IGES Draughting Concepts

(The URL for this page is http://www.WiZ-WORX.com/eng_dwg/)

Day Books: 20-Oct-95 - Freeman Lake

Today I found some plots I made, oh, probably ten years ago. I was working for Applicon in Ann Arbor, Michigan, and needed some illustrations to "raise the consciousness" of my management as to the problems of IGES translations. I made these GIF files by scanning prints made on a DEC LA-50 dot-matrix printer (72 d.p.i. resolution), so the quality Really Sucks. (Sorry about that, chief!)

This Web page is based on numerous lectures and presentations of the material to various audiences over the last ten years. These figures were used with overhead slides, and usually distributed with the hard-copy minutes of meetings.

Since some of the IGES community is from the United Kingdom, IGES discussions frequently use techno-babble derived from the Queen's English, such as "zed nought plane" for "Z-zero plane" (the XY-plane where Z has the value of zero) and "draughting" instead of "drafting".

First, some basic terms & concepts …

Product Definition Data (PDD) - This is what it's all about. Designers, engineers, and manufacturers use computers "to describe and communicate the characteristics of physical parts".

CAD/CAE/CAM a.k.a. CAx - I will use the term "CAD" to refer to Computer Aided/Assisted Design/Draughting. (A moment's thought should convince you that there are at least four "definitions" for the acronym "CAD".) The earliest Etch-A-Sketch® computerized draughting systems have evolved to suit the needs of Engineers and Manufacturers as well as Draughtsmen, so we also have "CAM" (Computer Aided Manufacturing) and "CAE" (Computer Aided Engineering).

Digital PDD - When you design your physical part using a computer, and save that product definition data in a computer file, it is called digital PDD. Since 1988, the U.S Department of Defense has required that all digital PDD for weapons systems be supplied in IGES format.

Physical Part - According to a friend of mine who is a Real Engineer, "A part is a thing that if you drop it on your foot, it hurts." I need to make this distinction because in the International Standards Organization, the sub-units of a standard is called a part, and in a recent discussion of the conceptual model for Part 203 of ISO-10303, there was confusion as to the fundamental point of the discussion … "the modeling of parts", "the models used to construct the parts", and "models made from parts".

Real Engineer - Someone who has lost a portion of their anatomy (i.e., a finger tip) in a manufacturing accident using a motorized tool like a lathe, rotary saw, or drill-press.

Product Life Cycle - The entire history of a physical part's evolution "from cradle to grave". (We used to say "from womb to tomb" in the industry, but the gender barrier was broken years ago, so now the Politically Correct expression is "from art to part". :-) This is the domain of Product Data Managers (PDMs), which use CAD/CAE/CAM as one component, along with revision history, bill-of-materials, parts lists, etc. … all the things that are beyond the scope of a mere CAD package.

Product Data Exchange (PDX) - Getting your digital PDD from vendor X's CAD system to vendor Y's CAD system. IGES is the most robust vehicle for doing this on the largest number of dissimilar CAD systems, and manufacturers have been using it since 1980.

This is yer basic engineering drawing of a part … there are four views displayed on the drawing sheet. Remember that a drawing may consist of multiple sheets.

If this were a technical illustration, then everything would exist in a 2D plane, so if you rotate the image in 3D space, you would see this.

The reality is that CAD models are usually 3D, and are rotated in space through a view matrix and placed on the drawing sheet, so a rotated view actually looks like this.

In the Land of Do The Right Thing, moving a view moves the dimensions as well as the part. This happens when the dimensions are in model space as opposed to drawing space. When the dimensions change automagically if the geometry is modified, this is called associative dimensions, and it is a feature which distinguishes High End from Garden Variety CAD systems.

More often than not, if you move the view, then the geometry moves but the dimensions stay in the same place. This happens when dimensions are located in drawing space. This is the most common type of CAD system, one with non-associative dimensions.

Lo and behold, this is what the dimensions look like in model space. Pretty confusing, ain't it? Well, the dimensions on each plane actually have a visibility attribute such that they can only be seen in the correct view.

This is what you see from the front view, which on this system is defined as looking down the Z-axis towards the XY-plane. On some systems, this might be called the top view.

This difference in terminology is one of the fundamental problems in CAD data exchange … the people at company A call it "FRONT", and those at company B call it "TOP". This terminology might be imposed by company A's (or company B's) Corporate Draughting Standards in spite of the terminology used by CAD vendor X and vendor Y.

Things you should have learned in school: A cube has six sides. 3D models use the X, Y, and Z axis. The six standard orthogonal views are looking either up (from zero in a positive direction) or down (from zero in a negative direction) one of these axis. On some systems they are called front, aft, left, right, top, and bottom. What makes it complicated is that even left and right may be defined in relation to top on one system and front on the other.

Another terminology problem which can cloud the discussion of data exchange is layers and views. By taking the discussion into the realm of The Big IGES in the Sky, and using the IGES definitions of the terms, the behavior and functionality of CAD systems from vendor X and vendor Y can be compared and contrasted.

1st Rule of PDX: Learn the similarities and differences between System One and System Two by comparing them with System IGES.

The IGES data models are based on a virtual CAD system that incorporates the non-contradictory features of many real-world systems. Since some CAD systems support dimensions in drawing space and other support them in model space with implicit view visibility, IGES supports both models. This means that the sending (preprocessor) system can use the representation in IGES that most closely matches their internal form, but this places a burden on the receiving (postprocessing) system to be prepared to map all of the possible IGES representations to their internal form.

Note that this is a generic PDX issue, and not unique to IGES … the implementers of STEP have discovered that there are 13 different ways to represent a trimmed parametric surface, where IGES only defined two ways. And then there's the subtle distinction between offsetting a trimmed surface and trimming an offset surface, but let's not go there right now.

Uh, listen … I've been doing this stuff for about twenty years now, and I'd be happy to come on-site and share my vast accumulation of knowledge and my internationally recognized expertise with you for only $50.00 per hour (plus travel, meals, and lodging), but this is about as much as I'm willing to give away for free.

That is all I have to say. -=DAH=- (20-Oct-95)

Last update: 2001-02-19 by <Dennette@WiZ-WORX.com>