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Is that computer doing what you think it's doing?


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Is that computer doing what you think it's doing?

I spent part of this week drawing a set of ballast castings for a new sailboat. I'm using the same three-dimensional parametric modelling system I used back in January to design the hull. A toolmaker should be able to put my 3D model into a computer-controlled machining system which will automatically produce the mould. A boatbuilder can do the same thing with the hull. In theory, this eliminates a whole lot of problems. If I specify in my 3D model that the hull is 3.997 metres long, the computer-controlled milling machine should make it exactly that length.

Boat hullEarly last year I asked an engineer at a high-tech boat-building yard in Northland how she was finding the various 3D modelling systems. I wasn't surprised when she said: "We use CATIA." CATIA has been one of the gold standard computer-aided design systems for some decades. "Some of the others are..." she trailed off. I wasn't surprised by that, either. These systems are complex. There is plenty to go wrong.

In the world of computer-aided manufacturing, it's not unusual to find engineers riding shotgun on systems that you might think would be fully automated. A three-dimensional model that looks great on a computer screen can have defects that cause trouble on the shop floor. This is especially problematic with complex curved surfaces, such as you find on a boat.

For example, you can finish up with horrible little gouges, especially around the edges of complex surfaces. The machine carves the surface by moving from place to place, each place defined in three-dimensions. If the list of "places" isn't sorted properly, the cutter might slice through the surface instead of moving around it. To prevent this kind of thing, manufacturing engineers check for problems in the 3D model, using diagnostic software that I won't try to describe because I'm still struggling to understand what it does.

Not all problems are caused by substandard software. A yacht bow is a typical example. We used to just draw the lines and leave it up to the boat-builder to shape the front few inches by eye, especially if we were building the boat ourselves. You can't do that with computer-aided manufacturing. The 3D model must define every last detail. Although we call the bow, "the sharp end", it's not really sharp. On my current project, it has to be wide enough to lay the fibreglass cloth and the resin into the "female" mould. And it must have enough slope or "draw" to allow the boatbuilder to pop the finished hull moulding out of the mould. I think in the past few years I have probably spent more time thinking about how to draw the front few inches of a boat hull, than I've spent thinking about everything else to do with hull design.

Setting up a computer-aided manufacturing facility takes gobs of capital. How do we know it's going to work? We could talk to people who are using the same software and machinery to manufacture similar products. Sounds simple, eh? Except that the person whose system most closely resembles the one you hope to build is probably a direct competitor. In any case, you can't get a competitive edge by doing what everyone else is already doing.

So I guess in the final analysis this is like most things in life. Do the homework, and then build it and see what happens.

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