Crystal Radio--Air Variable Capacitors (4K) 

It's only recently that I've begun to appreciate that. I've always taken them for granted.

In these, I 3d printed the knobs. They're just plain plastic no mechanism.

@@tsbrownieDIY knobs, excellent! :) I have repurposed screw caps from bottles as knobs before, using a little Milliput (epoxy modelling clay) inside the cap to accommodate the spindle and a little drill hole in the side for a securing screw :) Perhaps bearings around the rota are responsible for the slow tuning in the tuning capacitor you featured? Vernier tuning dials tend to be horribly expensive.

@MrKarlGP My son's 3d printer has made things I only dreamed of before, possible. In the past I'd drill 2 holes in a piece of plastic and carve out the space in between. It was slow and inaccurate. I have no guess how that 2 speed dial works, and don't know what it is called, so I can't look it up.

I'm going to try milliput. There is no way I can do 3d printing!@@MrKarlGP

The ball Bearings at the front work as planetary gears to link the two shafts together. So simple. Plus those Banks are actually called "Gangs".