Detailed Design Specifications

The stone used is an exceptionally rare "premium black granite." This stone is characterized by its lack of crystalline grains and its higher specific gravity compared to other granites. In general, matcha mills require users to add tea leaves gradually while grinding. However, if the amount added is not consistent, the particle size of the matcha can vary. My matcha mill is specifically designed for grinding matcha only, ensuring that even those unfamiliar with using stone mills can produce uniformly ground matcha. To use, simply add all the tea leaves (tencha) you wish to grind into the top hole of the mill. Then, continue turning the upper millstone. The ground matcha is collected by an attached brush and directed into the container, making the process efficient and user-friendly.

The position of the hole relative to the center point also plays a role in grinding performance and efficiency. While it might seem that ground powder exits from all around the millstone's circumference, observation reveals that it only discharges from one specific spot on the upper millstone. Because this spot rotates, it creates the appearance of uniform discharge across the entire circumference. This specific spot on the upper millstone is located between 45 and 90 degrees counterclockwise from the handle attachment point. The reason for this range lies in the pressure distribution on the grinding surfaces. When the millstone is stationary, the pressure is evenly distributed. However, when the handle is attached and held with the right hand, additional force is applied to the handle attachment area, increasing pressure in that region. This increased pressure at one spot causes a decrease in pressure on the opposite side of the grinding surface. When the millstone rotates, the high-pressure area prevents the powder from entering, so the powder is pushed out slightly before reaching the high-pressure zone. The area where the coffee beans are first crushed often causes the upper millstone to lift slightly. To counteract this, the crushing force must be aligned with the line connecting the handle and the center point. Since the handle generates force not directly downward but angled toward the rotation direction, the hole must be located 90 degrees forward in the direction of rotation from the handle attachment. Both the size and position of the hole are determined based on careful observation, imagining the internal flow of the powder, and verification through prototypes and testing.

A common misconception is that the upper and lower grinding surfaces are entirely in contact. In reality, only a portion near the outer edge is in contact, with a gradually widening space extending inward toward the center. Materials fed into the millstone are crushed into smaller pieces step by step, eventually turning into powder at the outer edge and being discharged. This step-by-step grinding process is influenced by the structure of the "fukumi" (space), which is designed to narrow progressively to ensure materials are crushed into increasingly smaller particles. When grinding larger materials such as soybeans, the depth of the fukumi needs to be greater, ideally about half the diameter of the material. For smaller materials like sesame seeds, the depth of the fukumi is minimal or nearly nonexistent. Understanding the function of the fukumi and designing it with precise and adequate dimensions is crucial. If the fukumi is not properly designed, it can lead to issues such as large amounts of material remaining inside the millstone after grinding, or materials not entering the grinding space at all, causing them to simply spin around at the bottom of the "material feed hole" without being ground.

A brush, hidden beneath the handle of the upper millstone, collects the ground powder during rotation. The powder is automatically deposited into a container through a hole in the groove designed to catch it. When grinding coffee beans, a small amount of powder may scatter. To prevent this, the grinding surface is recessed below the outer edge of the powder groove. This requires precise machining, but it effectively minimizes powder scattering around the millstone. Note: For matcha millstones, there is no powder scattering, so the grinding surface does not require this recessed design.