Eccentric axe uses physics to make splitting firewood easier

If you've ever tried to split your own firewood, you know it's kind of a pain in the tookus. Swinging the axe with enough force to drive the wedge into the wood and also split said wood (rather than just getting the axe head stuck) is not easy. That's why lumberjacks have big arms.

So Finnish inventor Heikki Kärnä redesigned the axe. Instead of working as a wedge, his axe is a lever. And it's sort of mesmerizing to watch.

It works because the Kärnä axe's center of gravity is to the side, rather than in the center, of the blade.

Upon hitting the top of the log and penetrating it slightly, the leading edge of the axe head begins to slow down. Where the axe blade widens sharply it stops the axe's penetration. However, the mass of the axe head still has kinetic energy and the off line center of gravity forces it to rotate eccentrically down towards the wood. This rotational movement causes the leading edge, or sharp edge of the blade to turn in a lever action, forcing a split with all the force of the kinetic energy of the axe multiplied by the leverage of the axehead. The widening blade edge also has a benefit in that it helps to prevent the axe from penetrating into the wood and getting stuck there as is often the case with traditional axes.

The 1.9kg axe head has a significant amount of kinetic energy when it begins the rotational movement. While the centre of gravity of the head continues first to the right and then downwards the edge moves in a rotational direction to the left. This movement uses the rotational torque to split a log and push it away from the wood. In total the edge opens the wood by 8 cm. When the axe has rotated sideways it has used most of its energy and ends on top of the log on the in a sideways fashion. This safety feature ensures that the axe does not continue towards your legs and the axe remains totally in control. In addition, the axe holds the log steady on the chopping block ready for the next swing.

Also, the official company "Tale of the Vipukirves Axe" is sort of hilarious, in a Lake Wobegon kind of way.

Throughout his arduous work the axe often swung close to the hard working man's calves. The axe struck him more than once, but luckily the man was wearing protective overalls with his hems stiff into his rubber boots. After receiving a few mighty blows from the axe, he was forced to toss his boots into the trash. When the hard day's work was over, the man collected all the resinous branches into one pile and the trunks cut with a power saw in the other. They would wait to be cut into firewood.

"Darn it!" the man said in despair. "Making firewood is so much work, and it's dangerous too!"

He sat down on a stump, threw his gloves in the moss, wiped the sweat from his forehead and started cogitating. He grabbed the axe that the hardware salesman proclaimed to be the best on the market and began to examine the blade and the handle, turning the piece of metal in his sap-covered hands. Then it came to him.

"Eureka! I need to work on this!"

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