[DonRotolo]

You need a shaft adapter.

To adapt a 1/2" pulley hole to a 1/8" shaft, you take a 1/2" piece of shaft and drill a 1/8" hole in it. Add setscrews to hold stuff together.

A 1/8" shaft is very small, and any pulley with a 1/2" bore is kinda big. Plus if you're using a motor like the RS550 or similar, the top speed is many thousands of RPM, making very precise balance of all components very important.

In other words, you can't just slap it together and expect it to be good.