As a simple test project to get the hang of this, I made a small pulley from HDPE plastic. Since this material is very slippery, I hope to be able to use the pulley without any need for bearings etc. My main goal is to illustrate how simple parts can be made without fancy tools.
I used a piece of cutting board, but you could melt down some milk jugs, find some plastic scrap or just buy some HDPE rod (Although that would sort of defeat the point). I also used a small m3 bolt (any small screw or bolt should work)..
Tools: Rotary tool or drill, hacksaw, file, screwdriver and some sandpaper.
We're going to shape the HDPE into a rough circle, and then spin it with the rotary tool while removing material (like a lathe) until we get the desired shape. The picture shows the end result, before I polished it and removed the threaded rod.
Step 1: Planning, Rough Shape
Plan out the rough shape of the pulley. I drew a rough circle near the corner of my chopping board. Use a hacksaw to cut out the shape, leaving some margin around your drawing. If you have a coping saw, this step is even easier since you'll be able to cut a nice smooth curve.
Once you have the rough shape, drill a hole as close to the center as you can. Make it smaller than the screw you are going to use. The you can screw the screw into the hole, and it will dig it's own threads (try putting the palstic into boiling water to make it a little softer). You'll find that it locks nice and tight.
Step 2: Using It Like a Lathe
Now for the fun part! On with the safety goggles... Chuck the part of the screw that sticks out into your drill or dremel, and spin it up. If your part is too off-balance, it might vibrate wildly. If this worries you, sand it down closer to the final shape before spinning it.
Once it's spinning, you can begin to shape the final part. Sandpaper works, and I was also able to brace a screwdriver against something and use it to remove material. It's a dirty hack but it works! After a short while, you should have a nice, perfectly round piece of plastic whizzing round. Now, you can use a file or a sharp screwdriver to remove material to form the channel in the middle. It's almost like we're real machinists! Although I suspect anyone who has actually used a lathe will wince at my photos :P
By gently applying some finer grit sandpaper, you can get a really nice finish (sadly, I have no photos to document this). Unscrew the screw, and you're done - enjoy the object you've just created and bear this technique in mind when you're in a pinch and want to make something round :)
I hope you've enjoyed this instructable. I'm going to try to make more, upping the quality as I go. Feedback appreciated