I'm so sorry for your loss. Losing a grandparent is so hard.
That sounds like a pretty powerful spell, though. It may take quite a bit of practice before anything happens. If you don't mind my asking, what specifically are you hoping for? A spell will most likely not create a full-body apparition outside of something like a vision unless you are already prone to seeing that kind of thing or in touch with that side.
Also, did you create this spell yourself or are you using one that you have found somewhere else? A few more details would be really helpful so that we know if what you are trying to accomplish is even possible.
Personally, I have never really used a spell to say goodbye. When I lost my grandmother a couple years ago, I dealt with my grief through creative writing, documenting pieces I remember of her, my relationship with her, how I felt about her, that sort of thing, and it really helped me to come to terms with the loss.
Writing might not be for everyone, but I always recommend trying a creative outlet of some sort. If you still want to try a spell to communicate with him, that is perfectly understandable too. So just fill in a few more details and I'm sure you'll get more helpful answers
One thing to bear in mind is that communication doesn't always happen the way one expects. It is actually comparatively rare for a person to have the chance to 'converse' with a spirit. Most often interaction comes in ideas and messages, or signs and hints.
For example it is unlikely (though not impossible) to be able to do a calling, feel your grandfather's presence, and have a chat. Instead you might get a sudden sensation of warmth, or an indistinct word but recognizeable as his voice. Or an idea of comfort. Or a sudden mental picture that pops in intuitively. Like suddenly imagining him sitting in a chair nearby. Or, he might make his presence known to you in dreams where he comes to visit you or where you might suddenly find him while doing other things.
It could even be something really simple, but familiar like a lingering scent. Or an urge to pick up and read one of his journals or favorite books. The idea is to work at letting go of expectations and allowing him to contact you in his own way, trusting that he will find a way that you recognize.