I received a question recently about how to improve ribbing cast-ons. This knitter was tightening tension as much as possible but still getting a flared, rippled cast-on edge. This problem is common, and how much trouble you have seems to depend on the yarn and the machine.
Why does this happen? It's hard to get a tight tension because the yarn has to travel the distance between the ribber and the main bed, needle to needle, for the first zigzag row. Also, the ribber comb's teeth, which are a fixed distance apart, tend to hold the edge open and make it flare.
What can you do? Well, there are several things that greatly improve the ribbing edges.
First of all, you could start your ribbing with waste yarn. Cast on and knit several rows with the waste yarn, getting an inch or two of edge on your work. Switch to circular knitting and knit 2 rows with ravel cord, which makes is easy to get the waste yarn off later. Then set the carriages for your regular circular cast-on with your garment yarn. The comb and weights are hanging down from the waste yarn before your "real" knitting, and your garment cast-on is being pulled together, nice-and-neat instead of sideways-and-wavy. The difference from that strategy alone is just wonderful.
Second strategy: Do the broken-toe cast-on in this old video (gosh, this is from before I did hi-def videos!). Especially check out the part at the end where I showed the nice edges. This is great information, and so easy.
And here's one more trick to put up your sleeve: If you're still not thrilled with the edge after your piece is knitted and off the machine, you can slip a blocking or ribber wire into that cast-on edge, gather up the edge, and hit it very briefly with steam. Heck, if your whole project is assembled, you can still do this to tidy up ribbing edges. Do not steam the whole ribbing, just the very edge, and briefly.
If you did the Broken Toe Cast-On in the video, you could leave in that starter row that I pull out at the end of the video. Pull on it to gather up the ribbing and then do the blocking trick, above. After the piece is thoroughly dry, pull that thread out.
Surprise! Your edge is beautiful and perfect! In a messy life, with difficult people and broken plumbing, where so many problems are achingly complex, I get great personal satisfaction from one little perfect cast-on edge...