I've got 4 inches of steel in my right lower arm and it makes thing go off from time to time. You can oftne control this yourself; i.e. go through the detector quickly and keep yourself compact to maximise the mass of yourself to pass through int he shortest time (hands along body, set for down just in the middle of detector, etc.) and it will always go off, metal or not. Conversly, get one arm out in front, make sure you set your foot beyond the detector as you walk through and one arm out back, etc.

It's fun, sometimes it's a cure girl doing the pat downs and being able to set off (or avoid setting off) the detector at will is a nice feature.

-A