November 1, 2008
Columnist Gerald Davis discusses the importance of delivering a clear illustration and how it can make an impact on the final fabricated product.
Fabricating is all about meeting the customer's expectation. That is hard to do if you don't know what they want. One of the best tools for communicating those expectations is with a drawing.
That insight has been around for a while. After all, our ancient foremothers drew on the walls of caves.
Technology, however, has not exactly enhanced one's ability to communicate with drawings. State-of-the- art CAD software has both helped and hindered the process of generating understandable mechanical drawings.
Figure 1 shows the projected views of a mechanical drawing for a sheet metal bracket. Here in the U.S., this drawing would be regarded as correct. It uses third-angle projection to establish the relationships between the various views. I was taught to think of the part sliding along the inside surface of an imaginary bowl as it transitions from one view to the next.
Figure 2 shows a drawing for the same part using first-angle projection. This is more of a European standard. In my youth, I heard this standard referred to as "Dutch projection." (I have no idea of how the Dutch feel about this.) The idea is to flip the part as a domino would fall.
Either standard is excellent, so long as both the draftsman and those trying to understand the drawing agree on which standard to use.
The thrill for me as a 3-D CAD jockey is that generating these two examples only took a few mouse clicks. The computer software did all of the hard work of figuring out which way to flip the 3-D CAD model to generate the 2-D view projections of the part.
The hindrance that 3-D CAD has introduced comes in a couple of varieties—an unrealistic expectation of perfect manufacturing and laziness in communicating the features of the design. Both of these problems are due to inadequate training and are not inherent in the 3-D CAD tool.
I first met a drafting table in 1969. That's where I learned the difference between a dashed line and a solid line, what view projections are, and how to draw dimensions between features.
When I was able to use a computer to draw a dashed line, I was ecstatic! Sure, you can draw a dashed line with a ruler and a wheel, but it is tedious. The computer made the drafting job a lot easier.
However, I still needed all of my drafting training in order to know how to draw the projections. That time spent on the drafting board was terrific preparation for using the CAD software.
Is 2-D CAD drafting obsolete? To prepare the illustrations for this article, I used a 3-D CAD system and completed the task in a few minutes and made a dozen revisions. I would not consider doing this with a 2-D CAD system. However, I know several very successful manufacturers and designers who thrive by using 2-D drafting software. Use the tool that best satisfies your business goal.
Figure 3 features an isometric projection of the part. Generating this kind of view by hand can be done, but it is tedious. With a 3-D CAD system, dropping an extremely accurate view like this into the drawing takes just a couple of mouse clicks.
The isometric projection is a terrific way to remove ambiguity from the drawing. The isometric view makes it instantly clear which side gets the countersinks. It also resolves a bit of a mystery with a slot that appears in only one flange instead of in both flanges.
Figure 3 also shows one of the drawbacks of the power of isometric projections. A good draftsman would have created a flat projection that would have presented the mystery slot without hidden (dashed) lines. Instead, the lazy draftsman (aka me) relied on the isometric view to explain what his short cut meant.
Deviations from drafting standards are always despicable. Out of the thousands of mechanical drawings that I've handled over the years, only a few have actually conformed to ANSI 14.4. The perfect drawing is a rare and cherished event in my world. Most of the time, the drawing is detailed only enough to get the part built.
It gladdens my heart every time I see members of the Fabricators & Manufacturers Association taking advantage of the available training to improve the skill level of their draftsmen and designers. Our jobs as fabricators would be much easier if all drawings were produced using the same drafting standard.
What is the best way to send a drawing to manufacturing? In the ancient days before the Internet, we used parchment rolls of vellum. That, pretty much, was awful. Big sheets of paper are hard to fold and expensive to transport and store. Sending electronic files is much more efficient.
But what kind of electronic file is best? Adobe® has created a great tool with its PDF format. Nearly every computer is set up with the Adobe Reader® software. It is a nearly indelible file (which prevents accidental edits) and, if properly published, allows for color and zooming to examine fine detail.
Figure 4 shows a screen capture of an eDrawings® document. eDrawings resembles the PDF idea: someone publishes the document while someone else reads the document. The reader is free; the publisher software is purchased. eDrawings documents allow the reader to measure, rotate, zoom, and play animations. Shaded views or line drawings can be selected. Because the software offers so many additional viewing options, the user needs to be at least a little bit willing to click on menu toolbar icons.
Several other excellent 3-D publication tools are available, but these two represent the mainstream options for a simple, flat document or an animated, 3-D document.
I like PDFs when I'm distributing the design for fabrication. They work great for quality control.
When I'm trying to communicate a design concept, the eDrawings tool is hard to beat. Just watching the animation of the transition of views from projection to projection is both educational and informative.
Sometimes I send out the original CAD models and drawings. That gives the fabricator maximum flexibility in adjusting my design for flat layouts, bend reliefs, or other details to make the part easier to manufacture. This gives great power to my manufacturing crew. With great power comes great responsibility. If they change anything, I need to know about it. Otherwise, they might build something that I don't expect.
Regardless of the electronic format used, I feel that the judicious use of color in the drawing is important. For my production drawings, I show the part in black and the dimensions in dark blue. While this color may not make it to paper, it is useful when using a viewer to study the drawing.
Drafting standards are vital. National and international standards do an excellent job of establishing rules for how drawings should be generated and interpreted.
In the odd event that your organization has its own evolving tradition in drafting standards, I offer the following recommendations. Figure 5a shows a bad example of scaling the drawing relative to the paper size. The views appear tiny with respect to the drawing border.Figure 5b has changed the scale of the drawing so that the views of the part are large—making it much easier to see the fine details.
The isometric view is probably the only view that should show tangent lines in a sheet metal part. Because I use the isometric view to resolve ambiguity and seldom dimension it, I keep it relatively small—just large enough to show the overall shape of the part.
In the standard projection views, I hardly ever show tangent lines. Because solid lines and dashed lines convey specific meaning about flange direction and bend information, showing the tangent lines in these views would simply confuse the issue.
The decision whether to use ordinate dimensions is a perpetual puzzle for me.
Ordinate dimensions are convenient and efficient; they dovetail nicely with quality control and with coordinate measuring machines.
The drawback is that the ordinate scheme does not do a very efficient job of conveying tolerance requirements. A very wise person taught me years ago to dimension my drawings the way that they are used. If I have a pattern of holes for mounting a motor, I dimension the relationship between the holes, and then I dimension the pattern to the edges of the part.
That works very well, but results in a cluttered drawing or a drawing with lots of views and sheets.
Perhaps the best answer is GD&T (geometric dimensioning and tolerancing). However, it is a skill that must be learned by both the draftsman and the fabrication staff.
Gerald would love to have you send him your comments and questions. You are not alone, and the problems you face often are shared by others. Share the grief, and perhaps we will all share in the joy of finding answers. Please send your questions and comments to firstname.lastname@example.org.
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