What is an interference fit when machining?

An example of an "interference fit" is when you have a pin that needs to go into a slightly smaller hole. The difference may be only 2/1000 inch (machinists measure parts in thousandths of an inch). In this case, the pin won't just slide into the hole, but needs to be forced, usually with a devise called a "press." . The usual purpose of an interference fit is to join two mating parts together in a way that they won't come apart without having to use other, more normal, fastening methods. For an elementary example of an interference fit and purpose, if you drive a nail half-way into a wall so you can hang something onto the part of the nail that sticks out of the wall, there is an "interference fit" between the nail and the new hole in the wall that the nail made.