As we move further into the realm of 3D CAD software, something that is still catching on is the idea of driving all specifications directly from the model file, instead of having a separate drawing. There are various terms for this, but I’ve seen Model Based Definition (MBD) most recently. I personally am not critical of this idea. I am critical of moving 100% to this form of documentation without better support from our 3D CAD packages and ASME/ISO standards.
Models are generally considered basic. All this means is that the tolerance is derived from some “other” specification. This is normally in the form of associated Geometric Tolerances. To fully define a part in MBD, you’ll need a GD&T scheme, often supplemented by traditionally dimensioning and tolerancing where needed. The difference is that if drawings are not used, this has to be done within the model itself and then is somehow communicated to the manufacturer. The task to communicate this information to the manufacturer via the model is harder than it might seem as first glance. This is due to the myriad of 3D CAD formats and versions now available. GD&T information may not translate to other formats, such as STEP and IGES.
Additionally, any information that would’ve appeared on the drawings now has to appear within the model itself. So, shortcutting the drawing step doesn’t mean one gets to ignore the information that would’ve been included on a drawing. It just means all of that now needs to appear in the model.
With that said, ASME Y14.41 supposedly standardizes this effort. In my opinion (and yes I’ve read it and “own” a copy), it is lacking right now.
If considering a MBD program, just make sure everyone understands that the model is now the drawing; and that means it will need to be as accurately detailed as the drawing would’ve been; and since this information is now in the model, a method of communication will have to be established with the manufacturer if they don’t have the ability to use the format where the GD&T information resides.
An alternative is to use the drawing in conjuction with the model, which together provide the complete specification. In this case, the drawing will still be the primary specification (usually for critical-to-function specifications), but it makes use model to complete the specificaiton. The model can either be basic, or used with some traditional tolerance. Where the model is basic, I’ve seen companies place a generic profile feature control frame in the general notes. This FCF is applied to the model for any dimensions that are unspecified on the drawing. If such as system is employed, it is important to clearly state this on the drawing to prevent ambiguities.
