So I was minding my own business building some bookshelves (pine planks screwed together), when I reached into the box of screws and found one that was smooth. And blunt. Comparing it to one of the other 99 in the box, I find the flat, philips head to be the same, along with the 1/4 inch of smooth metal shaft below that. But this unfinished screw is nothing but the continuation of that shaft. A finished screw (#6 - 1 1/2 inch - purchased at Home Depot) has two threads starting at that 1/4 inch mark, going all the way to a point (which extends about 1/16th beyond the unfinished screw’s length). The threads are wider than the unifished shaft.
One of the first things that popped into my head was the old Crosstime Engineer series of sci-fi novels, in which the question “how do you make a screw without a lathe, but how do you make a lathe without screws?” was asked, and, if I remember correctly, the answer was conveniently skipped (“these ingenious people figured out a way…”). So much for getting practical advice from fiction.
Anyway, my “first draft” at an answer to “how are modern woodscrews manufactured?” has been “in a really good mold, because I can’t see any seams.” This unfinished screw has ripped that possibility to shreds.
It now appears to me that they are, actually, lathed into existence. Perhaps. The threads on a finished screw appear to pop out at a 90-degree angle from a slightly smaller shaft. If some of the metal is “scraped away” from the original shaft and bent outwards in the milling process, it sounds like a tough way to do it. I also have trouble conceiving of a machine capable of doing so to the precision that appears to be the norm. Also, the extra 1/16th inch of metal on the tip is puzzling.
So millwork only? Or is the shaft trimmed and then threads added via some precision welding (which seems even more difficult)? Or something else?
They are formed by rolling.
The blank wire is spun in something resembling a large drill chuck, with a short section sticking out. Two or three “dies” are forced against the wire, pressing the thread pattern into the wire. The metal- typically in an annealed/soft state or even still red hot- flows like clay. This is why the “shaft” diameter appears smaller than the thread outside diameter.
Modern “screw machines” can crank out literally thousands of such screws an hour. Your “blank” screw could have been when an operator started a new roll of wire, changed dies, whatever.
My machine shop instructor had a unique example of a screw machine… er, screwup: It was a three-foot long section of “all-thread”, basically a long rod that’s nothing but threaded shank. Except this one wasn’t threaded, it was grooved. It had the typical peaks and valleys of threading… but the threads didn’t go anywhere. It wasn’t a single progressive groove, it was a stack of concentric rings.
For some reason, that thread title made me think of blue balls.
(damn misleading thread titles)
[sub]This was a test. This was only a test. In the event of an actual hijack, Cecil Adams would have personally emailed you. Okay, maybe not.[/sub]
For information about all things about screws and screwdrivers, read One Good Turn: A Natural History of the Screwdriver and the Screw by Witold Rybczynski.
I have seen studs that are used for setting into concrete that have the ring pattern you describe, these are used to put a stress into the rod and when the concrete sets the rod are unclamped which then results in pre-stressed concrete sections.
Parts of “The Woodwright’s Shop” TV series have described making a portable treadle lathe that used a springy tree branch as a return force against the downstroke of the user’s foot. A rope was wrapped a few turns around the work, which spun one way on the downstroke and reversed when the branch pulled back up.
Another episode showed how the wooden screw of a bench vise was made without a lathe, but I can’t remember how it was done.
Doc Nickel and RealityChuck: Thanks for the info.
AskNott: It’s been so long, I’d forgotten about that show and the lathe. Of course, there’s still plenty of room for screws in the bits of the lathe holding the piece being worked. At least, if you don’t want the work to catch fire from friction. There’s a picture of it here in the show’s glossary. (It would have been really nice of them to include a full list of shows with subject matter somewhere on that site.)
[sub]Mikahw: At least I didn’t use the term ‘abortion’.[/sub]
I found one of those threadless screws too (it was a plain steel slotted one), funny thing is, I only noticed the thread was missing after sweating ang grunting trying to get it to ‘bite’ (the front piece of timber was pre-drilled, so I didn’t notice the absence of the thread)