In this the very first post I will show you how to make cogs for a gear train as seen in my model 'Grandfather Clock'.
Open Blender, remove the default cube and enter top view.
Keyboard Shortcut: Keypad 7
Now add a circle.
Keyboard Shortcut: SHIFT+A
Now for this step you need to know how many teeth your cog is going to have because in the Operator panel (left hand side) you need to set the Vertices count. You require twice the number of planed teeth. I decided on 30 teeth so I put in 60.
Press Tab to Edit mode and double tap A. This will select the entire circle.
Make sure you are in vertex select mode then Scale inward until you have the outer ring of the cog. Keyboard Shortcut: S
It is now time to make your ring 3d. Adjust view to see it on it's side the Extrude along the Z-Dimension.
Keyboard Shortcut: E
Now you need to make the cross-section. Start by selecting 1 or 2 Faces on the inner side of the ring and removing them. ENSURE YOU SELECT FACES when prompted or you could damage your model.
Keyboard Shortcut: X
Then select the faces that are directly opposite and remove them.
Now select the opposing vertices and Fill.
Keyboard Shortcut: F
At this point you can skip to the next step or repeat the process to add another beam. If you choose to add a second beam you will need to slice your 1st beam in order allow the 2nd through.
Keyboard Shortcut: CTRL+R (CMD+R for MAC)
After slicing your beam remove the faces that will be intersected and Fill as before.
It's time to add the teeth on the outer edge, go to Face select mode and select every second face. You can select multiple faces by holding SHIFT.
Return to top view and tap Extrude, press Scale so the new faces will expand from the center. Scale them to be approximately the distance from the ring as the space between them.
Without deselecting the faces change the pivot point to Individual Origins. Then Scale down about 70%.
Your basic Cog is finished but it looks a little flat.
Return to Object Mode and go to Modifiers (Wrench Tool). Add 2 Sub-Surf Modifiers, set them to two and set the FIRST one to Simple.
This way allows you to turn the modifiers off if you want a lower poly count.
Rename your Cog to something descriptive and Save.
Keyboard Shortcut: SHIFT+S
At this point you will want to add materials and textures to give it some life.
Note that if you make more cogs with different amounts of teeth you will have to scale them appropriated to ensure the teeth are the same size. Guiding rule is that a cog with twice the teeth needs to be twice the scale, half the teeth, half the scale.
I hope you are able to use this technique to make some cool things.