Flatness on both sides + Parallelism vs. Flatness on one side + Parallelism 1

[Vah1D]

Hello everyone,

I have a welded table with all the mounting pads on the top Blanchard Ground to 0.005".

My first question is:
I assume specifying a parallelism tolerance on mounting pads (relative to datum A on floor pads) is complete waste of money since the table will be sitting on uneven floor and in to order to achieve parallelism, we can just level ta table using adjustable legs, etc. Am I correct about that?

I also have a 1" A36 steel table top design that I want to send out for flame cutting & blanchard grinding.

My second question is:
I have designated bottom face of table top as Datum A. What's the benefit of applying flatness tol. on both top and bottom faces vs. flatness only on top face (while in both cases I have parallelism to Datum A)? The only thing I can think of is that in first case I can run int,o let's say potentially waved/curved bottom surface, so when we bolt down the plate to the the mounting pads, the plate will be forced to flatten a bit, then I will lose some of the flatness and parallelism that I specified? Is that correct? What if I just leave it on the pads without bolting it, what are the difference between table top#1 and table top#2?

Third question: the company I work for, specified on their standard, table tops should be "flame cut", "blanchard ground both sides to -/+ 0.005" tolerance thickness", and "-/+ 0.005" flatness and a parallelism". But from what I understand after reading some other topics, the flatness and parallelism in this case are completely redundant, and unless that tight thickness is absolutely necessary all over the plate, we are most likely paying extra to achieve 0.005" of flatness and parallelism?

Sorry if these questions are considered basic, but I'm relatively young designer and I want to know exactly the meaning of what I'm putting on my drawings!

Thank you everyone for taking the time to answer my questions.

 

[Burunduk]

I have designated bottom face of table top as Datum A. What's the benefit of applying flatness tol. on both top and bottom faces vs. flatness only on top face (while in both cases I have parallelism to Datum A)?

Usually, flatness is applied to the primary planar datum feature. Your bottom face is datum feature A. You can apply flatness to it, and parallelism to the top face. Parallelism applied on a planar surface will also control its flatness to the specified parallelism tolerance value.

Third question: the company I work for, specified on their standard, table tops should be "flame cut", "blanchard ground both sides to -/+ 0.005" tolerance thickness", and "-/+ 0.005" flatness and a parallelism".

If you work according to ASME Y14.5, there is a rule called rule#1 which relates the allowed form variation of a feature of size to its size tolerance. As a result of this rule, if the size (thickness) tolerance of the table top is +/-.005 then by default the flatness variations of the top and bottom surfaces cannot exceed .01 (the total size tolerance). As a further result of these limitations, effectively the parallelism variation can not exceed .01 either.