A discussion on that post reminded me of this old project: replacing the chain pulleys in the midships chain tensioner on my Tour Easy recumbent.
The problem is that the original pulleys used steel bearings in a plastic race, for reasons that I cannot fathom. They last for a few thousand miles, then get very wobbly and noisy. The solution, as nearly as I can tell, is to replace them with pulleys using cartridge bearings.
This is what one looks like after four years slung below my bike. Surprisingly, the bearings still feel just fine, even though they’re not really sealed against the weather.
Gotcha: the OEM pulleys are not the same OD / number of teeth as pulleys found in rear derailleurs.
Soooo, after a bit of Quality Shop Time, I had these…
This is where you really want an additive machining process, as I turned most of a big slab of aluminum into swarf while extracting each pulley.
The first step is to drill holes around the perimeter where the chain rollers will fit, plus drill out as much of the center bore as possible. Then mill down to the finished thickness across the roller holes and helix-mill the bore to size.
Flip it over and mill the other side to the proper thickness.
Run it through the bandsaw to chop off all the material beyond the outer diameter.
Grab what’s left in the three-jaw chuck and mill around the perimeter to get a nice clean edge.
And then it Just Works. I made another for Mary’s bike, but she said it was too noisy (which is why they used plastic rather than aluminum) and I swapped it for a Terracycle idler.
This is from back in the Bad Old Days before EMC2’s version of G-Code supported loops. You don’t need to see that code, trust me on this.
The Smell of Molten Projects in the Morning 
I can understand how disappointing it is to reduce most of your raw material into swarf. When I was having some plastic laser cut, I decided to hide the small parts inside the large parts, reducing waste somewhat.
http://www.vitriol.com/spikenzie/minicut407.eps
Looks like artistry to me!
I keep looking at those 8-kilobuck laser cutters that look like printers, but my toy budget won’t stretch quite that far. Sigh…
Yeah, I look at those $350 CO2 laser tubes and think about it, but I’ve decided it’s more cost effective to just pay companies to laser-cut stuff for me. Less fun, tho. I am going to try lashing a 200mW laser diode to my old pen plotter and see if I can cut paper stencils with it, however.
Or carve neat patterns on your retinas…
I’ve done this. It works. You can get a 350mW diode off ebay for not much more, and it cuts a lot faster. Get a module with the lens and make sure it’s a glass lens: the plastic fast axis lenses melt with this kind of power, and if you just get a bare diode it’s a *bear* trying to build your own alignment setup. The fast axis optics seem to be mounted in a non-standard metric thread (0.9 x 0.1mm) so I had to make a tap (http://www.flickr.com/photos/9279759@N03/4305969694/) to get the diode collimated with the lens. Gah.
It’s also fiddly hooking up a switch so the pen down/up turns the diode on/off, but it sure is cool being able to dump HPGL straight into your printer and have it work.
Irfanview is a good program for converting between different strange output files if you need to make a postscript into hpgl or vice-versa.
I *meant* metric 9.0 x 0.5, but apparently I’m having an attack of the stupid today.
dump HPGL straight into your printer
I knew I should’a kept that old HP plotter. Drat!
That’s really cool!
There’s an argument that derailleur pulleys should have an odd number of teeth because chains have an even number of links and the inner and outer links wear differently, but I’m not convinced it’s a big issue to anyone who isn’t trying to sell odd-tooth pulleys.
The number of teeth really should be relatively prime with respect to the number of chain links, but that’s getting perilously close to wankery, even for me.
And probably close to impossible, given the different chain lengths on our bikes.
Yeah, I can see it now: custom-cut idler pulleys and tensioner frames for each bike. Spare me…