So I have made a small amount of progress this evening. With the laser mounted solidly to the mill head I was able to map the my machine frame. I clamped the laser solidly then left it well alone. This meant the machine frame had to be adjusted to the laser plane, which is much easier than vice versa. I lifted the frame at 3 points and shimmed each to get the best coincidence of the two planes, this was just done with a steel rule with the end on the frame and reading off where the laser hit it.

This is the frame and I'm trying to create a planar surface across the top two beams:

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I then mapped the surface, I have about 1mm total peak-peak error, basically each top beam slopes down 1mm from end-end but in opposite directions. This isn't too bad.

I also took delivery of a little Makita trim router, I think it is the same sort as used on some peoples cnc routers. Anyway for £100 it is a very nice piece of kit, no measurable up-down play in the spindle, I also got a 6mm collet for it.

The levelling process I'm going to attempt I outlined earlier in the thread, but basically it involves using a trim router and a laser aligned router jig to mill down material fixed to the top the frame to bring it into plane.

So what material to use? I mixed up some metal polyester filler of the type used to fix leaks on engine castings. This stuff has some nice properties but it is too brittle and I think too soft (definitely softer then unfilled epoxy).

So what about epoxying an aluminium strip to the top of the steel frame and machining that? Would probably work but more effort, I checked the little Makita is happy milling aluminium and it is fine.

Whilst I had the router in hand with a 6mm carbide endmill in it, I thought I may as well see what it can do against mild steel, not expecting anything other than chatter and heat. Anyway it cuts it like butter, very cleanly. I only tried 0.5mm and 1mm DOC and it was fine and this is within the range I need for levelling this frame.

So I will pick up some 30mmx4mm flat bar stock tomorrow and weld it to the top of the frame where the rails will be mounted, then build the laser/router jig and hopefully have a planar frame very soon.