Chemical And Electrochemical Machining

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Chemical machining (CHM) is a nontraditional process in which material removal occurs through contact with a strong chemical etchant. Applications as an industrial process began shortly after World War II in the aircraft industry. The use of chemicals to remove unwanted material from a workpart can be applied in several ways, and several different terms have been developed to distinguish the applications. The scope of this unit includes chemical milling, chemical blanking, chemical engraving, and photochemical machining (PCM). They all utilize the same mechanism of material removal, and it is appropriate to discuss the general characteristics of chemical machining before defining

An electrochemical machining process (ECM) is an important group of nontraditional processes use electrical energy to remove material. This group is identified by the term electrochemical processes, because electrical energy is used in combination with chemical reactions to accomplish material removal. In effect, these processes are the reverse of electroplating. The work material must be a conductor in electrochemical machining.

4.2 LEARNING OBJECTIVES

The objectives of this unit are to:

1. Recognize several basic Chemical and Electrochemical Machining processes.

2. Identify differences in applications for different form of CHM and ECM.

3. Understand the different principle of Chemical and Electrochemical Machining.

4.3 CHEMICAL MACHINING OPERATIONS AND MACHINES

The chemical machining operations consists of several steps. Differences in applications and the ways in which the steps are implemented account for the different forms of CHM:

1. CLEANING - The first step is a cleaning operation to ensure that material will be removed uniformly from the surfaces to be etched.

2. MASKING - A protective coating called a maskant is applied to certain portions of the part surface. This maskant is made of a material that is chemically resistant to the etchant (the term resist is used for this masking material). It is therefore applied to those portions of the work surface that are not to be etched.

3. ETCHING - This is the material removal step. The part is immersed in an etchant which chemically attacks those portions of the part surface that are not masked. The usual method of attacks is to convert the work material (e.g., a metal) into a salt that dissolves in the etchant and is thereby removed from the surface. When the desired amount of material has been removed, the part is withdrawn from the etchant and washed to stop the process.

4. DEMASKING - The maskant is removed from the part.

4.3.1 MECHANICS AND CHEMISTRY OF CHEMICAL MACHINING

The two steps in chemical machining that involve significant variations in methods, materials and process parameters are masking and etching. Maskant materials include neoprene, polyvinylchloride, polyethylene and other polymers. Masking can be accomplished by any three methods; cut and peel, photographic resists and screen resist.

The cut and peel method involves applications of the maskant over the entire part by dipping, painting or spraying. The resulting thickness of the maskant is 0.025 to 0.125 mm thick. After the maskant has hardened, it is cut using a scribing knife and peeled away in the areas of the work surface that are to be etched. The maskant cutting operation is performed by hand, usually guiding the knife with a template. The cut and peel method is generally used for large workparts, low production quantities, and where accuracy is not a critical factor. This method cannot hold tolerances tighter than ±0.125 mm except with extreme care.

The photographic resist method uses photographic techniques to perform the masking step. The masking materials contain photosensitive chemicals. They are applied to the work surface and exposed to light through a negative image of the desired areas to be etched. These areas of the maskant can then be removed from the surface using photographic developing techniques. This procedure leaves the desired surfaces of the part protected by the maskant and the remaining areas unprotected, vulnerable to chemical etching. Photoresists masking techniques are normally applied where small parts are produced in high quantities and close tolerances are required. Tolerances closer than ±0.0125 mm can be held.

The screen resist method applies the maskant by means of silk screening methods. In these methods, the maskant is painted onto the workpart surface through a silk or stainless steel mesh. Embedded in the mesh is a stencil that protects those areas to be etched from the painting applications. The maskant is thus painted onto the work areas that are not to be etched. The screen resist method is generally used in applications that are between the other two masking methods in terms of accuracy, part size, and production quantities. Tolerances of ± 0.075 mm can be achieved with this masking method.

Selection of the etchant depends on work material to be etched, desired depth and rate of material removal, and surface finish requirements. The etchant must also be matched with the type of maskant that is used to ensure that the maskant material is not chemically attacked by the etchant. Table 4.1 lists some of the work materials machine by CHM together with the etchant that are generally used on these materials. Also included in the table are penetration rates and etch factors.

Material removal rates in CHM are generally indicated as penetrations rates, mm/min, since rate of chemical attack of the work material by the etchant is directed into the surface. The penetration rate is unaffected by surface area. Penetration rates listed in Table 4.1 are typical values for the given material and etchant and Figure 4.1 shows a chemical machining.

Table 4.1: Common work material and etchants in CHM,

with typical penetration rates and etch factors.

