I don't recommend using a grinder. First off, it's something new and different, so it's scary. Then, it makes noise, so it's even more scary. If you really want to grind their nails down, then I suggest using an emery board nail file. It'll take longer, but it's not as big or as noisy.
What type of clippers are you using? Do they start screaming before or after you start trimming? (This question is for determining the issue.) Whenever you do their nails, you want the sharpest clippers possible to minimize nail cracking. For the pigs that I do, I use different types of nails clippers meant for humans, and not the clippers that one might use on a cat. (Toe nail clippers are the sharpest, though I don't recommend due to the space between blades is wide enough that a wrong move can be dangerous.)
Trimming their nails is something that is very scary to them and doesn't feel the greatest. If you've ever accidentally cut into the quick, it's even more scary now. As a previous poster said, try offering them veggies or treats to distract. (Lettuce is a good distraction for some.) How do you hold them when you do their nails? If you hold them like how you hold them everyday, they might not be so scared.
A thing to try is to hold them for ten minutes like you usually would, but instead of the usual, you're playing with their nails by just petting them or touching them. Get them used to their nails and feet being touched. (This might take a few days.) Then, let them smell the clippers and maybe after awhile, touch their nails with the clippers. Do this until they're feeling safer around the clippers. (This might also take a few days.) Once they are more comfortable, clip one nail halfway through a session of them with the clippers. Only do one nail in that way during each time you're holding them so to keep them from getting too stressed. Always offer treats while prepping, clipping, and after. (For added comfort and distraction, talk to them in your "treat" voice.)
