Using cylinder to represent robot joint

I'm developing a robot simulation GUI. Each robot link was represent by a stick as shown in figure below:

http://imageshack.us/photo/my-images/641/robotpg.jpg//

I wish to use cylinder to represent link joint,as shown in figure below:

http://imageshack.us/photo/my-images/21/robotjoint.jpg/

I have x,y,z coordinates for point2,point3 and point4. How can i manipulate this data to generate the cylinder?