What technical difficulty delayed the successful realization of reliable multiplication on the Stepped Reckoner?

Precise manufacturing and meshing of the stepped drums

Although Leibniz had the sound mathematical concept for multiplication via the stepped drum, transforming that idea into a functional reality was extremely difficult due to the engineering limitations of the era. The primary hurdle involved achieving the absolute precision required for the stepped cylinders. If the teeth were misaligned, or if the machining process left burrs on the metal, the machine risked mechanical failure, such as skipping digits or seizing up entirely when the carry mechanism engaged. This need for exact tolerances meant that a reliably working model capable of multiplication was not fully constructed until around 1694, more than two decades after the concept originated.