27.2 Chemical Machining

Question 1

Definition of chemical machining (CM)?

Answer 1

Shallow removal on large flat or curved surfaces, blanking of thin sheets

• low tooling and equipment costs
• suitable for low production runs

Question 2

Basic overview of chemical machining?

Answer 2

• CM is based on the observation that chemicals attack and etch( corrode) most materials, thereby removing small amounts of material from the surface.
• CM process is carried out by chemical dissolution using reagents or etchants eg acid and alkaline solutions
• shallow cavities are produced on ates, sheets forgings and extrusions, generally for an overall reduction of weight. Depths of removal can be up to 12mm on metals

Question 3

How is selective attacking achieved?

Answer 3

Selective attack by the chemical reagent on different areas of workpiece surfaces is accomplished by removable layers of material called masking or by partial immersion of the part in the reagent

Question 4

Process of chemical machining?

Answer 4

1) relieve the part of any residual stress prior to processing to prevent warping
2) the surface are degreased and cleaned thoroughly, to ensure both good adhesion of the masking material and uniform rate of material removal. Scale from prior heat treatment should be removed
3) the masking material, called maskant, is applied. Typically using tapes, paints, elastomers and plastics. Maskant should not react with chemical reagent
4) the areas in the maskant that will require etching is peeled off…. see book

Question 5

Examples of where CM used?

Answer 5

• In aerospace industry to remove shallow layers of material from large aircraft components, missile skin panels, and extruded part for airframes
• also used to fabricate microelectronic devices

See table for range of surface finish and tolerance obtained by chemical machining compared to other processes

Question 6

Design considerations for CM

Answer 6

• Avoid designs with sharp corners, deep and narrow cavities, severe tapers, folded seams or porous workpiece materials because the etchant attacks all exposed surfaces continuously
• Because the etchant attacks the material in both vertical and horizontal directions, undercuts may develop, in the areas under the edges of the maskant

Should be used if workpiece

• has complex shapes
• brittle materials
• difficult to clamp

Objective

• shallow cavities
• no stresses to the parts
• want a good surface finish

CM improves production rates on large samples