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Manufacturing Processes > Machining processes > Lapping

 

Lapping

 

Process description

The removal of very small amounts of material by the relative motion of fine abrasive particles, embedded in a soft material (the lap), with the aid of a lubricating and carrier fluid.

Materials

All materials, but materials of low hardness or high flexibility present problems.

Lapping

Process variations

  • Hand lapping: operator moves the workpiece over a grooved surface plate in an irregular rotary motion, turning the part frequently to ensure uniformity.
  • Machine lapping: horizontal and vertical lapping machines with variety of floating work holding devices that can carry many parts at once over the rotating plate lap.
  • Centreless lapping: used for internal and external cylindrical, spherical and contoured surfaces.
  • Pressure jet lapping: uses a low viscosity mix of abrasive grit and water applied at high speed to the surface using compressed air. Similar to Abrasive Jet Machining (AJM).
  • Range of lap materials, abrasive materials, grain size and carrier fluids are available for different materials.

Economic considerations

  • Production rates ranging 10–3000/h, depending on level of automation.
  • Lead times are short.
  • Very little material is removed.
  • Suitable for all quantities.
  • Tooling costs vary depending on degree of automation and size.
  • Equipment costs are moderate.
  • Direct labor costs are low to moderate. Operator skill required for hand lapping.
  • Finishing costs are very low. Cleaning only required.

Typical applications

  • Any component where superior surface finish is required on flat, cylindrical or contoured surfaces
  • Bearing surfaces
  • Gauge blocks
  • Piston rings
  • Balls for ball bearings
  • Piston pins
  • Valve seats
  • Glass lenses
  • Pump gears

Design aspects

  • Complexity is limited to nature of workpiece surface, i.e. flat, cylindrical (internal and external) or spherical.
  • Lapping is performed to remove the minimum amount of material, usually between 0.005 and 0.01mm.
  • Lapping should not be specified if the surface finish on the component is not critical and can be produced by other processes.
  • Lapping logically follows the grinding or honing process to produce precision surfaces.
  • Parts required to provide lapping pressure under their own weight should have a low center of gravity and be stable.
  • Surface features should be kept simple.
  • Sizes ranging 1–500mm for flat lapping.
  • Centerless lapping sizes ranging Ø0.75–Ø300 mm. Maximum lengths are 4+ for up to Ø75 mm.

Quality issues

  • Soft materials difficult to lap due to abrasive particles becoming embedded in workpiece material.
  • Low lapping speeds can introduce beneficial compressive residual stresses into the surface of workpiece to improve fatigue resistance.
  • Choice of abrasive, lap and carrier important for specific material types.
  • Surface detail excellent.
  • Surface roughness values in the range 0.012–0.8 µm Ra obtainable.
  • A process capability chart showing the achievable dimensional tolerances is provided.

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