I once interrupted my own eye exam so i could poke a sprocket to see what it was made of. The apparatus they use for moving the lens box closer/further from the

I once interrupted my own eye exam so i could poke a sprocket to see what it was made of. The apparatus they use for moving the lens box closer/further from the patient had these weird looking gears on them. I asked the doc if i could touch, he said yea but was confused why. Told him i was curious what it was made of. Turned out to be plastic though it looked like unpolished metal. The reason i was so curious was because of its design. Wasnt a typical sprocket, and instead had dimples where the teeth would be