Slipjoint Folding Knife

Most people are familiar with the slipjoint mechanism thanks to the venerable Swiss Army Knife. Opening and closing the knife requires the user to overcome the tension of a spring in the back of the knife. This is a nice design to cut out of plastic or timber to get an idea of how the mechanism operates.

This design is shown with a 1/8" pivot and 3/32" holes in the spring and liner. You can use metric dimensions if you wish (e.g. 3mm pivot and 2.5mm in the spring and liner), but it's worth noting that most folding knife hardware is imported from America and is most commonly in Imperial sizes. These knives require a bit of hand tuning to get running nicely, and you'll probably find yourself hand filing a bit of relief into the liners to accomodate the blade so it can close without scratching.