=== INTRODUCTION ===
There are 12 (twelve) fans in my printer and every now and then I break the blades. To spare time & money I have to print the spare blades frequently.

=== SIMILAR STAFF ===
Search on Thingiverse brought me here:
https://www.thingiverse.com/thing:3025985

As difference to olavl I sketch one blade and rotate it around center. This looks a bit more elegant for me. The other thing is sketches are not attached to surfaces but first datum planes are applied to surfaces and then sketches are applied on datum planes. This seems to be "lege artis" for FreeCAD 0.17.

For those who want to learn and exercise in FreeCAD you'll find all parameter explanations in Spreadsheet in the FreeCAD file.

=== WHAT WE'VE GOT HERE ===
Two things are provided here:
1/ the .stl file of 50mm fan blade which should fit to most 50mm fans.
2/ the FreeCAD file where you can input your desired parameters to produce the blades of needed size, blades number, their slope angle, curvature, interblades distance, thickness, motor shaft diameter, etc.

=== SLICING ===
When you will slice the model please pay attention to the layer width. The optimal width is half of the blade thickness. Than you will get the uninterrupted line connecting blades and the body (as you can see on the last picture).

It sounds reasonable to set the jitter to 360 degrees so that each new layer starts at random point (if your slicer supports this feature). Than you will have no seam and probably will have less vibration. Although it doesn't really matter at the speeds we use these fans.

=== FAN REPAIR SEQUENCE (as on pictures) ===:
1/ Remove the small teflon C-shaped washer from the back of the shaft. You will need 1-2 small screwdrivers. Cover it with your finger or it will jump away and you will have to make new.
2/ Remove small rubber ring from motor shaft. You WILL need it later.
3/ Pull the shaft out of blades body with pliers. Don't be scared you don't need the old broken blades part.
4/ With a flat screwdriver remove the magnet ring from the blades.
5/ Check if the shaft tightly fits into it's new place in the new blades. If the hole is too small you can drill it with appropriate drill. :-) If it is too big, you will need to change the radius and reprint it :-( Also check if magnet ring fits into the blades body.
6/ Use cyanoacrylate glue (superglue). You need to act fast. Put tiny drop of glue in the hole. Push the shaft in the hole. Use paper tower to wipe the excess of glue which will be squeezed out of the hole (otherwise it will glue the shaft to it's sleeve). Put the motor on the shaft. This will guarantee that the shaft will be kept perpendicular to the blades body. Push with something on the shaft for several seconds. U-u-u-f-f.. done.
7/ Put the rubber ring on the shaft.
8/ Push the magnet ring into the blades body.
9/ Assemble the fan.
10/ Put the C-shaped washer on the back of the shaft.
11/ Put a drop of oil on the back of the shaft.
12/ Cover the back of the shaft with a sticker.
13/ Celebrate!
14/ Optionally: tip the designer.