You might want to use this technique to know where to drill the hole in the middle or draw concentric circles on the surface.

I can't take any credit for this as I probably learnt it at school, many eons ago. I just thought I'd add it because I saw lots of methods for drawing circles, using bits of string, wooden battens etc. but nothing for reverse engineering the problem.

If it's not correct then I can only blame my very poor memory.

## Step 1: Draw a chord across the circle

If you can also make the chord a nice easy length i.e. 10, 20, 24, etc this might make life easier in the next step.

A simple and accurate way to find a right angle to any line is as follows :

- Take a divider (pair of compasses) and set it to a length greater than half the length of the line, it doesn't matter exactly how long as long as it is more than half, but you need to keep the divider set to the same length throughout.

- Set the point of the compass at one end of the line (in this case where the secant intersects with the circle and draw a circle.

-Repeat the process with the point set at the other end of the line so you now have two circles which intersect each other.

- Draw a straight line between the two points where the circles intersect, this will cross the original line at it's mid-point at 90 degrees to it.

This is generally a more accurate method than measuring the midpoint with a ruler and using a square to find the right angle and doesn't require any measurements or calculations.

Thank You. When I worked on the railroad we had tools and machines to do all that.

I had just made 57 2 inch round wood blocks and then couldn't figure how to find the centers. Thanks to this article I'll make a jig and drill them now.

you are awesome

Just used this. Thanks!

1. Draw a square around the circle.

2. Draw a line from top-left to bottom-right corner of square.

3. Draw a line from top-right to bottom-left corner of square.

4. Where the lines cross is the center.

I am starting a project in which I will need the centers of some circles.

I was going to do what you have done as part of my own instructable but now I will just give a link to this.

Thanks