chemical and electrochemical machining

CHEMICAL AND

PHOTO-CHEMICAL MACHINING

Manpreet Singh LittUID 16017

Department of Mechanical Engineering

Chemical Machining (CHM)

• Chemical Machining (CHM) was developed based on the observation that chemicals attack metals and etch them by using chemical dissolutions.

• Chemical machining (CHM) is the controlled chemical dissolution (CD) of the work piece material by contact with a strong reagent.

• Special coatings called maskants protect areas from which the metal is not to be removed.

• The work piece is either immersed in or exposed to a spray of chemical reagent.

• The process is the oldest of nontraditional machining processes, and has been used to engrave metals and hard stones and to remove materials from parts having a high strength-to-weight ratio.

Name of Discipline Mechanical Engineering 2

Steps in CHM

• Cleaning• Masking• Scribing• Etching• Demasking


Principle of CHM


Contour, Taper and Step Cut by CHM


Maskants

• Maskants are generally used to protect parts of the work piece where CD action is not needed. Synthetic or rubber base materials are frequently used. Maskants should, however, possess the following properties:

1. Be tough enough to withstand handling2. Adhere well to the work piece surface3. Scribe easily4. Be inert to the chemical reagent used5. Be able to withstand the heat generated by etching6. Be removed easily and inexpensively after etching


Types of maskants used in CHM

• Cut and Peel masks• Screen resist masks• Photo resist masks


Etchants

• Etchants are acid or alkaline solutions maintained within a controlled range of chemical composition and temperature. Their main technical goals are to achieve the following:

• Good surface finish• Uniformity of metal removal• Maintenance of personal safety• Maintenance of air quality and avoidance of possible environmental

problems• Low cost per unit weight dissolved• Ability to regenerate the etchant solution and/or readily neutralize and

dispose of its waste products


Maskant and Etchant Characteristics


Scribing Templates

• Scribing templates are used to define the areas for exposure to the chemical machining action.

• The most common work piece scribing method is to cut the mask with a sharp knife followed by careful peeling of the mask from the selected areas.


Advantages

• Simultaneous material removal, from all surfaces, improves productivity• No burrs are formed• No stress is introduced to the work piece, which minimizes the part

distortion• The capital cost of equipment, used for machining large components, is

relatively low.• The good surface quality in addition to the absence of burrs eliminates the

need for finishing operations• Tooling costs are minor


Disadvantages

• Handling and disposal of chemicals can be troublesome• Difficult to get sharp corner• Difficult to chemically machine thick materials• Scribing accuracy is limited, causing less dimensional accuracy• Welded areas frequently etch at rates that differ from the base metal.


Applications

High Precision Partsand Decorative Items• Gaskets• Washers • Sensors • Nameplates• Jewelry • Microprocessor Chips

• CHM is used to thin out walls, webs, and ribs of parts that have been produced by forging, casting, or sheet metal forming


Photochemical machining (PCM)

• Photochemical machining (PCM) is a variation of chemical machining (CHM) where the chemically resistant mask is applied to the work piece by photographic techniques.

• CHM is usually used on three dimensional parts originally formed by another manufacturing process, such as forging and casting of irregular shapes.

• Photochemical machining, creates new parts from thin materials, rather than simply smoothing or altering parts formed by other manufacturing methods.

• Sometimes photochemical machining is used to surface etch components with lettering or graphics where the etchant works its way to only a certain depth in the material.


Flow Chart for Photochemical machining


Steps in PCM


Electrochemical Machining (ECM)

• ECM can be thought of a controlled anodic dissolution at atomic level of the work piece that is electrically conductive by a shaped tool due to flow of high current at relatively low potential difference through an electrolyte which is quite often water based neutral salt solution.

• Electrical energy is used to produce a chemical reaction, therefore, the machining process based on this principle is known as Electrochemical machining (ECM). This process works on the principle of Faraday’s laws of electrolysis.

• Michael Faraday discovered that if the two electrodes are placed in a bath containing a conductive liquid and DC potential (5-25V) is applied across them, metal can be depleted from the anode and plated on the cathode This principle was in use for long time. ECM is the reverse of the electroplating.


Schematic of Process


Chemistry of process


Electrochemical Drilling Unit


•Power supply

• Electrolyte supply and cleaning system

• Tool and tool feed system

• Work piece and Work holding system.

Power Supply

• During ECM, a high value of direct current ( may be as high as 40000 A) and a low value of electric potential ( in range of 5-25 V) across IEG( Interelectrode gap) is desirable.

• The highest current density achieved so far is around 20,000 A/cm2.

• Hence , with the help of a rectifier and a transformer, three phase AC is converted to a low voltage, high current DC.

• Silicon controlled rectifier (SCRs) are used both for rectification as well as for voltage regulation because of their rapid response to the changes in the process load and their compactness.


Electrolyte supply and Cleaning system

• The electrolyte supply and cleaning system consisting of a pump, filter, pipings, control valves, heating or cooling coils, pressure gauges, and a storage tank ( or reservoir).

• Electrolyte supply ports may be made in the tool, work or fixture, depending upon the requirement of the mode of electrolyte flow.


Tool and Tool Feed system

• Use of anti corrosive material for tools and fixtures is important because they are required for a long period of time to operate in the corrosive environment of electrolyte.

• High thermal conductivity and high thermal conductivity are main requirements.

• Aluminum, Brass, Bronze, copper, carbon, stainless steel and monel are a few of the material used for this purpose.


Work piece and work holding system

• Only electrically conductive material can be machined by this process, The chemical properties of anode ( work) material largely govern the material removal rate (MRR).

• Work holding devices are made of electrically non conductive materials having good thermal stability, and low moisture absorption properties.

• For Example, graphite fibres reinforced plastics, plastics, Perspex,etc., are the materials used for fabricating the work holding device.


Applications


Electrochemical Grinding

• Electrochemical grinding is a process that removes electrically conductive material by grinding with a negatively charged abrasive grinding wheel, an electrolyte fluid, and a positively charged work piece.

• Electrochemical grinding and electrochemical machining are similar but a wheel is used instead of a tool shaped like the contour of the work piece.

• Process characteristics1. The wheels and work piece are electrically conductive.2. Wheels used last for many grindings - typically 90% of the metal is removed

by electrolysis and 10% from the abrasive grinding wheel.3. Capable of producing smooth edges without the burrs caused by mechanical

grinding.4. Does not produce appreciable heat that would distort work piece.5. Decomposes the work piece and deposits them into the electrolyte solution. The

most common electrolytes are sodium chloride and sodium nitrate.


Electrochemical Grinding


Uses

• Production of tungsten carbide cutting tools.• Burr-free sharpening of hypodermic needles• Grinding of super alloy turbine blades• Form grinding of aerospace honeycomb metals
