[mwibbels]

While it is a bit hard on the manual mill's quill and chuck - the method I like to get everything lined up and the gear in place:
1. make sure the bore of the gear is properly sized
2. mount a sacrificial Al block in the vice
3. mount the drill chuck in the spindle with a small center drill with a tip smaller than the 3.2 mm shaft
4. peck a divot in the Al block - less deep than the length of the shaft extension out the back of the motor
(then without moving the table of the mill)
5. remove the drill bit and open up the chuck until the outside edge of the chuck is ~ the same size as the outside of the gear
6. set the backend of the FP shaft in the Al block's divot and the gear loosely on the other end of the output shaft
7. press the quill down until it contacts the gear and holds it in place - then use your fingers to align the gear with the chuck teeth
8. press a bit more and the gear should slide onto the shaft without buckling the shaft

For small motors, big mills, and robust chucks - I don't worry too much about the stresses - for smaller equipment I would proceed with caution - could take the load off of the chuck by constructing at stepped shaft that will seat on the bottom of the chuck to align and press with.