Great idea Johnathan. thanks for the help. just what I wanted to do. After doing N scale for a few years I think my mind had forgotten all the HO scale stuff. Thanks again.

I know. I'm going to be running a yard style layout with tank engines and box cars, and basic freight train rolling stock. So the radius isn't was I'm worried about. 

thank you Johnathan.

you've cleared up some of my troubles. what I'm trying to do is have three 18" curves, with one on each end going into the curve. and in the middle of the curve having one 18" with a 1/3 section of 18" connected together. and between those three to have two 15" curves. I'm not to worried about it being a "perfect" circular curve. I'm a little limited on the space. I know that the 18" wouldn't work all the way, I'm trying to off center it just a little.

I wanted to know if the combination of three 18" curves, a short 18" curve section, and two 15" curves can make an half circle ans stay under 34" in width. I'm getting back into HO scale and I'm converting my N scale table to HO.