Originally Posted by Dave Brand
Just to clear up the confusion between screws & bolts: using correct engineering terminology, a screw has threads right up to the head, a bolt is only threaded over part of its length.
Dave, thats a very simplified explanation, and It's plain wrong.
Bolt versus Screw Definition
The correct definition of bolt and screw is as follows;
Bolts are defined as headed fasteners having external threads that meet an exacting, uniform bolt thread specification (such as M, MJ, UN, UNR, and UNJ) such that they can accept a nontapered nut. Screws are defined as headed, externally-threaded fasteners that do not meet the above definition of bolts.
Because various organizations have thoroughly confused everyone regarding this simple definition, the remainder of this article dispels several myths and discusses the full rationale explaining the above, correct definition.
(1) Several dictionaries, Machinery's Handbook, part specification sheets, military specifications, bolt manufacturers, and vendor catalogs are wrong and have botched the above, unambiguous definition. Their definitions of bolt versus screw are arbitrary, random, ambiguous, nondefinitive, and do not align with common sense. Having "credible" sources such as Machinery's Handbook, ASME, ISO, and military specification sheets misusing and arbitrarily misdefining the words throws the whole world off. Then eventually most dictionary authors follow suit and copy some arbitrary version of the incorrect definitions floating about. All of these incorrect definitions and misuses, no matter how credible the source may otherwise seem, should be completely ignored. It should be apparent to you that technical definitions that make no sense are not credible.
(2) Another major confusion factor is the fact that bolt threads are sometimes generically referred to as "screw" threads in specifications, even though they are actually bolt threads, in an attempt to generically refer to the threads themselves, whether internal or external and regardless of which part they exist in. Words can have more than one definition, and this particular usage of the word "screw" is an attempt to describe helical threads simply in reference to the act of screwing. Using the word "screw" when "screwing" is meant does not cause a bolt, whose threads meet the specification of bolt threads, to be suddenly metamorphosed into a screw. If the threads meet the specification of bolt threads, such that they can accept a nut (regardless of whether you install a nut or not), then the threads themselves would more aptly be called "bolt threads" in the specifications, instead of "screw threads," to dispel confusion, as the nut is no more a screw, by the strict definition, than a bolt is a screw.
(3) Let us get the record corrected with the previously-stated, correct definition, which stands unaffected by several other factors. And let us now, by stating the correct facts, dispel several myths that have no affect upon the above definition.
The extent to which the shank of a fastener is threaded, whether fully or partially, does not affect the above definition in any way. Bolts can come fully or partially threaded, as do screws.
Whether or not a nut is installed does not affect the above definition in any way. Common sense tells you a bolt is not suddenly metamorphosed into a screw each time you extract the nut (if you chose to install a nut instead of using the bolt in a threaded hole, insert, or nutplate); nor is it suddenly transformed into a screw each time you choose to not use the bolt but just let it sit there on a table unused.
Bolts are untapered. Screws are often tapered but can also be untapered.
Screws always cut their own internal threads when initially installed, as there is generally no tool meeting the arbitrary specification of their threads to tap out the internal threads beforehand. Conversely, however, it is possible for a bolt to be self-tapping. The only criterion in regard to the bolt versus screw definition is whether or not the self-tapping fastener, non-cutting threads meet the strict specification of bolt threads, meaning they can be correctly mated with a nut.
The type or size of head on a fastener does not affect the above definition in any way. Bolts come with almost every imaginable head; screws do also, including hexagonal. Likewise, the configuration of the driving (or holding) tool surfaces in the head, whether internal or external surfaces, does not affect the above definition in any way.
The fastener nominal diameter does not affect the above definition in any way. Bolts do not suddenly and mysteriously no longer accept nuts just because they become small, miniature, or micro.
The term "machine screw" is a misnomer. A bolt, clearly having bolt threads, is not suddenly metamorphosed into a screw just because someone arbitrarily misnamed it in a specification, book, organization, or industry.
Whether or not specifications incorrectly or loosely refer to bolt and nut threads as "screw threads," or even erroneously refer to bolts as "screws," does not suddenly transform the bolt into a screw. The inability of the technician who drafted the specification to master or understand language, grammar, measurement systems, coherent, unambiguous, internationally-standard units of measure, correct mathematical expressions, etc., does not mean the coherent engineers are suddenly thrown into an abyss without coherent definitions. Intelligent engineers must be able to sort out the technical facts from among the fiction and typos. Whenever someone has botched definitions, world class organizations such as ISO and IEEE need to step up to the plate and redirect the incoherent, aimless, arbitrary, lower-level entities. All of these incorrect definitions and misuses of the word "screw" should be completely ignored, as there is no need to continue to copy past mistakes and propagate confusion. The previously-stated, correct definition should be used in new specifications, standards, and publications.
Thank you for your time
PS. The above is a little C&P job