When promoting the virtues of additive or ‘direct’ manufacture, it’s common for enthusiasts (like me) to bang on about the fact that the processes impose far fewer design constraints than apply in traditional manufacturing. Of course it’s true: words and phrases like draft angle, undercut, sink mark, ejector pin, gate scar, witness line, flash, draw axis — and so on — that govern so cruelly the design of plastic parts, for example, mean little or nothing in Additive World. Direct technologies do impose constraints on design but they are fewer, and kinder.
To anticipate the ways that new, less constraining manufacturing technologies can and will influence our engineering and design, we need to understand how traditional methods — everything from the first-ever chipped rock to the latest micro injection-moulding machine or laser cutter — influence not just the design of the parts they’re making, but also the very engineering and design principles we employ.
Until additive came along, there were three main ways to make things: subtraction — cut, chip, grind material away from a lump of stuff; moulding — squish, squeeze, pour liquid phase material into shape before it becomes solid; then forming: take a sheet or linear material (that had probably been through a process already to get that way) and then bash, smash, bend or press it into shape. Do these by hand and you have considerable licence with the shape — or geometry, as it’s fashionable to say these days — but use an industrial, machine-age version of the process, like milling, die-casting or press-forming (respectively) and expect considerable restrictions on your geometric freedom.
Machines and tools impose geometrical frames of reference on the parts being made — the centre axis of a lathe-turned part being the simplest example. Mechanised subtraction, moulding and forming processes tend to set up a dominant plane that orients the part — a machine bed or tool face for example, along with dominant axes normal to that plane. This is shown for each process type in my illustration, above. This kind of geometrical stereotyping (if you can call it that) has a powerful influence, not just on the way products look, but also and much more importantly on how they fit together and how they work.
We can call this geometry a Cartesian frame of reference, and it strongly conditions the geometry of parts. In future posts I want to look at how it helped shape the very principles of engineering and design.