Who would have thought a screw and bolt could be so confusing? is it because they look so small and insignificant, or, because there's so many of them everywhere and so easy to get hold of that we take them for granted? Whatever, they are and I'm going to spell it out for you as succinctly as I can without being too boring. A screw fixing holds materials together by the form of the shank that ties/ locks itself to the material itself at one end and locks/ compresses the material tight at the other. A bolt holds materials through compression or clamp at each end of the shank. Both use helical coils (the thread) to adjust the effectiveness of the joint. Ah, that feels better to be able to write it down.
Can fix into a blind hole that is tapped or it can create it's own thread (self-tapper)
Can hold the greatest loads
Now, these classifications make sense until some plonka goes and names a, 'hex head, set screw' and a hex head, bolt. What's all this about screw being a bolt and vice-versa when I've quite clearly explained the difference already? People who don't make heavy structures won't know the difference between a hex head, set screw and a hex head, bolt. Holding them up a, 'normal', person would think they were basically the same, with the same function..... and they would be close to correct. The above classifications no longer apply.
Here are some images from the website.
You can see that one has a plain shank and the other does not. I hear you cry, 'what about wood screws that have a plain shank, they're never called bolts even by the most supreme anal, engineers?' Well, I'll explain that shank another day, for now, please see the 1st comment above (in bold) on the difference between a screw and a bolt. If it's not the shank that distinguishes a bolt from a screw, is it the way they material together that makes them different. I would have thought both points would be good to describe the difference but obviously not the case.
The 'cap-screw,' term is often used to describe a fastener that is fully threaded in lengths up to and around 1 inch (from the underside of the head). Over 1 inch in length the fastener will have a, 'shoulder,' or unthreaded area under the head as the rest of the fastener lies redundant within the material. This space of unthreaded area confirms that the fastener is now a bolt. A little confusing, no?
They both can screw into a tapped/ threaded hole, both can use a nut/ washer and both can withstand enormous loads. The only difference being that the plain shank will protect a hole if the material is prone to movement or vibrations. We can't use this majority difference because of the exceptions in fixings like the longer length timber screws that have a plain shank.
Hexagon headed, Zinc coated, High tensile, steel graded 8.8, bolt with bright, zinc plated, full nut.
Both a set screw and a bolt can be used with and without a nut. Shall we now call them set screw, bolts? Judging from the description of the fastener above, I'm not sure it needs a longer name.
Basically, use the short definitions at the beginning to describe the difference between a screw and a bolt. Then expect a headache when explaining them from an engineering perspective.
When is a screw, a bolt? When you let a crazy engineer deal with marketing!!!
Next time, I'm going to shed a little light on the names of fixings and fasteners......