Here's some sketches on motor encoders. The basic idea is that, back in the day when MCUs weren't affordable or readily available (the Motorola 68HC11 was popular, I recall), one had to plan for a pretty high chip count.
In this case, some type of counter that would track the number of slots in a disc that whizzed by an IR LED/Phototransistor pair. Remember I was still in college, with limited tools, etc.
So building any kind of precise disc with slots in it was not going to happen (since I didn't know any MechE's that could get me into their fancy machine shops).
But there are other ways to build encoders. You can print up an encoder wheel with a laserjet/inkjet printer and use an IR pair to read reflected light. Or you can print a transparency hoping that the printed spots are dark enough to block light. Or you can use one of these drain strainers or some other piece of hardware that has precisely predrilled holes.
Of course now, too, you can use an MCU to count pulses, and you can by tiny SMD-based IR circuits like those I used on Pokey. And what about broken inkjet printers? The two I've disassembled have excellent transparent encoder discs and IR sensors. (Not to mention a slew of motors, gears, rubber pieces, etc.)