Lapping

Lapping produces ultra-flat, ultra-smooth surfaces using abrasive slurry and low pressure, ideal for precision sealing and mating faces after prior machining or grinding.

Overview

Lapping is a precision finishing process that uses abrasive slurry between a moving lap plate and the workpiece to achieve extremely flat, smooth surfaces. It corrects minor shape errors from grinding or machining and can hit flatness in the microinch range with very low surface roughness (down to single-digit microinches Ra). It works best on rigid, relatively flat parts where both faces or critical sealing areas must mate perfectly.

Use lapping when you need tight flatness, controlled material removal, and consistent surface texture across a batch of parts. It is common as a final step after heat treatment or grinding, especially for sealing faces, optical components, and gauges. The tradeoffs: lapping is slow, requires specialized fixtures, and is usually practical for small to medium parts and moderate volumes. It is not suited for large stock removal or complex 3D shapes, and you must control cleanliness and part handling to avoid scratching and edge roll-off.

Common Materials

  • Tool steel
  • Stainless steel 316
  • Aluminum 6061
  • Tungsten carbide
  • Alumina ceramics
  • Optical glass

Tolerances

±0.0001" on thickness, flatness down to 0.000005" TIR, Ra down to ~2–4 µin

Applications

  • Mechanical seal faces
  • Valve seats and valve plates
  • Gauge blocks and precision shims
  • Optical flats and windows
  • Pump and compressor plates
  • Hydraulic spool and bore mating faces

When to Choose Lapping

Choose lapping when you need very high flatness and low surface roughness on already-machined or ground parts, especially for sealing or precision mating faces. It suits rigid, mostly flat parts in low to medium volumes where consistency across a batch matters more than raw throughput. Use it as a final finishing step, not for heavy stock removal.

vs Manual Polishing

Choose lapping over manual polishing when you need controlled flatness, thickness, and repeatable surface finish across many parts. Manual polishing can improve appearance but struggles to hold tight geometry and consistency, while lapping uses fixtures and defined processes to hit quantifiable specs on flatness and Ra.

vs Electropolishing

Choose lapping over electropolishing when flatness, tight thickness control, or precise sealing faces are critical. Electropolishing smooths and deburrs complex shapes uniformly but removes material isotropically and can distort flatness, whereas lapping targets specific faces with controlled, low material removal.

vs Grinding

Choose lapping over grinding when the ground surface already meets size but needs better flatness and a finer, more uniform finish. Grinding handles higher stock removal and larger surfaces, but lapping refines the last microns, improving seal performance, leak tightness, and optical or gauge quality.

vs CNC machining

Choose lapping over CNC machining when the machined surface cannot reliably hit the required flatness or surface finish, especially after heat treatment. CNC defines geometry and features efficiently, then lapping corrects residual distortion, tool marks, and waviness on critical faces.

vs Superfinishing

Choose lapping over superfinishing when you must control both flatness and parallelism on faces rather than just improving surface texture on cylindrical or curved surfaces. Superfinishing excels on bearing journals and rolls, while lapping delivers ultra-flat, tightly controlled planar interfaces.

Design Considerations

  • Design critical lapped areas as accessible, flat pads or rings rather than interrupted or highly segmented surfaces to simplify fixturing and improve uniformity
  • Limit required stock removal before lapping (typically 0.0001"–0.001") and leave grinding or machining to handle bulk material removal
  • Specify clear, measurable requirements such as flatness (e.g., 0.0001" over 2") and surface roughness (Ra/Rz) instead of vague terms like “mirror finish”
  • Avoid tall, thin, or highly flexible geometries in lapped regions; stiffness improves flatness control and reduces edge roll-off
  • Call out which faces must be lapped together as matched sets if you need controlled thickness or parallelism between them
  • Tell the shop the material hardness and heat-treatment condition, since very hard or gummy materials may require different abrasives, laps, and cycle times