Steph Peters wrote:

I just bought a coffee grinder from a reputable manufacturer. The only
specific requirement for spices is to have a small capacity so that the
ground spices don't get lost around a bigger grinding area. Consider how
easy it is to clean. I tend to brush out the bits from mine and give it a
wipe with a bit of damp kitchen towel. This is OK because mine is only ever
used for spices, so a bit of one spice blend in the next doesn't matter. I
don't think it's feasible to get them clean enough to swap between coffee
and spices in the same machine.

As far as the blade-type grinders go, when grinding something where a
tiny amount is needed (and when it's appropriate for the recipe) I add a
half teaspoon or teaspoon or so of coarse corn meal. This helps pick up
all the finer grit that might be lost sticking to the sides, and helps
distribute it throughout the dish being prepared.