There is a good deal of talk about MIM, and Tam points out that the barrel of her Bodyguard 380 is MIM. Not horribly long ago “Mim” was what “Made in Japan” was in the 50’s. MIM parts in revolvers and automatics, put where they ought not to have been put, caused a non inconsequential amount of difficulty to people, and it put a lot of people off on the process.

TL: DR warning. Unless you took apart an alarm clock with a fork when you were five and managed to get it back running again, or you’ve done fourier transforms for fun, you might want to go read back issues of Field and Stream for a while, this is gonna be for geeks.

Everyone has seen some form of sintered metal. If you had an aquarium as a kid, you probably had a “Stone filter” or bubble producer that was a small cylinder of tiny beads of brass bound together by heat into a pretty resilient solid. This type of material is used even now for filtration, and a similar process is used to make the socket part of an artificial hip. The porosity allows the bone to grow into the material, making it as solid as solid can be.

Another type of sintered metal in very common use for a long time is ferrite- the magnets in your slot car didn’t grow like that, they are cast as a fine powder and sintered. The magnetic field is applied either afterwards, or during the heat as long as the ceramic/metal alloy has cooled beyond it’s eutectic phase. (The point at which it cools to a solid)

Here’s a really good video on the current state of the art in powdered metallurgy, which shows both cold press manufacture of green (unfired) parts, and injection molded parts.

This is all pretty simple stuff, but why make something out of powder when you could make it some other way? To understand that we need to look at gears.

When you want to make, say, an aluminum wheel for a car, you get a casting, you put it in a lathe, and turn it to the desired contours. Drill several holes in it for the lugs and you’re done. If you had enough aluminum lying about, you could do it yourself, with a big enough lathe. While similar kinds of things are possible with gears, for the most part, gear teeth are not cut, they are generated. Here is a quick and dirty visual of how gear teeth are designed. Now, whjat’s the big deal, right? Make a cutter that shape and cut the shape. It doesn’t work like that; the shape of teeth is not cut, but generated. Sometims in shapers, but most often in hobs, and to understand how a hob works you almost have to see it. Here’s a really descriptive video of the process of hobbing. Best thing is to watch the first minute or two and skip to the end. it’s very crude, but this is how the first gears were commercially hobbed.

here is a commercially made hob which is tiny, used to cut small gears for engine lathes and etc.

Hob contouring for gear cutting is a bloody difficult science, and could maybe even be thought of as an art. A cutter which has a shape that looks nothing like the final contour of the gear is rotated through the workpiece, moving in a radial and axial direction while the hob AND the gear blank are being rotated at the appropriate speeds in relation to one another. Here is a simplified cutaway view of that process. If you enlarge you can see what I mean about the shape of the cutter not even resembling the shape of the gear tooth, but it readily becomes apparent how the teeth are made.

Grinding a hobbing cutter to the appropriate contours is such an art and such a difficult process that the gear by god better be worth it. And not a lot are, frankly. Look at the gears in a car transmission. Millions are made and they take a beating, but few people know that a lot of inner trans parts have been sintered material for a long, long time.

To make a transmission not scream as you drive it, the gear teeth have to be helical. Helical gear teeth are orders of magnitude more difficult to hob. Also, helical gears act like ‘Screws” and provide not only rotation but axial thrust which must be countered. In order to prevent issues the herringbone gear was developed, that allows the axial thrust forces to cancel one another out.

Gears of this type are so bloody difficult and expensive to manufacture that another manufacturing method was needed, and that is the powdered metal process. The correct contour can be machined into a die, the metal compacted, and the gear sintered. Once done the part is “Net”, that is, ready to use, or “near net”, ready to use with some finishing.

Gears and gear components were a very good first use for powdered metallurgy, because it allowed parts to be made very quickly and at very low cost. And your home is full of them, absolutely full, though you might not have any idea. Vacuum cleaner? full of sintered metal parts. mixmaster? Blender? dishwasher? dryer? garbage disposal? oscillating fan? DVD player? CD drive? self propelled lawnmower? All are absolutely chock full of sintered metal gears and bushings and other parts. Because the stresses on gears are linear or circular and can be easily calculated, and so can the relative strength of the materials, it is very easy to tell how a gear needs to be designed to withstand the appropriate amount of stress. And there are many gear profiles that a hob simply cannot cut- not all gears are round, you know.

Gun parts, not so much. The stresses in firearms are much different, and that is a discussion for later.