A search on the topic will reveal a lot of old threads on this.
When you grind, the grinder,blade, and you become a Van de Graaff generator. You end up being the reservoir for the charge because you are the most ungrounded object. When your hand gets close to the grinder ( or any earth grounded object), the charge neutralizes with a mini lightning bolt. As the weather gets colder and/or dryer, this becomes more of a problem.
The solutions are all different ways to do the same thing.....ground yourself better. The simplest and safest way is to change your shoes, and maybe put down an anti-static floor mat. This will allow the charge to drain away as it builds, making the sparks stop. Leather soles are the best, rubber the worst. The static strips worn by people in the operating room and other places where sparks are a bad thing do the same thing. In the worst of winter,I now stick a small piece of chain ( pull cord chain, but any thin chain will work) in one shoe, with just enough out to drag on the floor about an inch. I used to use a ground clip that was a 3 foot piece of the same type of pull chain with an small alligator clip on it. I would tuck the chain in my belt, ,and clip the end to the grinder table. There was a 1 megohm resistor in the line for safety. This worked well, but became unnecessary once I started using the ground strap in one shoe.
Stacy