Will steel angle work for a decent machine?

[routercnc]

amd7000

I agree with Silyski. Reviewing what you want to start with:
> 50mm angle iron baseframe and gantry
> Simple bolt together, no lathe/mill etc.
> Steppers and electronics from (big) photocopier
.
And what you want it to do as a specification:
> 1200x800mm
> Cut wood, aluminium, possibly thin steel
> Hold 0.05mm tolerance
> Be a 'decent' machine
.
I think you cannot even come close to meeting that specification with the approach you are looking to take. So you have 2 choices -
1. Make a simple 'learning machine' with the 'wrong' bits and get yourself up to speed with what is important etc. Be prepared to write off the hours and costs involved. There is something to be said for this if you go in with that in mind.
.
2. Actually build a machine capable of what you want (and by the way 0.05mm is tough at this size, even if you do a really good DIY build) by getting some of the bits made for you.
.
My experience is that I have made 3 major evolutions of cnc machines over the years, with tens of minor improvements to each of them along the way, and I can tell you that I started with a machine a little better than you are proposing (but would not come close to what you want it to do) and worked my way up to a pretty solid machine now (which would almost match what you want to do).
.
Hope this helps and I know it can be disappointing to be told that something won't work but it is said with the best intentions of having been there, or done that, or seen it done that way and failed.