Relapping

Relapping is the process of refreshing or restoring the surface finish and flatness of a previously lapped surface by performing additional lapping with finer abrasives. It is used when a surface has degraded from wear, contamination, or minor prior rework, and when achieving very tight tolerances or ultra-smooth finishes is required without removing large amounts of material.

Process: The workpiece is cleaned and prepared on a lapping setup. A fresh abrasive slurry or polishing compound is applied to a lap or the workpiece, and the surface is re-lapped with controlled pressure and motion. Material removal is kept small by using fine grits and shorter dwell times. The choice of lap material and abrasive depends on the substrate; common laps include cast iron, steel, copper, or polymer, and abrasives range from aluminum oxide and silicon carbide to diamond suspensions for very hard materials. The surface is periodically checked with precision instruments such as surface plates, straight edges, optical flats, or profilometers to verify flatness and surface roughness.

Applications: Relapping is used to restore precision flats on gauge blocks and optical flats, machine tool ways, bearing surfaces, or valve seats, and to refresh ultra-smooth optical or metrological surfaces. It is also employed in the maintenance of high-precision rotary or linear guides and in the preparation of fine optical surfaces.

Limitations and considerations: Relapping removes material and can alter tolerances if not controlled; it requires clean conditions and proper slurry management to prevent scratches or contamination. It is time-intensive and typically performed by trained technicians with appropriate equipment such as lapping machines, precision surfaces, and metrology tools.

Safety: use appropriate PPE and handle abrasive slurries carefully to avoid inhalation and skin contact.