I've always called them set screws (fully threaded length) rather than machine screws. Conversely, I've always thought of machine screws as rather small diameter set screws with a c/sunk head.
Wikipedia told me that machine screws have thread on the whole length, but it didn't specify what is classed as a bolt - so I figured it may be something to do with the thread length.
That's what I thought. I think we're basically right, but another common definition seems to be that a screw is intended to screw into a part, whereas a bolt is designed to be usee with a nut. Like most things, it can get much more complicated.
This page says that some very short bolts can have threads all the way to the head.
There's even a 21 page document to help US customs decide whether to use the duty rate for bolts or screws, which are apparently different!
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