One common joinery method used in wood working is the mortise and tenon. A mortise is basically a hole in a piece of wood and the tenon is another piece of wood cut to tightly fit in that hole. The tenon is usually secured in place with either glue or a wooden pin or wedge.
The folks over at [WayOutWest] were building a fence and needed a way to cut a bunch of mortises in 4×4 inch posts to accept 2×6 inch rails. Although they had a chainsaw, trying to cut a mortise with it by hand turned out to be super dangerous because the chainsaw would kick up every time the tip of the blade touched the wood. The team had some parts kicking around so they made a fixture to hold the chainsaw as it is plunged into the 4×4’s.
The contraption’s frame is made from an old scaffolding stand and the slides are just pipes inside of pipes. The chainsaw is bolted to the slide and a lever moves it forward and back. A second lever moves the piece of wood getting mortised up and down so that the mortise can be cut to any width. This is a pretty ingenious build that only cost a little effort and will end up saving a bunch of time mortising countless fence posts.