Mythbusters workshop, Keep it real

Image credit: Josh Cochan
Date:30 September 2009 Tags:, , ,

This mythbuster thinks computer design is no substitute for handson experience.

These days, CAD – or computer-aided design – touches nearly every aspect of our lives, from the cars we drive to the tools we use. These graphics programs enable engineers and designers to create parts or entire machines in the virtual world of the computer before building them in the physical world. CAD is one of the most important inventions of our time – a powerful tool for testing materials, experimenting with configurations and, ultimately, turning ideas into reality.

I boot up CAD programs regularly for builds on MythBusters, and consider them to be indispensable tools. For one episode, I used SolidWorks CAD software to analyse a seesaw design with a lightweight but extremely complex truss structure. That seesaw ended up withstanding 135 582 N.m, and yet I could pick it up and walk around with it.

Note the word “aided” in the name, though. Computers don’t do the designing; engineers and designers do. If people don’t bring good stuff to the project, the results will usually not be good. The problem isn’t “junk in, junk out”. Even if you start out with a junk concept, CAD can help generate something that works. It may be overly complicated and impractical to build or service, but it can still be functional.

This is a classic example of the killingsnakes- with-a-shovel school of design, where you figure out solutions to problems as they arise, instead of becoming familiar with and internalising the project as a whole first, and then coming up with a clean, elegant design.

We ran into a problem like this on the show. I needed to change the battery on a car we were using to run some tests. Because the cooling system and other hardware were located over the battery, I had the choice of removing the right-front wheel and inner fender or dismantling the cooling system to get to the battery.

This wasn’t an exotic car that you might expect to have some impractical aspects. This was your average American mid-size sedan. I decided to remove the wheel, and it took me about 10 times longer to replace the battery than it should have. I was astonished – the person who designed that car clearly had never changed a battery. The battery fitted in CAD, and it fitted in the car, but what was an elegant fit in the computer was a problem in the real world. I can just see the designer in front of his computer stuffing the car’s components into available spaces without a clue as to what he was doing.

To my way of thinking, an engineer’s work is better if he has a foundation of hands-on experience. If he has memories of his own blood smeared on his tools, his approach to a mechanical problem would not be the same as it would if most of his insights came while sitting in front of a computer.

Let’s look at a staple of home-repair toolboxes: the pipe wrench. You just know it was designed by a guy who needed to get a job done in close quarters, and it was based on bleeding knuckles. The jaw is at 90 degrees to the handle, which is unlike any standard wrench. The heft of the tool, the rounded shapes of the pieces – all of these features were informed by users with years of experience in the field.

And there’s a great deal of slop – loose tolerances of all the moving parts – so that the wrench still works with rust, dirt and gunk all over it. The slop also means that the more torque you apply, the more the components shift, and the tighter the jaws bite into the pipe. Sometimes slop is our friend, but I’ve never seen it on a pull-down menu.

Extremely complex designs, such as those for airliners, do benefit from CAD. The software is well-suited for focusing on individual parts and refining them – how thin can you make this component without it breaking? You run a finite element analysis on it and see that you need to make it a little thicker here, but thinner there, and all told you can cut the weight of that component by 50 per cent. That’s wonderful. CAD can help you save fuel or make a plane safer.

Airliners are chock-full of CAD components, so it’s seductive to think that solutions presented by software are the only way to go. But here again, plumberwith- wrench insights are crucial. The plane as a whole has evolved out of decades of flight experience that had nothing to do with CAD. What we know about structural weak points, aerodynamics and peak loads on engines came from thousands of people who contributed over decades to that design with the fear of falling out of the sky forefront in their thoughts.

Those people internalised whole sections of the aircraft and understood how all of its various components worked together, and that knowledge has informed modernday design. Just because we now have CAD doesn’t mean all that hands-on experience is moot.

CAD is just another tool, like a pipe wrench. There are things it’s good for, and things it’s not. The point is that the designer needs to understand the job a pipe wrench is intended to do, in his head, before going to CAD. I think it ought to be obligatory for anyone engineering parts for an airliner to go skydiving at least once before he sits down in front of his computer. I mean, who really understands what a hammer is if he hasn’t hit his thumb with one a couple of times?

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