To always have fresh coffee I put in the grinder only the amount I want to drink at that moment. Then I just grind until no more coffee comes out - but that causes the grinder to run empty for a few seconds every time.
Can it cause harm to the grinder?
I have the Rancilio Rocky (without doser)
No it's not a problem. Producers know that people at home often single dose their coffee, so I assume they consider that when producing a grinder (in fact most coffee shops that do pour over grind single doses as well). Maybe in some manuals it's written that you shouldn't let it run empty on very fine settings, but that's more to avoid warranty claims than because it would actually hurt your grinder and I assume they are referring to letting it run for minutes on end, which you probably shouldn't do. And why would you? The burrs in any case shouldn't touch each other at all, even on fine settings (or only at the flat part of the burrs, but certainly not the blades). I have yet to seen a grinder that was getting damaged due to the few seconds of running empty after your dose went through.
No it won't be a problem. Prosumer and commercial grinders will not be damaged by running empty for short time periods like you describe. Your Rocky grinder will be fine.
And the vast majority of inexpensive consumer grinders that I have seen would also be OK in this situation.
It could stress the motor and mechanics of the grinder because of the high rotary speed the grinder will achieve when not under load. That high-pitched whine doesn't sound healthy, in any case.
But i agree with the other answers that short periods of empty running must be expected by the designers.
