Likely depth-adjustment mechanisms first evolved into into the use of levers. These levers were of one of two types: static and moveable (allowing for lateral adjustment of the blade). The lever mechanism usually has some form of teeth which engage in the back of groves cut into the blade. This form of depth adjustment mechanism generally relies on three forms of lever cap to lock down the blade once the optimal position is found: (i) a disc-type screw clamping mechanism; (ii) a cam-type clamping mechanism or (iii) a butterfly-style clamping mechanism.
The first form of lever is a one often seen in early Stanley planes, with the lever encased inside a cage: pulling the lever up causes the blade to retract, pushing the lever down causes the lever to insert.
These mechanisms relied on fine groves on the back of the blade meshing with corresponding grooves on the saddle of the “cage”. The lever “cage” is held in place at the pivoting point by a ball-and-socket type joint, and at the rear by a pin which anchors it to the planes sole. These mechanisms are very jittery, and only offer a small range of movement, typically 1/8″. Because the cages saddle is curved to allow for movement, there is also limited contact point between it and the grooves in the blade.
The next form of lever-based mechanism, uses a pivot attached to an integral saddle. These levers often suffer from overhanging the end of the plane, thereby decreasing their usability as it is easy to accidentally knock the lever and put the blade out of sync.
The levers on these pivot mechanisms, often have 2-3 large teeth which engage with the back of the blade. The blade either has deep grooves, as found in the Birmingham.or a rack-and-pinion like mechanism, as found in the Ohio No.103.
The final, and very unique depth-adjustment mechanism is that of the Hobbies No. 8. It uses two pivotal levers to control both depth adjustment and lateral positioning of the blade. As the top lever is moved right, the blade inserts, when moved back to the neutral centre position, it retracts. The lower lever is used to make minor lateral adjustments.
A closer look, shows a blade with holes in the blade that meshes with a pin on the adjustment mechanism. This mechanism works quite well, as once the wing-nut locks the laver cap down, it is very difficult to accidentally move the levers. The entire lever mechanism sits on a “rearwardly and upwardly inclinded bracket“.
This lever-based depth adjustment mechanism is one of the few which has a patent. It’s British patent (No.208,401) was received in 1923, so it was a late development, when most block planes had moved to alternate mechanisms. The objective of the patent was to provide an “improved and simplified means for effecting the adjustment of the plane iron”.