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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. With prolonged use, coffee beans' oils can leave stains on the millstone. However, due to the deep black color of the stone, such stains are barely noticeable. The exterior design is divided into 16 equal sections around the circumference, forming a twisted rhombus shape. The surface is textured with a chisel to represent the waves of the Amakusa Sea, and then further refined and polished to reflect light. This process results in a subtle reflection across the entire surface, and the irregular chisel marks give it a unique and distinctive appearance. The upper millstone is designed to hold enough beans for two servings (approximately 20g) at a time. However, it can be used repeatedly to grind larger quantities without generating heat that could cause "bean scorching." (Exposure to extreme heat is said to increase bitterness in coffee.) The diameter of the grinding surface was determined through trials to ensure sufficient grinding capacity without over-grinding, and it is set at 5.0 sun (approximately 15 cm). The height of the upper millstone was calculated based on the necessary weight. The lower millstone features a donut-shaped groove around the outer edge of the 5.0 sun (15 cm) grinding surface. This groove is 8-9 bu wide (24-27 mm) and 5 bu deep (15 mm), designed to collect the ground coffee. The donut-shaped groove has a single "hole," through which the ground coffee is swept by a "brush" attached to the underside of the upper millstone. With each rotation, the coffee falls through the hole into the groove. The "hole" in the lower millstone is crafted to accommodate a "tray," with an opening measuring 10 cm wide, 10 cm deep, and 6 cm high. This design determines the height of the lower millstone.

Detailed Design Specifications

The position of the hole at the top of the upper millstone is set as close to the center as possible. This design maximizes the working distance inside the millstone. If the hole were positioned further outward, the working distance would decrease, which would be disadvantageous for grinding. The diameter of the hole determines the amount of material supplied inside the millstone. If the diameter is too large, material is supplied too quickly, increasing efficiency but causing the powder to become coarser if the grinding surface's capacity is exceeded. Conversely, if the diameter is too small, the grinding surface cannot be used to its full potential, resulting in unnecessary additional rotations. For coffee beans, there are variations in bean size depending on the variety. To prevent clogging, this factor is also taken into consideration when determining the hole size.

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.