Thor, or should that be Zeus?

[Chaz]

Hi Chas
Sorry but i can't get my head round how you are going to fix the rails to the concrete or set the rails perfectly straight and co planar prior to injecting the epoxy, could you run through it step by step (for a dummy)
Mike

Sure. It's probably a slight different way to do it, but Ill try and explain as best I can.

1. Cast the concrete, try and get it reasonably flat. Is it 'good enough', no.
2. Clean the concrete, grind the area to prepare for the DWH putty (or liquid, there are 2 options).
3. Place the putty along the location where the rail will sit (so in my case, roughly 50mm wide x 2 lengths).
4. Place a reference surface against the putty. The reference surface has to be perfectly flat (well, to whatever tolerance you need / want). The reference surface should have a release agent so that it does not stick to the putty.
5. Putty dries, remove the reference surface, the putty is now hard and has formed against the reference shape.
6. Either drill holes or remove the material used to block the holes (wood in my case). The putty would have cured over this.
7. Epoxy the M12 nuts into place, using the rails in location and putting a release agent around the bolts and threads of the nut.
8. Wait for the epoxy to cure, then tighten up.

The 'key' here is the reference surface. This has to be perfect. Ideally the X and Y are done by the structure and at the same time. Im still working on how best to do this. I have a granite surface plate which Ill likely use. I can make some surfaces on my mill (perhaps not as accurate) or I can cast putty against mild steel which is then 'correct' against the granite surface plate which is then in turn used for the machine.

Here is a vid of similar. They use a putty, drop the surface onto it. This is moglice .... not the same, but the principle is the same. The putty will form a 'perfect' copy of whatever you place against it and its as hard as concrete once cured. Its fairly expensive stuff. You also get it in liquid form which can be poured between two surfaces.

https://www.youtube.com/watch?v=BZuEH4nyBtw at 4min35 you will see what they have, once they lift the top surface, is a perfectly aligned surface. Once cured, its as hard as the concrete and has a similar hardness rating.

https://www.youtube.com/watch?v=dKdanHZmV_k this is a similar idea. The key is to have the 'master' to reference against.