This material details the multi-stage lapping process required to achieve an ultra-precision surface finish, targeting a flatness specification of less than one micron. The methodology employed a coarse lapping stage using a cast iron plate followed by a fine lapping stage utilizing an epoxy resin plate charged with diamond liquid. The procedure emphasizes the critical role of lapping rings for maintaining plate flatness and outlines the use of both qualitative (optical flat/monochromatic light) and quantitative (laser interferometer) metrology for validation. Initial inspection revealed poor flatness, necessitating plate correction before the final processing run. Final metrological inspection confirmed an achieved flatness of $0.6$ microns.
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Achieving Sub-Micron Flatness: A Precision Lapping and Metrology Protocol
0:00 Project Objective: The goal is to lap a specific surface to an ultra-precision standard, targeting a flatness tolerance within one micron ($1.0 \mu \text{m}$).
0:18 Two-Stage Lapping Process: The process is divided into two distinct stages: a coarse stage (performed on a green machine) and a fine lapping stage (performed on a blue machine).
0:35 Coarse Lapping Equipment: The coarse stage utilizes a machine featuring a cast iron lapping plate and three floating lapping rings. These rings are essential for continuously correcting the plate's flatness, as a convex plate results in a concave part (an inverse copy of the plate's shape).
1:41 Coarse Finish Result: Following the first stage, the part exhibits a uniform matte surface finish, estimated at an RA (average roughness) of approximately $4$ microns.
2:03 Fine Lapping Stage: The second stage employs an epoxy resin (plastic) lapping plate. Diamond liquid slurry is sprayed onto this plate to act as the abrasive medium.
2:15 Fine Finish Result: The fine lapping produces a highly reflective, shiny surface finish. The resulting surface roughness is reported as $0.005$ microns RA (or $5$ nanometers).
2:36 Intermediate Flatness Inspection: Flatness is initially checked using an optical flat placed under a monochromatic light source, generating optical interference fringes. Straight, parallel fringes indicate optimal flatness ($3:16$).
3:06 Process Correction: The initial inspection showed the surface flatness was "awful," necessitating a correction to the flatness of the main lapping plate before proceeding with further material removal.
3:36 Final Metrological Verification: The finished part is taken to a dedicated Metrology lab for quantitative measurement using a laser interferometer.
3:55 Final Achieved Flatness: The laser interferometer measurement confirmed the part achieved a flatness of $0.6$ microns, successfully meeting the project specification of being within one micron.