HERE is a fantastic paper on the Swedish engineer C.E. Johansson who invented a way of machining steel Gages that were able to offer flexible standardisation to about 1 millionth of an inch. They were known a Jo Blocks in honour of their inventor. He commentated that: “My sets of gauge blocks have become for the engineering industry what types are for the printing industry.”
One of the most remarkable features of Jo Blocks is that when two of them are placed together they stick (this is known as wringing) and cannot be pulled apart easily - they need to be twisted off each other.
