This is the Reluctant Sorcerer trilogy by Simon Hawke.
The protagonist, Dr. Marvin Brewster, is an absentminded American professor who lives in London with his fiancée, Dr. Pamela Fairbum, and is employed by a multinational conglomerate.
In the first book, he invents a time machine that uses buckyballs as one of its components.
Externally, the time machine did not appear much different from a helicopter with the rotor blades and tail removed, except for one particular, distinguishing feature. Encircling the entire assembly and the frame, positioned diagonally so that it ran around the top of the bubble and behind the back skids, was a stainless-steel tube three inches in diameter, a torus encircled by loops of superconducting wire, the interior of which was filled with a small amount of a rare substance known by the innocuous name of Buckyballs.
He activates the time machine, planning to travel ten minutes back in time as a test run, but instead lands in medieval fantasy-style realm in a parallel universe, where he meets a leprechaun named Mick O’Fallon, who mistakes him for a sorcerer.
Mick is an armourer but aspires to become an alchemist, and believes that if he can discover the secret behind creating the precious metal known as nickallirium -- AKA aluminium -- he'll finally be granted membership in the Sorcerers Guild.
The way Mick saw it, if he could convince Brewster to take him on as an apprentice, then he would have a sponsor, and that would get him over the first hurdle. Once Brewster accepted him as an apprentice, then perhaps he'd help him learn the secret of the Philosopher's Stone, which Mick was certain Brewster knew. And, in fact, he did. Brewster knew what nickallirium was, you see. He merely knew it by another name. Aluminum.
Mick is particularly impressed by Brewster's Swiss Army Knife and wants Brewster to help him make more of them.
Now, this wasn't one of the cheaper models, but a deluxe one, with two regular knife blades, a screwdriver, a can opener, a bottle opener, a saw, a magnifying glass, a scissors, an awl, a corkscrew, a toothpick, and, of course, tweezers. In other words, the whole shebang. It had red plastic handles with the authentic Swiss cross emblem on one side that marked it as the genuine article. Mick, naturally, took it to be Brewster's crest.
Brewster also suggests making a distillery to speed up Mick's production of wine.
"Well, actually, there's a much easier way," Brewster replied. "You could make a still." "A still what?" asked Mick.
"No, still is what it's called," Brewster explained. He saw Mick's frown and added, "It's short for distillery... an apparatus for brewing. It would greatly speed up the process and allow you to have a greater yield."
One of Mick's friends, a large warrior named Bloody Bob, is nearsighted and keeps losing his swords, so Brewster makes him a visor to improve his vision, for which Bob is extremely grateful.
Worst of all for Bloody Bob was the embarrassment, the sheer mortification, of losing his swords. To a true warrior, nothing was more important than his sword. He ate with it, he slept with it, but he never, ever misplaced it. It was the worst possible sin. And Bob had done it more than once. He couldn't help it. He'd put his sword down somewhere and then be unable to find it again because he couldn't see well enough. The other brigands had learned to be considerate and if they happened upon his missing blade, they'd surreptitiously place it within his reach and then arrange for him to notice it.
("Ooops! Sorry, Bob. Didn't mean to trip over your sword. Didn't see it lying on the floor there, right next to your chair. Nay, on the other side of your chair. Bob.")
The main antagonist, Warrick Morgannan, is the Grand Director of the Sorcerers Guild and has a dim-witted troll familiar named Teddy. He can also sense the narrator as a voice in the ether, and occasionally responds to him.
Well, that last chapter gave your narrator a rather nasty turn. Everyone knows fictional characters are not supposed to be able to detect the presence of the narrator and start talking back to him. (This is against all the rules of good writing, just like "breaking the rule of the fourth wall," which is what happens when an actor breaks character and starts talking to the audience, or when a narrator addresses the reader directly, which is exactly what I’m doing now, so I suppose it serves me right.) Anyway, in all the books I've written, I've never had this kind of experience before, and I don’t mind telling you, I’m not quite sure what to do about it. It's pretty weird. (Not to mention potentially confusing.)
