Gubar probably whats happening with you is that under the Create UV's menu you have Assign Shader to each Projection checked, if you uncheck that then it will keep the shader that is currently assigned to the object.
What I do is first create a numbered texture (you could use the checkered shader if you want )in the hypershade, then select the faces on the object I want to map then right click and assign that shader just to those faces and then uv map them.Continue selecting faces and assigning the texture and mapping as you go , because it's the same texture as you UV map the pieces of the same object the texture will just continue seamless, once you have all the pieces mapped sew them as best you can to make a complete UV map, it might be necessary to mabe leave one or even two pieces unsewn but on the same map but genrally you can sew together pretty much everthing leaving seams where you can see them too much.