Work Material

Etchant

Penetration rates (mm/min)

Etch Factor

Aluminium and alloys

FeCl3, NaOH

0.020

1.75

Copper and alloys

FeCl3

0.050

2.75

Magnesium and alloys

H2SO4

0.038

1.0

Silicon

HNO3 : HF:H2O

0.025

NA

Mild steel

HCl : HNO3

0.025

2.0

Titanium and alloys

HF : HNO3

0.025

1.0

Figure 4.1: Chemical machining (CHM)

Example 4.1:

What are the three methods of performing the masking step in chemical machining?

Solution:

1. Cut and peel

2. Photographic resist

3. Screen resist

Example 4.2:

What is photographic resist in chemical machining?

Solution:

The photographic resist method uses photographic techniques to perform the masking step. The masking materials contain photosensitive chemicals. They are applied to the work surface and exposed to light through a negative image of the desired areas to be etched. These areas of the maskant can then be removed from the surface using photographic developing techniques. This procedure leaves the desired surfaces of the part protected by the maskant and the remaining areas unprotected, vulnerable to chemical etching.

4.3.2 CHEMICAL MACHINING OPERATIONS AND MACHINES

In this section, chemical machining processes are divided into 4 types:

1. Chemical milling

2. Chemical blanking

3. Chemical engraving

4. Photochemical machining

Chemical Milling was the first CHM process to be commercialized During WORLD WAR II; a U.S. aircraft company began to use chemical milling to remove metal from aircraft components. Chemical milling is still used largely in the aircraft industry, to remove material from aircraft wing and fuselage panels for weight reduction. It is applicable to large parts where substantial amounts of metal are removed during the process. The cut and peel maskant method is employed. Chemical milling produces a surface finish that varies with different work materials. Surface finish depends on depth of penetration. Metallurgical damage from chemical milling is very small, around 0.005 mm into work surface.

Chemical Blanking uses chemical erosion to cut very thin sheet metal parts down to 0.025 mm thick and /or for intricate cutting patterns. In both instances, conventional punch and die methods do not work because the stamping forces damage the sheet metal or the tooling cost would be prohibitive or both. Chemical blanking produces parts that are burr free, an advantage over conventional shearing operations. Methods used for applying the maskant in chemical blanking are either the photoresists method or the screen resist method. For small and intricate cutting patterns and close tolerances, the photoresist method is used; otherwise, the screen resist method is used.

The small size of the work in chemical blanking excludes the cut and peel maskant method. Application of chemical blanking is generally limited to thin materiala and intricate patterns. Maximum stock thickness is around 0.75 mm. Also, hardened and brittle materials can be processed by chemical blanking where mechanical methods would surely fracture the work.

Tolerances as close as ±0.0025 mm can be held on 0.025 mm thick stock when the photoresist method of masking is used. As stock thickness increases, more generous tolerances must be allowed. Screen resist masking methods are not nearly so accurate as photoresist. Accordingly, when close tolerances on the part are required, the photoresist method should be used to perform the masking step.

Chemical Engraving is a chemical machining process for making name plates and other flat panels that have lettering and artwork on one side. These plates and panels would otherwise be made using a conventional engraving machine or similar process. Chemical engraving can be used to make panels with either recessed lettering or raised lettering, simply by reversing the portions of the panel to be etched. Masking is done by either the photoresists or screen resist methods. The sequence in chemical engraving is similar to the other CHM processes, except that a filling operation follows etching. The purpose of filling is to apply paint or other coating into the recessed areas that have been created by etching. Then, the panel is immersed in a solution which dissolves the resist but does not attack the coating material.

Photochemical machining (PCM) is chemical machining in which the photoresist method of masking is used. The term can therefore be applied correctly to chemical blanking and chemical engraving when these methods use the photographic resist method. PCM is employed in metal working when close tolerances and intricate patterns are required on flat parts. Photochemical processes are also used in electronic industry to produce intricate circuit designs on semiconductor wafers. Figure 4.2 shows a material removal in Chemical Machining

Figure 4.2: Material removal in chemical machining (CHM)

Example 4.3:

What is different between Chemical Engraving and Photochemical Machining?

Solution:

Chemical Engraving is a chemical machining process for making name plates and other flat panels that have lettering and artwork on one side. These plates and panels would otherwise be made using a conventional engraving machine or similar process. Chemical engraving can be used to make panels with either recessed lettering or raised lettering, simply by reversing the portions of the panel to be etched. Masking is done by either the photoresists or screen resist methods whereas Photochemical machining (PCM) is chemical machining in which the photoresist method of masking is used. The term can therefore be applied correctly to chemical blanking and chemical engraving when these methods use the photographic resist method.

Example 4.4:

What is the limitation of Chemical blanking process?

