Ridges can be caused by two factors :
- If your lathe felt on the ground or something similar, there is a change that your screw is bent. To check that tighten the grub screws of the moving part of the lathe (the screws that hold on the delrin bit that goes on the guides) until you have to put some effort to get any movement (notice, you don't have to put a lot of effort, but you shouldn't be able to turn the screw with 1 finger). If there is a hard spot when you turn the screw, then it's bent and you should replace it.
- Otherwise, the grub screws that holds the delrin bit are too loose. As I said before, you shouldn't be able to turn the screw with one finger. If that is the case your lathe is set too loose and the guide isn't doing its job properly. Remember that with a lathe you're playing with microns, so a play that will end up with ridges is impossible to notice by just looking at it. You also have to do this for both directions (there are two screws and both need setting up). On my lathe (which is quite old now), the best setting I found is when I need to put one finger on the base to keep it steady whilst turning the screw. It's also a good idea to put a tiny bit of black grease on the guides.