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Manufacturing Processes > Non-Traditional Machining > Chemical Machining(CM)

 

Chemical Machining(CM)

 

Process description

Selective chemical dissolution of the workpiece material by immersion in a bath containing an etchant (usually acid or alkali solution). The areas that are not required to be etched are masked with ‘cut and peel’ tapes, paints or polymeric materials.

Materials

Most materials can be chemically machined with the correct chemical etchant selection, commonly: ferrous, nickel, titanium, magnesium and copper alloys, and silicon.

CM

Process variations

  • Chemical milling: chemical removal of material to a specified depth on large areas.
  • Chemical blanking: used for thin parts requiring penetration through thickness.
  • Photochemical blanking: uses photographic techniques to blank very thin sheets of metal, primarily for the production of printed circuit boards.
  • Thermochemical machining: uses a hot corrosive gas.
  • Electropolishing: for removal of residual stresses in surfaces.
  • Chemical jet machining: uses a single jet of etchant.

Economic considerations

  • Production rates low to moderate. Can be improved by machining a large sheet before cutting out the individual parts. Parts can also be etched on both sides simultaneously.
  • Linear penetration rate very slow, typically 0.0025–0.1 mm/min, but dependent on material.
  • Lead times short.
  • Setup times short.
  • Material utilization poor. Scrap material cannot be recycled.
  • Disposal of chemicals used can be costly.
  • Economical for low production runs. Least economical quantity is 1.
  • Tooling costs low.
  • Equipment costs generally low.
  • Direct labor costs low.

Typical applications

  • Primarily used for weight reduction in aerospace components, panels, extrusions and forgings by producing shallow cavities
  • Printed circuit board tracks
  • Features in silicon wafers for the electronics industry
  • Decorative panels
  • Printing plates
  • Honeycomb structures
  • Irregular contours and stepped cavities
  • Burr free parts

Design aspects

  • High degree of shape complexity possible in two-dimensions.
  • Suitable for parts affected by thermal processes.
  • Undercuts always present. The etch factor for a material is the ratio of the etched depth to the size of undercut.
  • Controlling the size of small holes in thin sheet difficult.
  • Compensation for the undercut should be taken into account when designing the masking template.
  • Inside edges always have radii. Outside edges have sharp corners.
  • Possible to machine thin and delicate sections due to no processing forces.
  • Minimum thickness =0.013 mm.
  • Maximum depth of cut =13 mm.
  • Maximum size =3.7m×15 m, but dependent on bath size.

Quality issues

  • Residual stresses in the part should be removed before processing to prevent distortion.
  • Surfaces need to be clean and free from grease and scale to allow good masking adhesion and uniform material removal.
  • Masking material should not react with the chemical etchant.
  • Parts should be washed thoroughly after processing to prevent further chemical reactions.
  • Porosity in castings/welds and intergranular defects are preferentially attacked by the etchant. This causes surface irregularities and non-uniformities.
  • Room temperature and humidity, bath temperature and stirring need to be controlled to obtain uniform material removal.
  • Surface detail is good.
  • Surface roughness values ranging 0.4–6.3µm Ra and are dependent on the material being processed.
  • Achievable dimensional tolerances for selected process and material combinations are provided.

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