Or, “Keystock, we hardly knew ye. . .”
So, back in April of last year I was eulogizing Keystock; at least the longer length, higher strength, plus-tolerance kind, that for years had been a “go-to” product for a lot of people. It seemed Keystock would be going the way of the Dodo bird. But, I was apparently wrong. More correctly, it was going the way of Coke, or New Coke, or Original Coke, whatever. The point is . . . “It’s baaaack.” And, that’s a good thing.
As I mentioned then, in the steel business in most general terms, anytime you see the word “stock” attached to another word; think, “stock used to make”. Bar-stock, Bushing-stock, Brake-Die-Stock, Rifle-Barrel-Stock, Pump-Shaft-Stock, etc., would be stock that might typically be used to make those items. While it may seem to infer that there are other attributes specifically suited for the particular application specified, it may simply mean this is a material some people have chosen to make this type of part. Details should be clarified; is it closer tolerance, harder, squarer, whatever? Facetiously, a tree might be “Tooth-Pic Stock”. Works the same with the term Quality, as in; Rifle barrel quality, or Military Quality, or Drawing Quality. The explanation and caution remain the same; It may mean the steel possesses a special grade certification, but it may as likely mean that “Someone, at some time, used it for that.” So, “Trust but verify”.
Keystock is generally a low to mid-strength mild or carbon steel square or flat bar, used to make keys. Those keys are inserted into shafts. A drive motor chuck will engage with the protruding key, and rotate the shaft. The idea is that the key is of less cost and strength than the expensive shaft. If something interrupts the motion of the shaft and causes failure, it should be the cheap little key that fails not the big expensive shaft. Clear the obstruction, put a new key in, and you’re good to go, hopefully with an undamaged shaft.
The perfect piece of keystock would have less mechanical strength than the shaft, but tuned to the actual application, close enough that the shaft doesn’t fail prematurely, contributing to expensive downtime. There is the conundrum; too soft of a key and you have frequent expensive delays. Too hard of a key and you risk damaging an expensive shaft.
Ideally, commercial Keystock would be a precision cold drawn bar possessing slightly elevated strength properties, with good cross-sectional accuracy and a close oversize tolerance. The ideal oversize tolerance for many years was considered to be +.002”/ .000”. That tolerance would allow you to cut a standard keyway that would fit nice and snug. Undersize tolerance resulted in sloppy, loose keys. Too much oversize required unwelcomed machining.
Unfortunately, over the past decade or so, tolerances on commercially available Keystock gradually opened up to +/- .004 to .007” (plus or minus); even with sources that promoted “Keystock”. For several years now, it has been common to only be able to source low property mild steel, undersize Keystock, in short lengths (12” and 36”).
Longer lengths, no matter how beneficial to the end user, require expensive additional processing to eliminate camber and bow.
Running multiple draughts (passing the bars repeatedly through drawing dies) is a good practice to increase strength (strain harden) and refine tolerance. However, that process adds cost, as does additional straightening. Those expensive practices were eliminated as the marketplace shrank.
Hardness (through multi-pass cold drawing/strain hardening), tolerance, cross-sectional accuracy and straightness, are all doable. Expensive, but doable. It requires a bit of fiddling in production for an item that does not represent significant tonnage. This current economic burst has allowed “The Return of Keystock”. Check with your vendor and take advantage of a simple perk that will make your life easier.
-Howard Thomas, September 15th 2021