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.\u00a0 There are various terms for this, but I’ve seen Model Based Definition (MBD)\u00a0most recently.\u00a0 I personally am not critical of this idea.\u00a0 I am critical of moving 100% to this form of documentation without better support from our 3D CAD packages and ASME\/ISO standards.<\/p>\n
Models are generally considered basic.\u00a0 All this means is that the tolerance is derived from some “other” specification.\u00a0 This is normally in the form of associated Geometric Tolerances. To\u00a0fully define a part in MBD, you’ll need a GD&T scheme, often supplemented by traditionally dimensioning and tolerancing where needed.\u00a0 The difference is that if drawings are not used, this\u00a0has to be done within the model itself and then is somehow communicated to the manufacturer.\u00a0 The task to communicate this information to the manufacturer via the model is harder than it might seem as first glance.\u00a0 This is due to the myriad of 3D CAD formats and versions now available.\u00a0 GD&T information may not translate to other formats, such as STEP and IGES.\u00a0<\/p>\n
Additionally, any information that would’ve appeared on the drawings now has to appear within the model itself. \u00a0So, shortcutting the drawing step doesn’t mean one gets to\u00a0ignore the information that would’ve been\u00a0included on a drawing.\u00a0It just means all of that now needs to appear in the model.<\/p>\n
With that said, ASME Y14.41 supposedly standardizes this effort. \u00a0In my opinion (and yes I’ve read it and “own” a copy), it is lacking right now.<\/p>\n
If considering a\u00a0MBD 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\u00a0established with the manufacturer if they don’t have the ability to use the format where the GD&T information resides.\u00a0<\/p>\n
An alternative is to use\u00a0the drawing in conjuction with the model, which together provide the complete specification.\u00a0 In this case, the drawing will still\u00a0be the primary specification (usually for critical-to-function specifications),\u00a0but it makes use\u00a0model to complete the specificaiton.\u00a0 The model can either be basic, or used with some traditional tolerance.\u00a0 Where the model is basic, I’ve seen companies place a generic profile feature control frame in the\u00a0general notes.\u00a0 This FCF is\u00a0applied to the model for any dimensions that are unspecified on the drawing.\u00a0 If such as system is employed, it is important to clearly state this on the drawing to prevent ambiguities.<\/p>\n","protected":false},"excerpt":{"rendered":"