This Instructable is a simple method of centering lathe cutting tools to the work. This also happens to be one of a series of Shop cheats I have been posting. If you would like to read any of the previous here are some links:
Quick change tool posts spoil people rotten. Until recently a single screw tool post was the only tool post I had to work with. So every time I changed tools I would have to center the tool to the work. And I had no idea about this! All I needed to do the hundreds of times I centered tools was pinch a steel ruler between the tool tip and work. If the ruler is anything but exactly vertical, the tool is NOT centered. This is governed by some good old trigonometry. Tangent functions to be exact. Here is the quick trigonometry refresher. A tangent is a line parallel to a single point on a curve. So on a circle any line parallel to a single point is also at a right angle to the center. That means that a tangent line that is at a 90 degree angle to the lathe bed, is also at the center line of a circle. And the steel rule is a neat tangible substitute for the esoteric mathematical concept of a tangent. Now that I have my math nerd hat off, I prefer to have my cutting tools slightly above center. So the top of the ruler leans toward the work piece a little, but that is my personal preference. This is another tip I learned from the "Home Shop Machinist" e-mail newsletter.