Here is the tutorial.
I will use the body created in previous tutorial on Creating helical gear in AutoCAD.
Enter MIRROR3D command.
Select the gear body to mirror and press enter. Now it asks for 3 points of the mirror plane.
We will select the three vertex of the gear body of the top plane. First one is like this.
Now we have the body mirrored.
Enter union command.
Select both bodies and press enter. Now we have herring bone gear created in AutoCAD.
Subtract a cylinder from the gear body and we have the required gear.