Hardness and brittleness of advanced sintered ceramics pose significant machining challenges, typically performed by abrasive processes. Widely used in industry, flat lapping reconciles shape correction and finishing of difficult-to-machine materials. Traditionally, lapping employs oils as a lapping medium that are harmful to the environment when not disposed of properly, which increases production costs. On the other hand, there is growing demand for sintered ceramic parts and components for the mechanical, electronic, and photonic industries that require sustainable processes to remove material without introducing microstructural defects. Within this context, this study investigates the flat lapping of polycrystalline alumina disks using a CNC lap-grinding machine reconfigured for precision ceramic lapping. The alumina workpieces, produced via single-action uniaxial cold pressing at 80 MPa and sintered at 1600 °C for two hours, exhibited a polycrystalline microstructure composed of grain clusters (~ 100 µm), individual grains (~ 5 µm), and closed pores (~ 25 µm). Porosity negatively affected surface roughness, increasing Ra, Rq, Rz, Rt, and Rp values, while reducing Rv and Rsk and significantly increasing Rku. Lapping tests were conducted using an aluminum plate, single-crystalline diamond abrasives (54 µm), and distilled water as the lapping medium. The best surface finish was achieved at 25 kPa pressure, with Ra = 0.894 ± 0.166 µm and Rku = 13.866 ± 7.964, indicating plateau-like surfaces. Material removal mechanisms were predominantly brittle, governed by three-body abrasion, with limited ductile contributions from two-body abrasion. Higher pressures enhanced plastic deformation and widened removal tracks, improving surface quality without compromising finish. Material removal rates (MRRs) were up to three times higher at 25 kPa compared to 20 kPa. The use of distilled water proved effective and sustainable, eliminating the need for solvent-based cleaning and reducing environmental impact. The Fiocchi CNC machine demonstrated high precision despite low structural rigidity, enabled by engineering solutions. The results confirm the feasibility of achieving deterministic, efficient, and sustainable ceramic machining.