Answer
Wave springs require mating surfaces to be flat and parallel within 0.05 mm per 25 mm of diameter. If the housing floor or the pressure plate is tilted, the wave spring will be compressed unevenly, meaning some waves will reach their work height while others are still relatively unloaded. This results in a 'stiffening' effect where the initial spring rate appears lower than calculated, followed by a sharp increase. Furthermore, the 'high waves' will experience stresses exceeding the design limit, leading to localized permanent set or fatigue failure. In precision optical assemblies, mating surfaces are often ground and lapped to ensure that the extremely low preloads required (often $<5$ N) are applied uniformly across the entire circumference.
