There are two core types of depth-adjustment mechanisms in block planes (apart from manual adjustment). The first involves a thumbscrew which travels vertically on a machine thread. The thumbscrew engages a lever which has pins that mesh with grooves in the back of the blade. As the thumbscrew is raised, the lever pivots up, extending the blade down. Lowering the thumbscrew retracts the blade. This mechanism is commonly found on block planes with 20º bedding angle.
The second mechanism is a screw mechanism that is attached to a “sled” that moves in and out extending or retracting the blade. The screw mechanism can be horizontal, or at an angle commensurate to that of the bedding angle of the plane. This mechanism appeared with the advent of low-angle block planes, where the bedding angle was 12º, and lacked the space necessary for the first mechanism.
Older block planes often exhibit a lever based depth adjustment mechanism. The following blog posts will discuss these blade depth adjustment mechanisms in more detail.