I want to attach something small to the smooth metal shaft of a small motor (2-3 mm in diameter), like fan blades, but I want it to be temporary.
If I make it too loose, then the fan won't turn at all, but if I glue or solder it on, then it will be too permanent and I want to be able to take it off.
Is there a way to firmly attach something temporary to a smooth, small shaft that has low friction while still being able to remove it when it needs to be?
I'd recommend searching for shaft couplings. These are a great option for connecting two rotating shafts in a non-permanent fashion.
A set screw (as mentioned) can work in low load cases, but is often best with a flat spot cut into the shaft so that you've got more than just friction between the screw end and shaft to constrain things. The setscrew may also scratch or dent the shaft.
Other designs of coupling will actually tighten and clamp to the shaft in question. They'll have a much better surface to keep static friction and may actually have more clamping force. Certainly better for a smooth shaft.
A further advantage of a coupling is that there are a number of more sophisticated designs that will transmit rotation even if the shafts are not well aligned. Options exist to allow small amounts of bending or translation.
A method which works extremely well and which does not require set screws and/or flats on the shaft and which can be added or removed easily and rapidly is a collet based connector.
While any one of these will only work for a limited range of shaft sizes, units are available off the shelf for a wide range of shaft diameters and you can make or modify them yourself.
Here is one example - many styles are available.
Assembled view at top.
In the middle row the "collet" is the slit part in the middle.
The nut at left is assembled onto the thread at right with the collet between them.
A conical shape inside the nut compresses the collet jaws onto the shaft as the nut is tightened.
The collet is fixed relative to shaft and nut and thread by friction.
If desired the collect could be keyed or pinned to the nut but this is seldom necessary.
I do not know how the threaded bolt at bottom is used - it is not essential for the above scheme but may optionally insert into the "nut" and provide a threaded shaft end at right.
The above image is from this page where there are numerous other versions.
While these versions are intended to attach items to the end of a shaft, the same principle allows a collar etc to be attached anywhere along a smooth rod - with a flange or bolted together discs or other means used to attach the "payload".
From the Stack Exchange robotics page:
Comes this version of the above:
The above and similar are available from here for a range of shaft sizes and similar are available from many other sellers.
Consider using something like a set screw.
As can be seen in the left hand side of this image:
The set screw is a small screw used to apply a high amount of friction and to keep a surrounding device in place.
Another option would be a dissolvable adhesive like this one. The idea being that you can use the adhesive to keep the fan blades in place but then dip the component in water to get the blades to release later.
There are two methods that immediately come to mind.
If you have access to a 3D printer and appropriate software, you could 3D print a coupler. You can adjust the radius of the hole to control how tight the coupler is to the shaft. I do this for connecting motors to LEGO axles. I have been surprised at how much friction I get from plastic against a smooth metal shaft.
