I would not recommend grinding

coffee beans with a mortar & pestle. It hurts your hands, takes forever, is very messy, and your espresso tastes bad. You should really just buy a coffee grinder to keep at your house instead of taking yours to work where everyone uses it and you can't take it home. Especially if it grinds coffee really well and you would occasionally like to use it at home where you have some awesome espresso beans you ordered off the internet. I guess this is just a reminder to myself. Good advise though.