There are two types of grinders in the world, blade grinders and burr grinders.
Blade grinders are everywhere and can be found for as little as $10-12 at your local supermarket, and believe me, if you’re paying more than that you’re paying way too much. They are suitable and recommended for spices, but aren’t the best thing for coffee.
They don’t grind the beans, they chop them up — and there’s a big difference. The rapidly rotating blade hacks the beans apart bit by bit with a very uneven final grind. If you use an auto-drip brewer you may notice larger pieces of beans on the top of the grounds when you go to throw the filter away (or when you compost it if you’re being earth-friendly).
You’re not getting the full potential out of your beans using a blade grinder. The solution is simple.
A burr grinder is the way to go. They are more expensive than blade grinders (as with many things you can spend as much or as little as you want on one), but the coffee you get from one is worth spending whatever you think you can afford.
A burr grinder pushes the beans between two surfaces, one stationary and one rotating, a set distance apart. The result is a very consistent grind that makes the best cup of coffee. The distance between the two surfaces can be widened and narrowed depending on the brewing method to be used and individual taste.
Do some research, find one in your price range and buy it.
In the words of the immortal Stan Lee, “‘Nuff said.”
By the way, I wouldn’t suggest using a mortar and pestle to grind your coffee. I tried it once and it sucked!