Types of ultra-precision machining

Lapping 

This is a form of optical sanding, where the surface roughness of the workpiece is ground to a level that allows for polishing. Materials such as alumina and SiC abrasives are used along with cast iron or aluminum lapping plates for processing glass lenses. Lapping typically uses cast iron plates with precise shape accuracy, with abrasives and lapping fluids supplied between the workpiece and the plate, transferring the shape of the plate onto the workpiece.

Lapping is divided into planar lapping, spherical lapping, and cylindrical lapping. If scratches form during lapping, the cause could be environmental contamination, foreign objects in the abrasive, or uneven grinding pressure.

Small parts such as precision molds, or non-simple flat or spherical shapes, which cannot be efficiently lapped and have rough surface finishes that cannot be crushed with loose abrasives, are ground with a whetstone. Whetstones are divided into WA (alumina) and SiC types, with the latter being particularly hard and prone to causing scratches if used with excessive force. To prevent scratches from the whetstone, start with a soft stone, avoid large stones, and adequately supply grinding oil while minimizing the whetstone’s movement stroke.

Polishing

This is the final step of grinding that generates gloss. The amount of material removed is minimal, and no lapping plate is required. Polishing tools are typically made from woods like Japanese oak, American pine, paulownia, or willow, and materials such as acrylic, nylon, or PBT can also be used. Steel polishing is generally done with diamond paste. Common reasons for workpiece deformation during polishing include microcracks from processing, impacts, material defects, and excessive grinding force. Care must be taken to avoid damaging the workpiece during polishing.

During or after grinding, three key aspects are typically evaluated:

• Appearance
• Surface accuracy
• Dimensional accuracy

Dimensional accuracy is often qualitatively assessed through visual inspection or under a microscope, with subjective evaluations based on experience. Precision measurement machines can also be used for strict quantitative assessments, and gauges or fit-feel methods are sometimes employed to determine pass/fail criteria. Surface inspections of ground pieces can be done using both general and optical methods.

Grinding regular molds to optical surface quality increases grinding costs. Even if a mold’s shape is slightly deformed, smoothness is often sufficient. As long as flow paths do not affect resin flow or demolding, leaving grinding marks is generally acceptable. In contrast, optical parts that are polished by feel alone may turn expensive workpieces into scrap. Understanding the workpiece’s material hardness, grindable volume, and shape precision requirements is crucial for efficiently achieving reasonable specifications.

Ultra-precision machining equipment

These can be broadly divided into two types. One is the aspheric generating machine, which can shape a workpiece into the required form and has grinding capabilities. The other is the ultra-precision mechanical processing machine, which achieves the final surface accuracy through grinding and cutting.

Aspheric generating machine

Aspheric generating machines are suitable for producing astronomical telescopes. Early control methods were copying, then punched tape control, and now computerized control. They are suited for large, simply-shaped, rotationally symmetrical workpieces with extremely high precision but slow processing speed. The equipment is expensive, immobile, and unsuitable for small workpieces.

Ultra-precision mechanical processing machine

These machines use computerized control and can process materials such as aluminum, stainless steel, gold, silver, various crystals, and plastics. The workpiece is often held in place by vacuum clamping, and the environment must be temperature-, humidity-, pressure-, and dust-controlled. The cutting tools used are typically single-crystal diamond cutters, and grinding is done with diamond or CBN vitrified wheels or coated wheels. However, due to slow processing speeds, limited material types, high equipment costs, and restrictive setup locations, processing marks are often unavoidable.