Before the planing machine was invented, making a long guide to provide a linear motion was difficult. This provided the incentive to design linkages with only revolute joints that can generate approximate or exact straight lines—such linkages are called the straight line mechanisms. Although today’s technology is excellent for manufacturing precise linear guides, the straight line mechanisms are still applicable due to the advantages, compared to the using of linear guides, of lower friction and better precision. For the shown mechanism, it is a straight line mechanism consisting of a six-bar linkage and a spur gear pair. For the spur gears, one gear is fixed on the frame and serves as a sun gear, and the other one is the planet gear that can rotate around the sun gear. For the six-bar linkage, a connecting rod is attached to the planet gear and another someone is constrained inside a sliding slot on the frame. When the planet gear is driven, the connecting rod is motivated and it therefore drives the overall linkage. And the link inside the sliding slot can make an exact straight line motion.