Drawings with GD&T callouts but no Basic Dimensions

[cmmguy75]

I see some drawings show gd&t without basic dimensions given. There is a little block onnthe drawing, says “basic dimensions are from CAD..”
My questions/concerns:
Most the time I’ve heard “drawings are amaster” so If now you rely on CAD.
The question: are all cad software reads/shows dimensions the same? Is the designer choosen 2 or 3 decimal on the basic dimensions? Should these numbers be allowed to round up?
As an inspector, we go with what are given on drawings then compare them with what on actual numbers on parts.
Another thing, now a day we see many drawings callout so crazy. Two surfaces, one at top and one at bottom, a perpendicular is used or an angularity is shown at one surface but without showing a reference datum... From these, I really want to see basic dimensions are shown on drawings when GD&T is used to ensure these calouts are correct and also help the designers/drafters know what they are doing.
For example, a FCF uses |A|D|F|, but there is no relationship between the feature to one of these datums. So if designers/drafters can get and put it on drawings.. then at least someone can figure it out.
What do you think/suggest?
Thanks,
Brandon

 

[3DDave]

Please delete the post I responded to if you are going to ask here as well.

 

[aniiben]

3DDave,
Could you tell us, please, where (which forum) this discussion took place?

 

[cmmguy75]

Yes, I deleted the other one.
Sorry for the confusion!

 

[mk3223]

IMO, the drafting specs on each company and/or Disciple can be different for each project. Instead using the generic terms, the Engineer may tailor the CAD software in some ways to fit the design needs, such as the units, decimals, etc.
In a perfect world, the design drafting would be clear for the content as a basic requirement. In the meantime, if the fabricator or inspector have any questions regarding the design drafting, they should send the request of information to the Designer for clarification before moving on the work.

 

[cmmguy75]

Thanks!

 

[Tunalover]

At one time there was a drawing note that read:
DIMENSIONS LOCATING TRUE POSITION ARE BASIC.
So, in other words, if a dimension leads to a feature with a geometric control then that dimension is basic. Any other dimension not enclosed in a rectangle is a bilateral tolerance with the positional tolerance depending on how many decimal places and shown in the boilerplate notes in the SH1 drawing format.
This practice could also be applied to a MBD where there is no drawing at all.


ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993