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Model Based Design Overview

Model Based Design, sometimes referred to as Model Based Definition ultimately in simple terms comes back to reducing the reliance on 2D drawings to communicate a parts PMI (Product Manufacturing Information). There are many resources on the internet which do a better job of explaining it in academic terms. This article is focused on explaining it from the perspective of a Creo Parametric user.

Unless you've been using Creo Parametric (aka Pro/ENGINEER) in a very unusual manner, you will have been making 3D models first and then likely documenting them second. Also unless you're very unusual, the typical process involves, taking the 3D, detailing it in a 2D Creo drawing but the master deliverable is likely considered the 2D PDF file. When inspection comes in and perhaps in worst case there is some kind of contact dispute, the 2D drawing is what the lawyers refer to regarding meets spec or not. All of this abstraction is of course done with the sole purpose of reproducing repeatably a physical part to your spec.


When i've presented MBD to Creo Parametric users, there's usually an interesting split in the room. Some folks fall into the first adopters, some are totally against the idea and of course the majority of folks sit squarely in the middle. Let's be honest for hundreds if not thousands of years a drawing has worked pretty well when it comes to doing this so why should you change? So any MBD initiative is going to be met by a laundry list of reasons why it won't work. In all lies there is some truth however so you'd better get your story straight.


Luckily for some time the standards committees have worked hard to produce something you can measure yourself against. ASME Y14.41 2012 is a fantastic read (really) and helps to flesh out the kinds of PMI you're going to need to teach yourself how to apply in a 3D world. It won't tell you how to do it in Creo of course, you have to make that leap yourself.


When working in a 3D mindset, metadata, parameters become more important. You want to capture information in a structured manner versus just floating a note over the top of something else. A core concept is semantics (i.e. the meaning of something, the standards talk at length about the need to be more semantically accurate in MBD than you probably were when making 2D PDF's). In Creo terms that means filling out a proper material, working in the proper units, entering the proper parameters.


Features would seem an obvious thing for any user of the worlds first production quality feature based solid modeller but regarding MBE, the key take away is that it pays to model the details you want to communicate. You're no longer going to draw a box in draft entities in the drawing to represent that label placement area, instead the model has to have that feature. The standard talks about features in a more semantic manner.


Dimensions also have a subtly different meaning when it comes to the standards use of the word. In the standard it implies the actual size of something versus the inputting parameter that Creo puts into it's model recipe to produce the geometry. This is important because you can model your Creo model at Nominal, Lower or Upper tolerances (or indeed anything in between if you're really pushing it). For that reason you'll need to tell someone receiving the model exactly how the model is represented (so they can make adjustments in their CAM program for example).


Geometry is pretty straightforward, any user of Creo will relate.


Annotation states are described as Annotation views in the standard. The best way to think of this is like it replacing a 2D drawing view. Like a drawing you can set up and control various aspects of an annotation state (prior to Creo 2 this was simply referred to as a Combo Rep [or an All Rep]). Each Annotation State is intended to communicate some or more aspects of the model. I like to describe the sequence of Annotation States as being the models story. All stories have a beginning, a middle and an end of course and it's the modeller/design/engineers job to populate that story.


Annotation Planes are planes onto which Annotations get placed and oriented. As a detailer, you're going to want to place Annotations onto planes such that they can be read without additional work on behalf of the viewer.


In Creo 2 the Annotation Feature is the feature into which the bulk of your PMI will get created. Until Creo 4, the Annotation Feature is the only way to impart additional (beyond direct references) semantics into your Annotation Element (An Annotation Feature is kindof like a collector of Annotation Elements).


Annotation Elements are the PMI that you wish to communicate. The elements themselves have the describing part and also the semantic referring to part. When highlighting the Positional Feature Control Frame below, see how the datums it refers to are also highlighted. PMI as it relates to models is thus more readily communicable than a drawing based scheme.


A big peice of anyones process is going to be handing off the design data either for review or to a supplier. It was really easy when you just sent them a 2D PDF but in the MBD world, you have many more options. At time of writing there are pros and cons to each from the perspective of a user of Creo Parametric 2.0. 3D PDF is probably going to 'win' if history has any say as Adobe Acrobat reader is everywhere. However the ISO STEP AP242 standard has won over a lot of proponents and when all the CAD systems support MBD this may be a perfectly valid and smooth exchange format.


MBD is still a somewhat work in progress for PTC though as from a user perspective creating PMI in 3D in Creo Parametric 2 is less intuitive than equivalent PMI in the 2D mode. Creo Parametric 4.0, expected out in 2016 promises to address the majority of current gaps with ASME Y14.41 (MBD) and ASME Y14.5 (Dimensioning and Tolerancing).


Next Steps>>>

This article talked about the abstracts of MBD, the next one talks to the tools that may make up a tech stack to support MBD in a business.


 Tech Stack>>