@graham-xrf
Most of the previous thread was diagnosing the calibration of an ultra sensitive level, which did not suit the intended purpose and current skill set of the owner.
Thanks
@Dabbler . You get it immediately what I was after. I never had a problem with calibrating. Finding the line that is level on a flat surface by rotating the level is easy. Turning the level through 180° and set back against the line provides enough. It can be used like that, if you keep in mind the offset. Or, you can start tweaking to get the bubble centered if it isn't already. I have three levels. The best is shown in the picture. The other two are lower cost Chinese with aluminium body. All are 0.02mm/m or about 4 arc-seconds). I need to get at least one a bit less sensitive.
What you say about the size/strength of the machine in relation to the accuracy it can hold is interesting. We come to the question of whether the machine simply "rests" on its supports, as opposed to gaining strength/rigity from being anchored solidly onto bigger stuff.
Starting with a lightweight machine, and by your guideline of 800lbs, mine are definitely featherweight, it will have some intrinsic strength to maintain it's shape even if just plonked down on a level surface.
It gains a whole lot more if it is hard bolted to "A" frame or other steel structure hidden in the bench under it. If the final anchor to the floor is by Lewis bolts, or resin-anchors into concrete with steel mesh and some rebar, the lightweight machine has become a heavyweight, and if the steel between it and the floor is designed right, it can also become a very stiff thing. I would not bother to go to such lengths. The accuracy(s) you mention are quite enough for my level of skill. Even so, any place I am going to bolt down a real machine is not going to rely on a springy set of floorboards!
The bench, if wood, has to be substantial, laminated, stiff, and of good design. I would not trust a lathe set down on a wooden bench that was not at least capable of having a level read the same from day to day, if set down against the same pencil line. As for floor. If it is wood, then consider at least setting the machine down on it's own strong single board, and perhaps putting some strategically placed bricks between floor and
terra firma.
In some ways, the bigger and more mighty the castings, the more one has to pay attention to the huge force effects of it's self-weight. I would guess that setting up "level" again when the machine has been working "bent" might look OK only for a short while, as the iron "un-distorts", and you may have to keep doing it over in smaller increments until things go stable.
Maybe someone with experience of having his iron distort from getting out of level can say what happens when he tries to get it to "go back".