CAD Craft

A resource to CAD Craft specialists, and a showcase of my work — in both material and virtual worlds

CAD: Process & Technique

Some users and others are coming to CAD fairly recently, and may be unfamiliar with its origins, how it got to be what it is today. Having been in the design field since well before its introduction, I thought I might be able to share some insights. This is pertinent because in many instances, the actual process and mindset one needs to bring to the CAD process is ensconced within the process of its evolution.

For an iteration of the technological/corporate developments is CAD, I have found The History of CAD to be an exceptional resource. One of the keys I found intriguing was in his section on 1970-1980, where Bozdoc asserts:

“MCS was founded in 1971 by Dr. Patrick J. Hanratty. Since the day it was founded in 1971, MCS has enjoyed an enviable reputation for technological leadership in mechanical CADD/CAM software. In addition to selling products under its own name, in its early years MCS also supplied the CADD/CAM software used by such companies as McDonnell Douglas (Unigraphics), Computervision (CADDS), AUTOTROL (AD380), and Control Data (CD-2000) as the core of their own products. In fact, industry analysts have estimated that 70% of all the 3-D mechanical CADD/CAM systems available today trace their roots back to MCS’s original code.”

I consider this to be important because, as many CAD polyglots will tell you, there are certain challenges which seem largely identical across certain types of CAD apps. This is also pertinent in that it further underscores the veracity of the premise that what you learn — in terms of technique/process — on one system is generally mappable to others.

One of the challenges we face as an industry is that so many of those coming into CAD do so without a grounding in drafting fundamentals, things those who started on the board could not have begun without. To the new wave, this may seem like old school grousing, but, as I am about to show, fundamentals are an essential foundation — in CAD, sports, language…in life. A key source in the genesis of CAD was an effort to deal with those who had challenges visualizing in 3D.

I will engage this more deeply in another related post, but for now, to the craft aspects. Most CAD jockeys who have worked in the aerospace/DOD world know that GD&T(geometric dimensioning & tolerancing — ASME Y14.5) is a crucial requisite tool, both conceptually and in practice. A simple example of this can be shown using the concept of datums.  Even if you don’t use GD&T directly in your CAD work, it is implicitly built into the software. A critical baseline in design & machining is the premise of the datum — generally a feature or surface of the part. Briefly, any 3 dimensional space is characterized by 3 planes — Front, Top & Side. A datum, in this case a datum surface, is one which has been identified within the drawing/model as being ‘synonymous’ with the ‘root’ datum plane by means of establishing relations with it — parallel, coincident, etc. Once this datum surface has been sufficiently defined such that it’s position/orientation is fully constrained, it becomes datum A, designated by a flag/box(on a drawing) extending from an extension line from its edge.

All dimensions taken from this edge or surface are thereby referenced to this feature as a baseline. This is important to remember, because a common mistake for the inexperienced is to select a datum which may be convenient, but which ends up being machined away — actually or virtually — in the manufacturing process. When this happens, it creates a problem for the machinist and inpsection as there is no no reference from which to base any subsequent measurements.

In the model, say in SolidWorks, this problem occurs often in a variety of ways. Say you have created a series of extrusions & cuts(Fig. 1). You then realize that you need to rollback(go back in) your design to add a feature you at first overlooked. Say this feature is a cut which eliminates a feature a later extrusion/cut was based on(Fig 2).

Figure 1

 

Figure 2

If you are looking at your Feature Tree, you will notice that the last feature you created now shows as an error, once you return the rollback. This is because the flat upper surface(highlighted green), which was selected to make the hole, can no longer be ‘found’ by the program. This is an example of what happens when a datum is machined away. How would an inspector determine if the hole was perpendicular to the surface which is now no longer there?

Similarly, as one gains experience in CAD, the machinists become our friends. Just like the phrase “If the cook’s not happy, nobody’s happy,” I feel this adage applies to the machinist in a manufacturing environment. It’s worth noting that CAD is not a stand alone tool/prmise/concept — it was always conceived as a partner in CAD/CAM = “Computer-Aided Design” & “Computer-Aided Manufacturing(or Machining)”. Consequently, as we plan/think through our designs, it’s important to factor in some critical elements.  So-called new catch terms like “DFM“, “DFT“, etc. are simply retreads of old standard best practices. One of the first things I do when starting a new contract is go down to the machine shop and introduce myself. With humility, I enquire about their machining capabilities, from the complexity of their multi-axis Sigma machines to the range of tooling they have, say for deep hog out cuts. Often, it doesn’t matter how creative/innovative your design may be, or, in fact, how many problems it solves, if, in the the end it is discovered that in order to fab your part it must be sent out to another shop, at greater cost.

For these and other reasons, it is most often the forethought and research  a CAD jockey does before they ever touch the mouse which most significantly impacts the simplicity of their model, and the efficiency of their designs. While 3D visualization my not be immediately critical here, its connection is implicit in the process of what we do, most certainly at the assembly stage.

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