[zudo]

Why can't you have teeth but have them get thinner in a way that another tooth can be fit in at each level? So at the tip say you have 5 teeth, then move one unit down, you'd have 6 teeth, but each would be 4/5 as wide as the last one, so the space between them is the same, eventually you'd come to a point where any thinner and it would break, but I don't see why it can't be done... There might be a small gap between the sliding gear's teeth and the cone teeth the farther down you go, but if you make the teeth long enough, they'd still contact.

Oh, and steel, it does matter which part of the cone it's on, because the radius is different, the circumference is different, one turn of the input shaft on the tip of the cone will spin the output shaft less if the sliding gear was on the base of the cone. If you could use friction to turn the gears it would be just as well, except it wouldn't slide, teeth are irrelevant on gears of the same radius.