i found that you have to use them like your a left handed person, hold it either by using it in front of your dog and cutting them, or you if your holding your dog on your lap then turn the unit so that your holding it in your left hand, as thats the only way it will properly cut the nails safetly, i have had one of these for years now and they work fantastic for me took me while to learn to use it properly , as the sandpaper wheel inside had to be turning the right direction to cut the nail, if your using it the wrong direction then it will defiently hurt the dogs nails, but my dogs love it much better then clippers i used to use, and my large chihuahua will now just lie on his belly for his nails to be cut by me lol. matter of fact i still have one little baby chihuahua male pure white not sold yet from the most recent litter still for sale if anyone is looking for one :), have ad for him on ebay local classifed page. but i even use it on the puppy and he doesnt even move or whine on me using mine on him, like i said you have to be holding it so that their nail goes in it with the unit either from the front of them and lift their paw and place inside but not to far, i do a little at a time until i get their nails down where i need them, never start cutting with this and keep their nail it in long as it does get warm on the wheel, so just do a little of the nail and take your time so as to not get their nail to hot or the wheel to warm as thats probably what caused him pain when you where using it. hope that helps and good luck on selling it , as they do work great once you figure out how to use them properly :)