Fixing normals after separating mesh into new objects I have small problem, let say I have this mesh:



At the end it needs to be split into separate chunks/objects (in this example it's split in five chunks) like this:



The problem is that normals are wrong as opposed to original mesh which makes shading go wrong (visible seams between objects). I've tried to use Auto Smooth and it's quite better, but still worse than original mesh.

This mesh is just an example of my original mesh which at the end will be exported in order to be used in external game engine with OpenGL as a backend renderer, so for OpenGL it doesn't matter whether it's rendered as one object or not (I'm NOT using triangle strips) as long as normals are the same as were before. Is there a possibility to separate mesh into objects while maintaining original normals?

Thank you in advance.
Unfortunately Blender has no method to adjust vertex normals by hand, so the only way to fix this (well, sorta...) is to separate your mesh in a way that the vertex normals don't change their direction at the cuts. 

In a nutshell this means cutting up your mesh in the areas where you have a more even geometry. Depending on your geometry this might not entirely fix your problem but it reduces the amount of difference in the shading significantly. Keep in mind that this also might increase the loopcuts, which results in more vertices/faces.

In the image below I marked the areas where I put the loopcuts with green arrows.