Solution:

Application of chemical blanking is generally limited to thin materials. Maximum stock thickness is around 0.75 mm. Also, hardened and brittle materials can be processed by chemical blanking where mechanical methods would surely fracture the work.

Exercise 4.1

1. What is the purpose of filling operation in Chemical engraving process?

2. What is the advantage of using photochemical machining in electronic industry?

3. Describe the Cut and Peel method that involves in chemical machining.

4. What type of applications that suitable for screen resist method?

4.4 ELECTROCHEMICAL MACHINING AND MACHINE

The basic process in this group is electrochemical machining (ECM). Electrochemical machining removes metal from and electrically conductive workpiece by anodic dissolution, in which the shape of the workpiece is obtained by a formed electrode tool in close proximity to, but separated from, the work by a rapidly flowing electrolyte. ECM is basically a deplating operation.

As illustrated in the Figure 4.3, the workpiece is the anode, and the tool is the cathode. The principle underlying the process is that material is deplated from the anode (the positive pole) and deposited onto the cathode (the negative pole) in the presence of an electrolyte bath. The difference in ECM is that the electrolyte bath flows rapidly between the two poles to carry off the deplated material, so that it does not become plated onto the tool.

Figure 4.3: Electrochemical machining diagram (ECM)

The electrode tool, usually made of copper, brass or stainless steel, is designed to process approximately the inverse of the desired final shape of the part. An allowance in the tool size must be provided for the gap that exists between the tool and the work. To accomplish metal removal, the electrode is fed into the work at a rate equal to the rate of metal removal from the work. Metal removal rate is determined by Faraday's First Law, which states that the amount of chemical change produced by an electric current (i.e., the amount of metal dissolved) is proportional to the quantity of electricity passed (current x time).

V = CIt (1)

Where V = volume of metal removed, mm3; C = a constant called the specific removal rate which depends on atomic weight; valence, and density of the work material, mm3/amp-s; I= current, amps; and t = time, s (min). Based on Ohm's law, current I = E/R, where E = voltage and R = resistance.

Under the conditions of the ECM operation, resistance is given by

gr

A

R

= (2)

Where g = gap between electrode and work (mm); r = resistivity of electrolyte, ohm-mm; and A = surface area between work and tool in the working frontal gap, mm2. Substituting this expression for R into Ohm's Law, we have

EA

gr

I

=

(3)

And substituting this equation back into the equation defining Faraday's Law

C(Eat)

gr

V

=

(4)

It is convenient to convert this equation into an expression for feed rate, the rate at which the electrode (tool) can be advanced into the work. This conversation can be accomplished in two steps. The first one is to convert volume of metal removed into a linear travel rate:

C(Eat)

gr

V

At

=

fr

=

(5)

Where fr = feed rate, mm/s. Second, let us substitute I/A in place of E/(gr), as

provided by equation 3. Thus, feed rate in ECM is

=

CI

A

fr

(6)

Where A = the frontal area of the electrode, mm2. This is the projected area of the tool in the direction of the feed into the work. We should note that this equation assumes 100% efficiency of metal removal. The actual efficiency is in the range 90% to 100% and depends on tool shape, voltage and current density, and other factors.

The preceding equations indicate the important process parameters for determining metal removal rate and feed rate in electrochemical machining: gape distance g, electrolyte resistivity r, current I, and electrode frontal area A. Gap distance needs to be controlled closely. If g becomes too large, the electrochemical process shows down. However, if the electrode touches the work, a short circuit occurs, which stops the process altogether. As a practical matter, gap distance is usually maintained within a range 0.0075 to 0.75 mm.

Waters is used as the base for the electrolyte in ECM. To reduce electrolyte resistivity, salts such as NaCl or NaNO3 are added in solution. In addition to carrying off the material that has been removed from the workpiece, the following electrolyte also serves the function of removing heat and hydrogen bubbles created in the chemical reactions of the process. The removed work material is in the form of microscopic particles, which must be separated from the electrolyte through centrifuge, sedimentation or other means. The separated particles form a thick sludge whose disposal is an environmental problem associated with ECM. Large amount of electrical power are required to perform ECM.

As the equations indicate, rate of metal removal is determined by electrical power, specially the current density that can be supplied to the operation. The voltage in ECM is kept relatively low to minimize arching across the gap.

Electrochemical machining is generally used in applications where the work metal is very hard or difficult to machine, or where the workpart geometry is difficult (or impossible) to accomplish by conventional machining methods. Work hardness makes difference in ECM, because the metal removal is not mechanical. Typical ECM applications include:

1) Die sinking, which involves the machining of irregular shapes and contours into forging dies, plastic molds and other shaping tools.

2) Multiple hole drilling, where many holes can be drilled simultaneously with ECM and conventional drilling would probably require the holes to be made sequentially.

