Clark,
I have made many springs including feather or frizzen springs that required adding welded metal for the finial. Dan is right. Buy Ravenshear's little book on springs. It is available at Track of the Wolf. A word of caution however, about his tempering method. I like strong springs (my bias) and I found his tempering method (which you have to do outside because of the smoke) to produce somewhat soft springs unless I beefed up the metal thickness. I am fortunate now to own an electric furnace that really allows me to experiment with temperatures for normalizing, hardening, and tempering to adjust the feel of the spring.
dave