Right up there with the dreaded “blank sheet of paper” is the inevitable question of “Can we use this instead?” It came as a real shock to me the first time I was asked, as I foolishly assumed that my employer had racks and racks of every possible gear material down in the warehouse. It was a silly thought then and even sillier now after forty years of pressure to reduce inventories.
Every gear shop has its own idea of what is the correct material for a particular part. Unfortunately, few can afford to have a full range of alloys and sizes on hand at all times. I will not be offended if your opinion differs from the “rules” I will outline in the next blog posting. You know your product best and should have confidence in what you have been using up to now.
For those without an established policy, here is some background on how I came to adopt these rules. My first employer had a three-inch-thick materials manual and a selection practice carved in stone. We never had to give a thought to what else might be used; in fact, our chart had only peripheral references to AISI grades; the primary designations were to our proprietary recipes.
My next stop was completely different and putting “gear alloy #1” on a drawing would have been a waste of time. Copying the specs off similar drawings had to suffice until that inevitable question arose and I had to do a bit of self-education before coming up with a substitute grade.
A few years later, I stumbled across a U.S. Navy materials substitution policy that had been developed for war time use. If you could not get what was on the drawing, you were authorized to move “down” the chart to “better” alloys and forbidden to move “up” to less capable choices. That document has since been withdrawn but the logic remains sound.
Cost variation between AISI grades is typically less than feared, as the actual change in the expensive ingredients is very small. Price has more to do with production quantity and many purchasers are pleasantly surprised when the “good stuff” is only a few cents a pound more than the “regular stuff,” because someone is buying it by the semi-truck load. What really drives up the cost is all the quality control operations, such as ultrasonic testing or tight controls on chemistry.
The next posting will contain my “rules of thumb” for through hardened and carburized parts.