3) Holes that are not round since ECM does not use a rotating drill.

4) Deburring.

4.4.1 Advantages of ECM include:

1) Little surface damage to the workpart

2) No burrs as in conventional machining

3) Low tool wear (the only tool wear results from the flowing electrolyte)

4) Relatively high metal removal rates for hard and difficult to machine metals.

4.4.2 Disadvantages of ECM are:

1) Significant cost of electrical power to drive the operation.

2) Problems of disposing of the electrolyte sludge.

Example 4.5:

Name the two types of electrochemical machining.

Solution:

1. Electrochemical Deburring.

2. Electrochemical Grinding.

Example 4.6:

Identify the significant disadvantages of electrochemical machining.

Solution:

1) Cost of electrical power is quit high.

2) Problems of disposing of the electrolyte sludge.

4.4.3 ELECTROCHEMICAL DEBURRING

Electrochemical deburring (ECD) is an adaptation of ECM designed to remove burrs or to round sharp corners on metal workparts by anodic dissolution. The hole in the workpart has a sharp burr of the type that is produced in a conventional through hole drilling operation. The electrode tool is designed to focus the metal removal action on burr. Portions of the tool not being used for machining are insulated. The electrolyte flows through the hole to carry away the burr particles. The same ECM principles of operation also apply to ECD. However, since much less material is removed in electrochemical deburring, cycle time are much shorter. A typical cycle time in ECD is less than a minute. The time can be increased if it is desired to round the corner in addition to removing the burr.

4.4.4 ELECTROCHEMICAL GRINDING

Electrochemical grinding (ECG) is a special form of ECM in which a rotating grinding wheel with a conductive bond material is used to augment the anodic dissolution of the metal workpart surface. Abrasives used in ECG include aluminium oxide and diamond. The bond material is either metallic (for diamond abrasives) or resin bond impregnated with metal particles to make it electrically conductive (for aluminium oxide). The abrasive grits protruding from the grinding wheel at the contact with the workpart establish the gap distance in ECG. The electrolyte flows through the gap between the grains to play its role in electrolysis.

Deplating is responsible for 95% or more of the metal removal in ECG and the abrasive action of the grinding wheel remove the remaining 5% or less, mostly in the form of salt films that have been formed during the electrochemical reactions at the work grinding wheel in ECG lasts much longer than a wheel in conventional grinding. The result is a much higher grinding ratio. In addition, dressing of the grinding wheel is required much less frequently. These are the significant advantages of the process. Applications of ECG include sharpening of cemented carbide tools and grinding of surgical needles, other thin wall tubes and fragile parts.

Example 4.6:

What is the purpose of using electrochemical deburring?

Solution:

Electrochemical deburring is used to remove burrs or to round sharp corners on metal workparts by anodic dissolution. The electrode tool is designed to focus the metal removal action on burr. Portions of the tool not being used for machining are insulated. The electrolyte flows through the hole to carry away the burr particles.

Example 4.7:

What is the significant advantage of the electrochemical grinding?

Solution:

The grinding wheel in ECG lasts much longer than a wheel in conventional grinding. The result is a much higher grinding ratio. In addition, dressing of the grinding wheel is required much less frequently.

EXERCISE 4.2

1. A square hole is to be cut using ECM through a plate of pure copper (valence = 1) that is 20 mm thick. The hole is 25 mm on each side, but the electrode that is used to cut the hole is slightly less than 25 mm on its sides to allow for overcut, and its shape includes a hole in its center to permit the flow of electrolyte and to reduce the area of the cut. This tool design results in a frontal area of 200 mm2. The applied current = 1000 amps. Using an efficiency of 95%, determine how long it will take to cut the hole.

2. In a certain chemical blanking operation, a sulfuric acid etchant is used to remove material from a sheet of magnesium alloy. The sheet is 0.25 mm thick. The screen resist method of masking permits high production rates to be achieved. As it turns out, the process is producing a large proportion of scrap. Specified tolerances of ±0.025 mm are not being achieved. The foreman in the CHM department complaints that there must be something wrong with the sulfuric acid. "Perhaps the concentration is incorrect," he suggests. Analyze the problem and recommend a solution.

SUMMARY

In this chapter we have studied the chemical and electrochemical machining process that involves in nontraditional process. The nontraditional processes are generally characterized by low material removal rates and high specific energies relative to conventional machining operations. The capabilities for dimensional control and surface finish of the nontraditional processes vary widely, with some of the processes providing high accuracies and good finishes, and others yielding poor accuracies and finishes. Surface damage is also a consideration. Some of these processes produce very little metallurgical damage at and immediately below the work surface, while others (mostly the thermal based processes) do considerable damage to the surface.

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