CHM

CHEMICAL MACHINING(CHM):

CHEMICAL MACHINING(CHM) Presented by: NAGESH N.M.

CONTENTS:

CONTENTS Introduction Principle Steps Processes Design considerations Characteristics Materials Advantages and Dis -advantages Applications Conclusion References

Introduction:

Introduction Chemical machining(CHM) Was developed based on the observation that chemicals attack metals and etch them by using chemical dissolutions. CHM is the removal of metal by chemical attack by a corrosive liquid. The areas of the work piece which are not to be machined are masked. The work piece is either immersed in or exposed to a spray of chemical reagent. CHM was basically developed for aerospace industry to maintain strength of part at reduced weight.

PRINCIPLE OF CHM:

PRINCIPLE OF CHM An etchant resistant mask, made typically of rubber or plastic is used to protect those parts of the component from which no material is to be removed. Part to be machined is defined by any one of the procedures: scribing & removing the coating, exposing to light & dissolving unexposed region, or silk screen printing. Strong acid or alkaline solution is used to dissolve materials selectively. Chemically resistant coating is applied to protect surfaces not to be machined.

Principle:

Principle Figure : (a) Schematic of chemical machining process (b) Stages in producing a profiled cavity by chemical machining

STEPS IN CHEMICAL MACHINING:

STEPS IN CHEMICAL MACHINING The 5 steps in chemical machining are as follows : Part preparation: cleaning. Masking: application of chemically resistant material Etching: dip or spray exposure to etchants Mask removal: stripping the maskant and cleaning the part. Finishing: inspection and other processing

Types of masks used in CHM:

Types of masks used in CHM Cut and peel masks. Screen resist masks. Photo resist masks . Cut and peel maskants Film of chemically resistant material is applied to the work piece by dipping, spraying or flow coating. Vinyl, styrene and buta diene are used. Rubbery film is then cut & peeled away selectively. Manual scribing of mask material usually achieves an accuracy of ±0.13 mm to ±0.75 mm.

Screen resist maskants:

Screen resist maskants Mask material is applied to the work piece surface by printing, using stencils and a fine polyester or stainless steel mesh screen. Relatively thin coatings with tolerances held to ±0.05 to ±0.18 mm are obtained by screen printing. Etching depths are restricted to about 1.5 mm because of thinness of the coating. Photo resist maskants Photo resist mask is quite widely used & is often referred to as photochemical machining. Produces intricate & finely detailed shapes using a light activated resist materials. WP coated with photo resist material & a master transparency is held against the WP, while exposure to UV rays takes place. Light activates the photo resist material in those areas corresponding to opaque parts. Tolerances of ±0.025 to 0.005 mm can be produced.

CHEMICAL MACHINING PROCESSES   :

CHEMICAL MACHINING PROCESSES Chemical milling Chemical blanking Chemical engraving Photochemical machining

Chemical milling:

Chemical milling In chemical milling, shallow cavities are produced on plates, sheets, forgings and extrusions. The two key materials used in chemical milling process are etchant and maskant . Etchants are acid or alkaline solutions maintained within controlled ranges of chemical composition and temperature. Maskants are specially designed elastomeric products that are hand strippable and chemically resistant to the harsh etchants. Chemical milling is used in the aerospace industry to remove shallow layers of material from large aircraft components missile skin panels extruded parts for airframes. Figure : Missile skin-panel section contoured by chemical milling to improve the stiffness- to- weight ratio of the part Chemical milling is used in the aerospace industry to remove shallow layers of material from large aircraft components missile skin panels (Figure 7), extruded parts for airframes.

Chemical blanking:

Chemical blanking Chemical blanking is used to etch entirely through a metal part. In chemical blanking, holes and slots that penetrate entirely through the material are produced, usually in thin sheet materials. Used to produce fine screens, flat springs, etc… Very cheap but efficient.

Figure : parts profiled by chemical blanking process:

Figure : parts profiled by chemical blanking process

Chemical Engraving:

Chemical Engraving Chemical Engraving is the practice of incising a design on to a hard, usually flat surface, by cutting grooves into it. The result may be a decorated object in itself, as when silver , gold , steel , or glass are engraved, or may provide an printing plate, of copper or another metal, for printing images on paper as prints or illustrations; these images. Types of engraving Wood Engraving Copper and Steel Engravings Laser engraving

Photo Chemical Machining (PCM):

Photo Chemical Machining (PCM) Photochemical machining (PCM), also known as photochemical milling or photo etching, is the process of fabricating sheet metal components using a photo resist and etchants to corrosively machine away selected areas. PCM can be used on virtually any commercially available metal or alloy, of any hardness. It is limited materials 0.0005 to 0.080 in (0.013 to 2.0 mm) thick. Metals include aluminium , brass , copper , inconel , manganese , nickel , silver , steel , stainless steel , zinc and titanium Process steps for photochemical machining

Design considerations:

Design considerations Designs involving sharp corners, deep & narrow cavities, severe tapers or porous work piece should be avoided. Undercuts may be developed because etchant attacks both in horizontal and vertical direction. To improve production rate the bulk of the work piece should be shaped by other machining process priorly . Dimensional variations can occur ,this can be minimized by proper selection of artwork media by controlling the environment. Many product designs are now made with computer aided design.

Characteristics of Chemical Machining:

Characteristics of Chemical Machining Shallow removal of material (up to 12mm) Blanking of thin sheets. Low tooling and equipment cost. Suitable for low production runs. Material removal rate(0.1mm/min)

Materials :

Materials Aluminum Chromium Copper (oxygen free, rolled…) Nickel Steel (carbon, stainless…) Lead Zinc

Advantages :

Advantages 1 . Easy weight reduction 2. No effect of work piece materials properties such as hardness 3. Simultaneous material removal operation 4. No burr formation 5. No stress introduction to the work piece 6. Low capital cost of equipment 7. Easy and quick design changes 8. Requirement of less skilled worker 9. Low tooling costs 10. The good surface quality 11. Using decorative part production 12. Low scrap rates (3%)

DIS-ADVANTAGES :

DIS-ADVANTAGES 1. Difficult to get sharp corner 2. Difficult to chemically machine thick material (limit is depended on work piece material, but the thickness should be around maximum 10 mm) 3. Scribing accuracy is very limited, causes less dimensional accuracy 4. Etchants are very dangerous for workers 5. Etchant disposals are very expensive

Applications :

Applications High Precision Parts and Decorative Items Gaskets Washers Sensors Nameplates Jewelry Microprocessor Chips

CONCLUSION:

CONCLUSION Chemical machining is widely used to produce complex machine parts for various application as well as decorative parts. The machining operation should be carried out carefully to produce a desired geometry. Environmental laws have important effects when chemical machining is used.

REFERENCES:

REFERENCES Modern machining process by P C Pandey . www.ieee.org www.chemicalmachining.org www.photochemicalmachining.com

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