The matching logic of flatness and roughness in the design of CNC sealing surface parts
For the design of precision CNC parts, especially in scenarios involving fluid sealing, the Flatness and Roughness of the sealing surface are not isolated parameters but core coupled variables that determine the leakage rate. Many engineers mark a flatness of 0.02mm or a roughness of Ra1.6μm on 2D drawings, but they overlook the mutual influence between the two in the assembly state, resulting in frequent micro-leaks in the finished products during pressure tests. Clearly defining the pressure, temperature of the sealing medium and the sealing form (contact type or fluid film) during the design stage, and then inferring the CNC processing path and post-treatment process, is the key to avoiding failure. For static seals, the requirement for flatness is usually higher than that for dynamic seals because dynamic seals allow tiny gaps to be compensated through the fluid film. However, for static seals such as the flange end face and the contact surface of the lens gasket, once the local depression exceeds the hardness limit of the material, it will form a long-term cold welding or leakage channel. Therefore, when CNC milling or turning the sealing surface, the tolerance budget must be set in accordance with the final sealing standard (such as ASME B16.5 or ISO 807).
The physical influence mechanism of flatness on the failure mode of seals
Microscopic contact Point and Leakage Channel Analysis
The failure of the sealing surface often results from the uneven distribution of microscopic unevenness. When the two sealing surfaces come into contact, the actual contact area is much smaller than the nominal area, and the main load-bearing area is concentrated on the microscopic peak. If the flatness tolerance is not properly controlled, for instance, if the allowable flatness deviation is ±0.01mm, in long shafts or multi-segment sealing structures, cumulative errors will lead to uneven pressure distribution at the edges. According to Hertz contact theory, local stress concentration will accelerate elastic or plastic deformation, causing the sealing material to extrude the Gasket under high pressure. For metal-to-metal hard seals, flatness directly determines the initial bonding rate. For non-metallic gaskets, flatness affects the compression and rebound characteristics of the gasket.
Tolerance allocation strategies under different pressure grades
Low-voltage system
