X is for Xref

I've been doing a bit of CAD drafting this week, producing a PFD. I find it easier to do this myself than employ a draffie to do it for me based on a hand sketch. There are those who believe that chemical engineers never draw anything, and some of my professional colleagues admittedly restrict their drafting activities to using MS Visio, but I'm basically competent with Autocad Mechanical. I don't use Autocad P+ID because it is too specialised (I do layout sketches as well as PFDs / P+IDs), and because it uses US symbology. When I did my chem eng degree, we were expected to be able to hand draft, and I still do a bit of that too.

I used to teach all of my students to use Autocad back when I taught process design courses. It's all very well academics telling students that they will have CAD Monkeys to draw for them when they practice, so they don't need to learn to draw, but CAD Monkeys aren't available in academia, (unless you count that least able member of the student design project team who gets to do all the scutwork).

Many of the academics who think it isn't worth spending the two hours it takes to teach basic CAD are happy to spend a whole module (or even two) teaching students how to use the Hysys, Matlab and Simulink programmes used in their research but unused by the vast majority of process design practitioners. Many chem eng courses don't even expose students to realistic P+IDs and PFDs, let alone layout drawings, as I have discussed before.

Before I get on to what an xref is, I've used a couple of terms "CAD Monkey" and "draffie" above. Draffie is a generic term, short for draughtsman (though there have been female draughtsmen for a very long time, as the 1938 pic above shows). There is a hierarchy of skill level within "draffies" - at the top of which is a "design draughtsman", a category into which I would place many "pipers".

Design draughtsmen may well have a degree or diploma in engineering, and will certainly (and more importantly) have many years of practical experience. They don't just draw what you tell them to - once they understand your aim, they can draw something better than you imagined. Some of these guys gave me the most useful stuff in my plant layout book.

At the bottom of the pile, there is the CAD operator, whose only technical qualification is in the operation of the CAD program. They may also not draw what you told them to, but their changes tend not to be improvements. The less their changes enhance the design, they more they are a CAD monkey.

Getting back to my drafting, I'm not even a CAD monkey when it comes to running the program. I taught myself how to (barely) use it, and enhanced my knowledge to the degree required to teach others basic competence. I did this only because Autocad drawings are the common currency of engineering, used to communicate electronically with design partners who may be anywhere in the world. Hand drafting is largely obsolete. Drawings no longer arrive in a cardboard tube. They are attached to an email, or picked up from the cloud.

The xref (external reference) function of Autocad is at the limit of my personal competence. It allows you to link to an external document from within a CAD file, stopping the file from getting too big to work with effectively, and making sure that the most recent version of the linked file is used, avoiding "Management of Change" problems. It is therefore a function which is based on an understanding of the practical issues of design in a professional engineering environment, and a term which belongs in my dictionary of terms used in professional practice.