Tolerance of assembly components logic

[Znjmech]

Hi All

I recently had an interview with a company and had to prepare a drawing sheet for a structure that was meant to be a support for another component like a winch to be mounted on it. this winch or a coil system had to be installed very precisely so there were pieces which needed to have well defined tolerances.

My mistake was that I didn't start from the lowermost part, which were four rectangular pads above which there were two long pieces of steel profiles were mounted (like a railway , parallel to eachother). instead started from the lower part of the steel profile and took different faces of it as a reference plane.

I was told that I had to started from the bottom of the pads, that also were supposed to also have some refining work, dimension to the topmost part, and then come back down, dimensioning part by part. cause this is the way a structure is assembled in practice.

While i think it is a correct statement, I wonder why it is not clearly indicated in any tolerancing or machine construction book or reference while everyone has to follow the same rule ?
or are there cases that this principle doesn't hold?

 

[Znjmech]

Is the question clear or I need to add more info ? please let me know, thank you very much

 

[emorrison]

Sorry, I'm having a difficult time picturing this. Any way you could upload a general sketch?