I own a manual coffee grinder which involves placing coffee beans in the top and then grinding it into powder. The mechanism is that the beans are pulverized by coming into contact with a burr blade inside the cylinder.
I know that grinding is generally forbidden on Shabbat but that there are also some exceptions.
Is my assumption that this is ‘assur’ correct?
You are correct, it can not be done on shabbos because it is tochain. Even if it would be done for immediate use, (which in itself is questionable here) it would still not be permitted because a specialized instruments (a grinder) is being used.