Bedeviled with the details

Balancing realistic detail with rational modeling discipline

THE FABRICATOR® OCTOBER 2009

October 8, 2009

Columnist Gerald Davis shows readers how to deliver just enough detail for visualization in a 3-D drawing, not for manufacturing.

shoulderbolt precision

Figure 1An extremely realistic model of a shoulder bolt—it is easy to visualize, hard to model.

A recurring theme in 3-D CAD modeling is design intent. What is the reason for creating the model?

Generally, the goal is some combination of visualization, virtual prototyping, and preparation for manufacturing. The trick is to satisfy those goals within a time budget.

Realistic Cost

Consider the shoulder bolt shown in Figure 1. It is dimensionally exact and includes the proper thread pitch and profile, knurling around the head, and exact hex key cutout size. This model is suitable for use in manufacturing this item. A machine shop could use this model directly for CNC programming.

It took me no time at all to model this part. I downloaded it from an online hardware store (http://www.mc master.com). If you want to fetch your own copy, use part No. 90298A615.

An expert CAD jockey starting from scratch could easily spend a couple of hours creating this model. If the goal is manufacturing, then this effort would of course be justified. In my practice, I more often use fasteners like this in an assembly for other reasons—primarily for visualization and generation of a bill of materials.

Speed Demon

For those purposes, the model shown in Figure 2 is more appropriate. I created this part in about 30 seconds by revolving a sketch of the profile of the shoulder bolt.

Why is Figure 2 "better" than Figure 1? One possible answer is shown in Figure 3a and Figure 3b. Compare the rebuild time my workstation reported (a 64-bit quad core, state-of-the-art monster). The realistic part took 3 seconds. The slug dummy rebuild time was too quick to detect by the timer software.

So, even though it took me very little time to download the realistic model, if I use more than a few of those fasteners in a CAD model of an assembly, then I'll be paying a time penalty as I wait for the model to update. Time is money. Quick CAD work is good CAD work.

Hasty Pudding

Another great use for 3-D CAD models is the generation of instructional videos for assembly personnel as well as the production of illustrations for manuals. The bolt shown in Figure 2 lacks so much detail that it would seem amateurish in those situations.

In Figure 4a we've edited the revolved sketch to include a semblance of threads. They are not true helical threads, but as you can see in Figure 4b, the casual observer won't be able to tell. The addition of a hex key tool hole and a chamfer results in a more finished appearance for the component. The rebuild time is now up to 0.05 second, which is still very efficient.

The component shown in Figure 4b has insufficient detail for manufacturing, but it does have enough detail for entertaining visualization. Because that is my design intent in this example, I'm happy.

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 dand@thefabricator.com.



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The FABRICATOR® is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971. Print subscriptions are free to qualified persons in North America involved in metal forming and fabricating.

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