So meanwhile I bolted the 10mm steel plate to the frame of the 6040. I have used the holes in the frame that have been used to bolt down the extrusion profiles.

I had to redrill 2 holes a bit bigger as the screws did not fit. I guess my fault as I did not have a good workspace for the marking and drilling of the holes. But overall I was happy with the result and the 6040 around 30kg heavier as before.

I already feared that the plate might not be perfectly straight and it turned out that there was some bending. But I don´t have the tools to straighten out a 10mm steel plate.
If you look from the side you can see the bending. It does not look that much, but from the front to the middle(high spot) of the plate it is nearly 1mm difference.

So I had the silly idea that the router could simply mill the surface flat.
Well, there are some problems with that:

1. The work area according to Mach3 is not 600x400mm. It is 566x385mm. So you have to live with the fact the everything outside is not flat and cannot be used to clamp something down properly.
2. This is a router, not a mill. A mill runs much slower and usually has a gear that provides enough torque to cut steel. The router spindle is a direct drive that has nearly no torque at speed below 5000rpm.
3. Which leads to the next problem. The High Speed Steel(HSS) mill bits are not designed to run with 5000 or 10000 or more rpm. As the ER11 collect cannot take any bit with more than 7,5mm shaft, I had only a 6mm flat end mill bit. The optimal speed for the size of the bit to cut steel is between 1000 and 1500 rpm depending on the stell and the lubricant/cooling. So this cannot work. Yes, there is carbide and others, but nothing really designed for cutting steel at 10000rpm and more in this size. 
4. There is a lot of force required to cut steel. I tried it anyway and thought I could take off maybe 0,1mm at a time until I am done. But the machine started to rattle as soon as the mill bit caught on the steel. The spindle was moving like crazy and I don´t mean the collet, the whole spindle.

So the result was not very good and I tried different speeds and feeds. But as the first screws dropped down from the z-axis assembly I knew that this was going nowhere. You can see the tool marks the wobbling spindle left on the surface.

I gave the idea up for now and I going to try to mill a flat surface on the fixture plates that will be bolted to the steel plate.

The holes you can see are actually drilled by the router. I used a center drill with a 2,5mm tip to drill those holes. And surprisingly that worked well with the help of WD40 as a lubricant.

In the next step, I tried to drill 4mm holes(at the time I had no 4,2mm) for preparation for tapping M5 threads. This was when I realized that not only drill bits are not designed to drill with that high rpm´s in steel and the spindle was stuck sometimes as even with 5000rpm there was not enough torque to drill holes in steel.
Another problem was the feed from the z-axis. The z-axis stepper motors lost always a lot of steps, so that mach3 was thinking the drill bit is at -10mm, but it was not even at -2mm.

I stopped the drilling and tried to think of a better way with the tools that I got. Also gave me time to order some 4,2mm drill bits.