One of the problems with using a treadmill motor is that they're often made overseas, and often have metric-sized shafts. Mine is 17mm, which I think isn't uncommon. I tried a couple of times to get a 2" pulley with a 5/8" bore bored out to 17mm -- the first time didn't work out due to lack of the appropriate cutting tool, and the second time (different place) I ended up with a 17.4mm bore. Oops. Happily I didn't have to pay for that. Also happily, the pulleys only cost $5.50, shipped, from Amazon.

I got a second pulley, this time with a 3/4" bore. 3/4" is almost exactly 19mm (19.05), so I needed to shim it by 1mm all around to fit the 17mm shaft.

It turns out that the walls of 1/2" EMT electrical conduit are just proud of 1mm thick -- the nominal wall thickness is .042", or 1.07mm. It also happens that 1/2" conduit is just about 5/8" diameter I.D.

So I was able to take a 5/8" shaft I have lying around (part of my Shopsmith accessories), and wedge a short length of conduit onto it. I chucked the shaft into my drill press, and used a hand file while it spun to thin the walls of the conduit a bit.

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image: Fri 19 Dec 2014 01:11:03 PM