I’m ready now to make the strutplates
I’m not sure what problem you are trying to show me. What did you do? What did you expect? What did you actually get?
I drived the router near the borders of the workvolume. Buth at the place where the 2 spoilboards comes together, there is a difference in height
How much difference is there? It doesn’t look like much.
Z error is hard to remove. Surfacing the spoil board (and a little extra) is one way to make it better. But a lot of tricks in CAM can make the tolerance acceptable.
For example, if you are cutting out parts, cutting an extra few mm isn’t going to hurt anything (except your spoil board). When cutting pockets that don’t go all the way through, you need to consider which reference face matters and set your Z nearby. If you are cutting mortises, then a little extra deep isn’t a problem and they are small, so you don’t have much error in a small area. If you are carving a design with a v bit, having a sharper bit will make the Z error have a smaller affect on the XY error.
MDF will “mushroom up” around screw heads if you don’t pre-drill and countersink or counterbore them.
You’ll want to get those screw heads buried pretty deep in the spoil board before you try to surface over them.
It looks to me like the screws are actually pulling the edges of the mdf down, so the bed doesn’t reach the surface near the border.
O they are deep, in fact my screws were to short. 
I’m gonna follow that idea. I had started a surface job. I wanted to stop it. I noticed disable the steppers with a Running router is a bad idea. The Z Drops down. Is there a better way to stop a Task?
I noticed another problem. Thé pc was already disconnected from the pc before the reset. Is it better to do a job with sd card?
Maybe you noticed : Cnc is totally new for me.
I’m experienced in 3d printing. Build my 3d printer 10 years ago and still printing with it.
That is hard core. Printers were not easy to use 10 years ago. Not many people who built them then are still using the same one.
You can pause, but there will be a few commands still in the queue, so it may take a minute to pause. Especially if the next few lines are very long moves. Then you can power off the router, and jog it to a safe parking place.
In an emergency, just kill the power. Your work may get ruined. But it probably already is if there is a need to kill it immediately.
Leaving a Gap between the spoilboards solved the problem.
Houston, we have a problem… I see shifting on the Y. I don’t now why…
Show us your Gcode.That is the typical issue.
I think I Found the problem: my cables in Collision with the belt. Last strutplate now. 
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