I've had fun watching them send one of my boxes back through the xray 5 or 6 times at the oversize luggage area.. It had a couple of nylon gears, and the rest of the box was packed tight with stainless steel shims, so for all purposes would have appeared totally opaque in those areas.
Surprisingly metal cased electronic control modules, cables and industrial joysticks in carryon didn't even get a second glance. But the steel ruler at the bottom of the laptop bag did.