I have copper coated Steel BB pellets and even after a google search, I can’t figure out how they make them so perfect.
In: Engineering
**EDIT**: I just realized that you asked about BBs. The process described below is overkill for those, but still used in way more things than you’d expect.
This is awesome. Perfectly spherical steel balls are actually one of the most important industrial products on Earth (for use in ball bearings).
Before reading on, you may prefer just watching [this video](https://www.youtube.com/watch?v=19duYMdiXi0) made by the Japan Science and Technology Agency’s youtube channel. It has pretty figures and animations that were super awesome in the 90s.
That said, I’m going to explain the process below, and draw attention to a couple details which the video glosses over.
There’s actually a few different ways, but in general they all have two stage: a “rough” stage and a “fine” stage.
The “rough” stage just sort of makes a ball. For small balls, it’ll usually take a little piece of wire and smush it into a mold to make it more-or-less spherical. Larger balls might have molten steel poured into a mold. The end result of this stage is something that’s, I dunno 95% of the way to being a ball. If you looked at it, you would say, “Hey! That’s a ball!”. But it might have weird edges or something from the mold processes, or a little “belt” where the two pieces of the smushing machinery met, stuff like that. Plus, over time the molds get worn and distorted so they couldn’t stay perfect even if you super duper high-precision machined them in the first place.
The “fine” stage takes these “Yeah, they’re pretty much balls!” and gets them to the super-duper high precision (typically a few dozen **nanometers**) that it ordinary in so many things today (like, say, a bicycle wheel).
There’s a few different ways for the “fine” stage to happen, but most of them exploit the spherical nature of what you’re trying to make basically rolling the balls through a polishing process over and over. The simplest example would be to imagine two big disks, one on top of the other. The disks are placed apart at ever so slightly bigger than the width you want your final balls. It needs to be wide enough that all “rough” balls can fit in, but other than that as close to the end width as you can get – so you see that the closeness of your “rough” is important! The more precise your rough ball, the fewer polishing stages you need.
One of the disks (usually the top disk) will slowly spin; eventually, this will polish away any bits that protrude past the desired width. A common variation is to make channels of the right size in the disks instead of just having two flat things; this works faster but the idea is the same. Note that we couldn’t employ the two-disks-rolling method like this if we were trying to make anything aside from a ball, since we can’t really control how the objects being polished move inside. But one way to define a sphere is “it’s the exact same width across no matter what direction you measure from”.
Anyway, you send your “rough” balls into this machine. For…some amount of time. How long? Well, you take balls that have been getting polished for some amount of time, and then you let them roll down a wide ramp. The balls that are perfectly round will roll straight down; the others will roll away to the side. You can then collect those imperfect balls and run them through the polisher again.
The balls are then often heat-treated, and then send through a similar polishing process as before, with the distance between disks closer to the desired one. The rolling/sorting thing is repeated, but with a narrowed “good path” to get finer results.
This process is repeated some number of times, getting closer and closer to the end requirement. Depending on the size, material, and precision needed, such a ball will usually go through at least 3 stages, and maybe many more. This is the same process that NASA used to make their [super-duper spherical spheres](https://en.wikipedia.org/wiki/Gravity_Probe_B#Experimental_setup) for an experiment about gravity.
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