It's worth noting that many of the smaller toothed pulleys seem to bear only a passing resemblance to the specified tooth profile. I'm talking about the things that are produced from lengths of toothed stock (made in a factory somewhere in China), then cut to the correct width and machined up with bores, bosses etc. I bought several of the HTD3 pulleys from reasonably reputable UK-based "bearing and pulley suppliers" recently and found that they all had narrower teeth / wider valleys than they should, with the result that I ended up with a lot of backlash. So this is another possible outcome from introducing a belt between the motor and ballscrew. The larger pitch pulleys from the likes of Fenner seem to be much more precise but there may not be room to fit one of those where it's needed. In the end, I had to make my own toothed pulley in order to get the final backlash down to a sensible level.