When I was a lad of twenty I spent a considerable amount of time with a pool stick in my hand. It was a passion and I quickly learned that physics had a great deal to do with the game. An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Newtons first law, and a powerful piece of information in understanding what happens on a billiards table. What I also learned is that there is someone out there that is always better than you and willing to take whatever amount of money you wish to lay down if you’re foolish enough to try him. No matter, you always have to try him at least once.
The same is true of a woodturning. There are three concepts of physics that I think about when when I’m turning. I think about two simple machines, the lever and the inclined plane, and Newtons first law. If you’re doing any turning, you’re thinking about them too but maybe you don’t know it.
More specifically, the type of lever that woodturner’s use is a first class lever. First class levers are where the fulcrum is located between the input effort and output load. In other words, the tool rest (the fulcrum) is between the turning piece of wood (the output load) and your hand on the tool handle (the input effort). By keeping the fulcrum as close to the output load as possible, you have a greater mechanical advantage and therefore exert less effort to keep the tool steady. When you move the fulcrum further from the cutting tip of the tool, you lose mechanical advantage and have to compensate by applying more input effort. You have to push down harder on the handle.
The key is to always keep the equation in your favor. Never give the spinning piece of wood more mechanical advantage than you have. There are a couple of ways to do this since sometimes you can’t put the tool rest or fulcrum close to the cutting edge. The first is to increase the length and weight of your tool handle. Increasing the length and weight of the handle balances the equation in your favor because the tool handle provides part of the input load instead of just your hand. Anatoly Tsiris uses this concept in turning his large pieces by using a 135+ lb boring bar that is 10 feet long.
Another method is to trap the tool handle in a captured hollowing rig. These are becoming very popular and both commercial products and home grown plans are available. This method effectively applies a large amount of input effort by keeping the handle static along the Y axis between two bars but allowing movement through the X axis. The captured hollowing rig will let you increase the distance the tool tip is from the tool rest a lot and give you better tool control.
The inclined plane is the second simple machine we use regularly when turning. It’s represented by the cutting edge and bevel that peels away the curly pieces of wood. This is what allows knives to work efficiently and the same principal is applied to the sharp edge of a bowl gouge. I mentioned this in a previous post discussing correlations with sports and woodturning. Here’s an exerpt from that post:
The fastest way to grasp this is to take your pocketknife out and use it to cut through a 1 inch square board by keeping the blade perpendicular (90 degrees) to the board’s surface and using a sawing motion. This will take a long time because you are cutting directly across the wood fibers and it isn’t an efficient cut. After you finish this long and strenuous exercise, angle your knife blade so that it is 30 degrees off of the board’s surface and whittle the wood away until you have cut the square piece in half. You’ve just demonstrated the advantage of riding the bevel. The angled inclined plane of the knifes surface lifts grain away from the rest of the board while riding the underlying fibers into the cut. This has a tremendous mechanical advantage to cutting across the grain. That’s why saws have serrated edges instead of just a sharp blade and you use a mallet with a chisel when cutting across the grain. You have to make up for the mechanical advantage that you’ve lost to get the cut you desire.
So next time you’re turning try to remember these simple physics concept and apply them to your technique. You’ll find that things are a lot easier. Good luck and happy turning and applied physics.