Thank you for your helpful answer Michael
I wasn't aware that "evaluateNormal()" function returns direction of the normal, not the normal itself.
This was the tricky part.
I took James' idea and took it further.
The results are satisfying. :)




Although, I had a problem on some extruded surfaces.
I extruded an "S shape" curve, and this is the result:



After rebuilding the surface with the network command problem solved.