IN everything I have written so far
I have really glossed over almost everything. Specifically gear manufacture, a subject so complex that even the old masters are often guessing. The tailrotor gearset in an Apache, for instance is manufactured one set at a time, by hand, on specific machines tailored specifically to the purpose, and a large percentage are scrapped due to tiny, tiny manufacturing defects. Each operation, each inspection can only reveal perfection, and that gearset cost as much as a well laid out home in a Chicago neighborhood. So there’s a lot more to this than I have mentioned.
The point of the whole diatribe has been to show how under controlled circumstances and in certain situations MIM has been a wonderful boon to a lot of things, especially in gears and small parts, because it is less expensive to produce, and when the engineering is done right, have a great lifespan.
Now: How do you make a cylinder latch for a Smith revolver? here’s one, just like the one in my Stainless 357.
this is an old one, so it’s a forging, most likely. Forging gives a good deal of strength to metal, because metal, like wood, has a grain. Shipbuilders looked for trees that had the right shape to make a keel, because if the grain followed the curve naturally it will be stronger. Wood can be bent, but it often tries to return to it’s natural shape.
Forging takes a piece of metal and forces the grains to flow in that metal. It makes the internal structure of the metal like a buttress and truss system, the dendrites aligning to form microscopic natural lines of strength, both tension and compression. Forgings have the ability to retain their shape and withstand heavy force, because they have the internal structure to do so.
Think of a baguette. You can hold a baguette like a sword and poke someone with it, and it will hurt. You can smack someone with it and it will, if not hurt, at least make a loud noise. That’s analogous to steel. Now, if you want to make that stronger, you make Challah. I have seen loaves of Challah that I think could be used to punish children with. That’s like a forging, you align the “grains” of the bread to make it more robust.
Now imagine cornbread. That’s sort of what people have always thought of as MIM. it has no grain of it’s own and it has no real strength except the strength of the molecular bonds. But those molecular bonds are very strong.
Problem is, are they strong enough? My Smith, apparently, had a MIM cylinder latch, because it snapped off leaving the very characteristic broken-mim surface. It had lasted under several years of apparent police work, only to be snapped when the 300+ pound mutha bore down on it. And that is the engineering issue: A gear with a known load, speed, and reduction can be engineered with a window of failure so small it should reasonably expect to last forever. But on parts that humans interact with, there is a great likelihood of one person’s strength being dramatically different than another. No amount of engineering can compensate for the difference in strength between Caspar and Bubba, and make MIM parts effective. And that has been the whole problem from day one.
The structure of MIM is similar, in some ways, to a casting- the “Cornbread” analogy. But wheras castings are poured and it is hoped that the molecules bond, MIM forces each piece of powder to bond with each adjacent piece of powder, no question. Cast bronze does that sort of automatically, whereas cast iron needs higher temps than was available to, for instance, early cannon makers. hence, a lot of the smaller cannon that survive are brass, and they can still be fired, some of them, after years of being out of service.
The stress exerted on a barrel is internal; a barrel has to resist exploding. In forged barrels the lines of forging are linear and align with the barrel, not across the grain(Except in roto-forging, which is another discussion) So the forging process is not likely to lend any actual strength to the barrel that the plain,unforged steel did not already have.
Using the MIM process, the barrel has the “Powder” welded together, particle by particle, almost molecule by molecule, in every direction at once. So it was a pretty fair bet that it would take the pressure, and apparently, it did fine. Nobody has abandoned expensively obtained empirical data, they only found some new data that work pretty well too.
Pascal says the left abandons any data if it doesn’t jibe with their social agenda, and he may well be right. Engineers cannot afford to, and for this reason engineering is a good thing.
Now, the paragraph this has all been leading up to.
You can do MIM yourself. It’s pretty easy, actually. The two tricks are to make a powder that is of the correct nature, and get a heat source from which oxygen can be expelled- or forced out. A small heat treat oven or ceramic kiln can be set to reach the appropriate temperatures and flush it with an inert gas, like a weld gas, so the metal doesn’t oxidize. You can also just leave combustibles in to use up all the oxygen, or if you have something small enough, draw a vacuum. No, you won’t make gun quality parts, not right away, but you can do this, it has been done all over the planet for ages.
Long before there were 3d printers there were MIM parts. I know a LOT of my readers are at least this smart. Think this is a skill you might like to have?
