Chemical Machining

Question 1

What is CM?

Answer 1

Chemical Machining

Question 2

Chemical Machining- Method

Answer 2

• Etching of selected workpiece material by means of a strong chemical (etchant)
• Chemical machining covers a variety of processes that use the same principle to remove material

Question 3

Chemical Machining- Process mechanism

Answer 3

• Cleaning, create a clean surface for uniform material removal when etching
• Masking, application of a protective and chemically resistant coating called maskant or resist to certain parts of the workpiece surface
• Etching, immersing the part in an etchant (acid or alkaline) which dissolves material from the unmasked workpiece surface
• Demasking, Maskant is removed from the workpiece and any residues washed off

Question 4

Chemical machining-
applying maskant (cut & peel)

Answer 4

• Maskant is applied over entire workpiece by dipping, painting or spraying (thickness 0.025-0.125mm);
• Cut with a knife using a template
• accuracy +/-0.125mm

Question 5

Chemical machining- applying maskant (screen resist)

Answer 5

• maskant is painted onto workpiece surface through a silk or stainless steel mesh
• embedded in the mesh is a stencil that protects the areas to be etched
• accuracy +/- 0.075mm

Question 6

Chemical machining-
applying maskant (photographic resist)

Answer 6

• maskant containing photosensitive material is applied and exposed to UV light through a negative image of the desired shape
• desired area of the maskant is removed using photographic developing techniques
• accuracy +/-0.0125mm

Question 7

Chemical machining- process characteristics (MMR)

Answer 7

Referred to as penetration rate; ranges from 0.01-2.00mm/min since rate of chemical attack is directed into surface
-penetration rate is unaffected by surface area

Question 8

Chemical machining- process characteristics (surface finish)

Answer 8

• ranges from 2micrometers Ra for aluminium, 1.5 micrometers Ra for steel and 0.6 micrometers Ra for titanium
• material removal with depth of up to 12mm
• hard and soft materials can be machined with equal speed and precision
• stress and burr-free surfaces

Question 9

Chemical machining- process characteristics(undercuts)

Answer 9

-etching also occurs sideways to the maskant

Fe=d/u

Question 10

Chemical machining variations- chemical milling

Answer 10

• good for shallow cavities on panels, sheets and forgings
• residual stress relieve at the beginning to avoid warping after the etching process
• good temperature and stirring is important to ensure uniform depth of material removal
• surface finish varies with the workpiece material and depends on the depth of penetration; increasing depth worsens surface finish
• small metallurgical damage (roughly 5micrometers)

Question 11

Chemical machining variations- chemical blanking

Answer 11

• used for chemical erosion of very thin sheet metal parts (0.025mm, max 0.75mm max 0.75) where conventional punch and die processes would fail due to stamping forces
• burr-free etching
• alignment of maskant critical as etched on both sides
• methods of applying maskant by using screen-resist or photographic resist
• tolerances of +/- 0.0025mm for a workpiece thickness of 0.025mm

Question 12

Chemical machining variations- photochemical machining

Answer 12

• low tooling cost
• easy to automate
• economical for medium to high volume production
• good for fragile and small parts
• etch factor is called anisotropy
• used for production of printed circuit boards, electric motor lamination and masks for TV and PC screens

Question 13

Applications of Chemical machining

Answer 13

• alternative to laser cutting and fine blanking thin foils and sheets and to conventional milling, EDM and ECM for cavities and forms
• weight reduction by thinning out walls, made by forging, casting or sheet metal forming
• aircraft industry, wings, fuselage panels for weight reduction
• electronics - printed circuit boards and intricate designs, semiconductors wafers

Question 14

Pros of Chemical machining

Answer 14

• low investment costs for tooling and equipment
• almost all metals and alloys can be manufactured, including hardened materials
• no distortion as no thermal or mechanical stress
• no burrs
• good for low volume production and one-offs
• short manufacturing lead times

Question 15

Cons of Chemical machining

Answer 15

• limited to metals only
• high volume and automation may become expensive
• production rates are slow compared to conventional machining operations
• not suited for sharp profiles
• environmental impact with the disposal chemicals
• high risk to health and safety
• safe handling of corrosive chemicals and fume distraction can be problematic