Take a look at this gimbal motor GM3506 (ebay). All affordable gimbal motors I can find have a similar type of hollow shaft. My question is how do I actually attach a shaft there? My initial idea was to connect a BLDC to a DC using a shaft, so I could do some simple classic experiments such as step torque load transient. DC motors kind of look like this (ebay). I do not have a 3D printer. I do not have ideas either 
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Do you have any practical suggestions?
Without acces to a maker shop or a friend with a 3D printer, you could try to find an RC-prop adapter that matches the hole pattern of the GM3506.
The shaft of the prop adapter is probably thicker than the DC motor shaft, so you’ll need a (flexible) coupler, too. They come in many sizes, metric or inches.
make sure you have the right screws. GM3506 has M2.5 threads, not M3.
I bought a whole set of assorted screws M2-M3 for “laptop-repair”
It may be possible to modify this kind of flexible shaft coupler https://www.ebay.com/itm/322125182601
Pull it apart, take out one of the clamping screws, and either drill holes to match the spacing of the screw holes on top of the motor, or file grooves in the sides of it if the motor holes are too far apart. As long as you can get enough contact on the screw heads it should work. Use button heads since they don’t stick up as much. You could carve some divots in the plastic part to allow it to fully seat over the screw heads, or just leave a little gap.
I think they’re only available down to 5mm shaft size though, so it wouldn’t work for that 4mm shaft DC motor.
This may or may not work. It would be a dirty solution by all means.
You could stick a M5 screw through the hollow shaft and maybe use small O-rings to center it within the shaft. Then use a M5 shim and M5 nut to tighten it. Maybe you can find a short piece of i.e aquarium hose in your local hardware store to use as coupler to the DC motor.
(If you know somebody owning a lathe that person could produce a much better solution in no time)
