Strong transparent plastic. Quick, simple method to determine the centre of round or square stock up to 200mm (8") dia and hexagonal stock up to 146mm (5-3/4") dia.
 
There are many ways to find the centre of a square blank or circular object in a workshop but this Centre Finder is very handy indeed.  Put the end of the circular object or cylinder into the V and then draw a pencil line down the straight side of the slot. rotate the object by 90 degrees and repeat. Where the lines cross is the centre of the circle.
 
For an approximate centre on a less than perfect circle, repeat at different angles and you'll get an idea of the middle. This is especially true for greenwood blanks that were rounded and then left to dry. They are no longer round when the moisture is gone and this device will help you to determine best centre point.
 
I have several in my workshop and they are always there when I need them.