Bevan writes -
An old machinist and carpenter trick is to make an object which is free to move but trapped permanently within another object. This site has pictures of two variations on that theme, with instructions on how to make one’s own using basic manual machine tools. Suggestions for other variations are included, and mentions of possible improvements on the fabrication method.
A captivating piece of sculpture from a relatively simple process – more on this and other machinist adventures over at Bevan’s site.
They’re known as “Turner’s Cubes” since they were often assigned to apprentice machinists and lathe operators (aka turners) as a project to prove their skill.