Okay, I had a fun discussion with a co-worker about this yesterday. He called something a bolt, and I said, "What are you talking about? That's a screw!" So off to our computers we went looking for evidence to back up our position. In the end, I was right, but not for the reason I thought.
So, do you know the difference - without looking it up?
I confess to verifying what I thought the difference was. I was right.
Bolts are attached to nuts, screws thread into the part they're holding.
screws have threads from tip to mounting surface. Bolts have a smooth shank between the threads and the mounting surface
mad_machine wrote: screws have threads from tip to mounting surface. Bolts have a smooth shank between the threads and the mounting surface
That's what I was taught in school.
I prefer to screw, then bolt.
Wait... what are we talking about?
Grizz wrote: I prefer to screw, then bolt. Wait... what are we talking about?
You only need to bolt if you are working with a nut. If you are not, take your time and do the job correctly; using three or four screws is usually a better way to do it
akamcfly wrote:mad_machine wrote: screws have threads from tip to mounting surface. Bolts have a smooth shank between the threads and the mounting surfaceThat's what I was taught in school.
Then what about wood screws? Many of them have bare shanks before the head.
I always thought screws were installed with a screwdriver, bolts with a wrench.
One you round the head of with a socket or wrench, the other you use a driver.
Anything above 1/4" diameter is a bolt.
Anything below (numbered sizes) is a screw.
That's why machine screws look like little bolts and lag bolts look like giant screws.
I thought there were some engineers on here.
Shawn
stuart in mn wrote: I always thought screws were installed with a screwdriver, bolts with a wrench.
I did too... until yesterday.
I always thought a bolt used a nut to clamp to parts together, a screw does not.
Trans_Maro wrote: Anything above 1/4" diameter is a bolt. Anything below (numbered sizes) is a screw. That's why machine screws look like little bolts and lag bolts look like giant screws. I thought there were some engineers on here. Shawn
This is not correct.
A machine screw is a bolt, no matter what you call it.
A lag bolt is more correctly known as a lag screw, and the generally accepted nomenclature is changing to correctly reflect this.
It also makes no difference what tool you use to drive it. A bolt can have a slotted or phillips head, and a screw can have a hex head.
The correct answer (I believe) is that a screw has a tapered shank and always taps it's own threads into the material it is connected to. A bolt has a non-tapered shank and threads into a component which was threaded to match, although there is such a thing as a self-taping bolt (which is a hybrid piece of hardware with screw threads at the end, however it ultimately relies on non tapered threads).
The tapered shank difference means that the screw is essentially a wedge, getting tighter as it is driven (both shank and thread depth/ spacing). The bolt is a clamp which relies on the friction of the compressed faces of the mating surfaces.
Hmm, That's the best explanation I've heard.
More bolt types:
BTW, there are also two different kinds of tap and dies.
Most of us have taps and dies designed to cut what are known as continuous threads. If you keep going, you can make a threaded rod.
Plumbers, however, use tapered dies for threaded water, steam and gas lines. It's a screw. That way, the tighter you tighten the pipe, the tighter the joint gets, and more resistant to leakage (until you over-tighten it).
In reply to Trans_Maro:
I like that chart. 
Trans_Maro wrote: Anything above 1/4" diameter is a bolt. Anything below (numbered sizes) is a screw. That's why machine screws look like little bolts and lag bolts look like giant screws. I thought there were some engineers on here. Shawn
We do, and they'll reference the ASME standard, which says MadScientistMatt is correct.
I learned that fact from McMaster-Carr! Is there anything that website doesn't have?
I beg to differ.
MadScientistMatt wrote: Bolts are attached to nuts, screws thread into the part they're holding.
Bolts are still bolts, whether or not they are attached to nuts. Additionally, some bolts thread into threaded blocks or plates, and are NEVER attached to a nut.
And a self tapping bolt threads into the material it is holding just fine.
Osterkraut, that definition doesn't work (and oversimplifies the ASME standard).
http://en.wikipedia.org/wiki/Screw, with reference to the Machinery's Handbook.
SkinnyG wrote: http://en.wikipedia.org/wiki/Screw, with reference to the Machinery's Handbook.
That's essentially a very good article, but there are a lot of errors in it (like the name "cap screw" next to a hex bolt, or the truss head screw referred to as a "sheet metal screw", or the assertion that sheet metal screws make excellent fasteners for attaching to wood, which they don't).
Interestingly, the reference to Machinery's Handbook notes that it fails to completely resolve the issue of what is a screw and what is a bolt. 
SkinnyG wrote: http://en.wikipedia.org/wiki/Screw, with reference to the Machinery's Handbook.
Thread. berkeleying. Over.
I hosted it fullsized here.
SVreX wrote: BTW, there are also two different kinds of tap and dies. Most of us have taps and dies designed to cut what are known as continuous threads. If you keep going, you can make a threaded rod. Plumbers, however, use tapered dies for threaded water, steam and gas lines. It's a screw. That way, the tighter you tighten the pipe, the tighter the joint gets, and more resistant to leakage (until you over-tighten it).
Not just plumbers. That's what a pipe thread is, as in NPT. or BPT. As found on many aftermarket brake parts, for example.
Osterkraut wrote:SkinnyG wrote: http://en.wikipedia.org/wiki/Screw, with reference to the Machinery's Handbook.
By that definition, a head bolt (or any other bolt that threads into something other than a nut) is a screw.
In fact, virtually every bolt on a car would be defined as a screw.
Sorry, don't buy it.
Machinery's Handbook definition is unsatisfactory.
SVreX wrote: By that definition, a head bolt (or any other bolt that threads into something other than a nut) is a screw. In fact, virtually every bolt on a car would be defined as a screw.
Machinery's Handbook said: If a fastener does not meet the primary criteria and does not conform to a fastener industry standard for a bolt, then it is probably a screw.
Yeah, that's about the short of it.
SVreX wrote: Sorry, don't buy it. Machinery's Handbook definition is unsatisfactory.
When I was a child, I'd argue with the dictionary, too. My mom would always razz me for it, till my grandfather informed me that she used to do it too.
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