green box) on the Z, you should be able to home all axes, then move the bit to the spoilboard as it will be at Z zero

Type in G92.1Z, press enter to remove user Z offset and then move bit to spoilboard

Any chance there is a loose connection in your Z or ballscrew? That is the only thing I can think of that would be affected by removing air pressure to the counterbalance.