The basic gears
For this the easiest way is to start with an effect included in Inkscape (for the current version 0.46 you can find it in Effects > Render > Gears and in the upcoming 0.47 it was moved to Extensions > Render > Gears).
A few parameters to adjust (with Live Preview enabled to see their effect in real time) and we get a toothed wheel:
Now add a circle:
And use the Align and Distribute dialog to align it to the center (I used here "e;Relative to: Biggest Item"):
Then substract the circle from the wheel (you may need to ungroup once, as the Gear effect created the wheel as a group):
Now to create spokes. Add a rectangle and align it to the center:
Duplicate the rectangle and rotate it 90°:
Select both rectangles and rotate them freely:
Select everything and do an union:
For the middle of the gear create a small circle, align it to the center and do another union:
The hole for the axis is another circle aligned to the center and substracted from the wheel:
And the first gear is done!
Another gears coupled with it must have similar teeth, so use the Gear effect and change only the number of teeth:
And do a complex mechanism:
You may want to increase the complexity further by adding some parallel gears, which may have their own parameters (as long as they are not coupled with the initial gears):
Coloring the gears - Golden
Now we will make the gears "real", emulating a metallic surface, like gold (or call it bronze, the process is about the same).
The first step is to define the color, and metallic is not a color, the metallic look of a surface is given by light reflection, so we will use a multistop gradient (a gradient with more than two colors). For gold it should contain a succession of lighter and darker shades of yellow, maybe also a bit of orange, for bronze also yellows with a shade of green (copper oxidation is green), for steel it should contains greys, the chrome is also greys but more reflective (more contrast, from almost black to almost white), silver is less reflective grey and so on.
Here is my gold:
Then take one wheel and apply the gradient to it:
For a 3D look add a drop shadow (duplicate, make it black, move a few pixels down and right, move it under the wheel, add a bit of blur and maybe decrease the opacity):
The gear does not say on air, we'll put on a background, and I used the same golden gradient for the sake of simplicity, you can use a different one, maybe darker:
Add some more gears (all your golden gears). Note the usefulness of the drop shadow, without it it would be hard to set apart the gear from the background, now they are distinct objects:
To make the image more vivid (and because so looks the pocket watch I'm using as a reference, I add some steel gears. Start this by defining the gradient (multistop, greys, with a shade of blue):
And apply the gradient to some wheels:
Here is one trick to get some of the wheels richer, not that plain and boring: add a groove - two smaller circles, aligned to the center of the gear, filled with the same grey gradient, the larger in an opposite direction, the smaller in the same direction as the rest of the wheel:
Put the steel gears in the device (just take care to not couple steel gears with gold gears: steel with steel and gold with gold):
Now for some axles: small circles, made from gold, steel, ruby or sapphire. Do not forget the drop shadow and consider a white highlight:
Place the axles in the center of the gears and we are set:
But I often have a tendency to go overboard and will do now the same: add some screws holding the device. They are easy to do: create a steel circle, substract a rectangle to create the groove, add a darker steel rectangle, the bottom of the groove, rotate the screw to a random angle (we don't want all the screws to have parallel grooves, that would be repeating and boring), fix the gradient and add a drop shadow. Maybe a hole: a larger circle colored with the same gradient as the background but with an opposed orientation. (I increased the zoom level in this step for a clearer illustration)
Distribute the screws evenly (or randomly it you feel like too) and it's done:
Coloring the gears - On paper
Now is the time to try about the completely different approach, making the gears look like old schematics, old writing on old paper, where we will work on the strokes.
Go back to the black and white drawing:
If we set the stroke color and unset the fill color will get something like this, with overlapping contours, we will have to get rid of:
So select the gear (gears if we have more) suffering due to this unwanted overlap and convert the stroke to path:
The go to another gear which covers it, duplicate, select the duplicate and the former stroke and do a difference operation:
Repeat with all the gears covering it until we get to something like this:
Then convert all the remaining strokes to paths.
Now we want the drawing to look rough. But it has a large number of nodes, it will take quite a while to edit them manually for the desired rough look, so, as usual, I will cheat and use an automatic simplify operation (shown at an increased zoom level):
Repeat for all your gears and get something like:
Now define a multistop gradient for the paper - light brown/yellow for old paper or dark blues if we want to go with blueprint (I have not decided yet about the way to go).
A multistop gradient is needed for ink too (not shown), and it has to have fitting colors but good contrast with the paper (like browns for old paper and light blue for blueprints). Apply the gradients:
Then add some texture to the paper: draw a random blob with the freehand tool, fill it in a color similar with the background (but slightly darker or lighter), unset the stroke, simplify if needed and blur a lot:
Add some more until you are happy with the texture:
The images is still too sharp for an old drawing on old paper, so we will have to soften the focus. Select all the gears, duplicate, make the duplicate darker (black), apply some blur and decrease the opacity:
And this is all for now: