Chemical machining ( CHM ) is a untraditional procedure in which stuff remotion occurs through contact with a strong chemical etchant. Applications as an industrial procedure began shortly after World War II in the aircraft industry. The usage of chemicals to take unwanted stuff from a workpart can be applied in several ways, and several different footings have been developed to separate the applications. The range of this unit includes chemical milling, chemical blanking, chemical engraving, and photochemical machining ( PCM ) . They all utilize the same mechanism of stuff remotion, and it is appropriate to discourse the general features of chemical machining before specifying
An electrochemical machining procedure ( ECM ) is an of import group of untraditional procedures use electrical energy to take stuff. This group is identified by the term electrochemical procedures, because electrical energy is used in combination with chemical reactions to carry through stuff remotion. In consequence, these procedures are the contrary of electroplating. The work stuff must be a music director in electrochemical machining.
4.2 LEARNING OBJECTIVES
The aims of this unit are to:
1. Acknowledge several basic Chemical and Electrochemical Machining procedures.
2. Identify differences in applications for different signifier of CHM and ECM.
3. Understand the different rule of Chemical and Electrochemical Machining.
4.3 Chemical MACHINING OPERATIONS AND MACHINES
The chemical machining operations consists of several stairss. Differences in applications and the ways in which the stairss are implemented history for the different signifiers of CHM:
1. Cleaning – The first measure is a cleansing operation to guarantee that stuff will be removed uniformly from the surfaces to be etched.
2. MASKING – A protective coating called a maskant is applied to certain parts of the portion surface. This maskant is made of a stuff that is chemically immune to the etchant ( the term resist is used for this dissembling stuff ) . It is hence applied to those parts of the work surface that are non to be etched.
3. ETCHING – This is the material removal measure. The portion is immersed in an etchant which chemically attacks those parts of the portion surface that are non masked. The usual method of onslaughts is to change over the work stuff ( e.g. , a metal ) into a salt that dissolves in the etchant and is thereby removed from the surface. When the coveted sum of stuff has been removed, the portion is withdrawn from the etchant and washed to halt the procedure.
4. DEMASKING – The maskant is removed from the portion.
4.3.1 MECHANICS AND CHEMISTRY OF CHEMICAL MACHINING
The two stairss in chemical machining that involve important fluctuations in methods, stuffs and procedure parametric quantities are dissembling and etching. Maskant stuffs include neoprene, polyvinylchloride, polythene and other polymers. Dissembling can be accomplished by any three methods ; cut and skin, photographic resists and screen resist.
The cut and skin method involves applications of the maskant over the full portion by dunking, painting or spraying. The ensuing thickness of the maskant is 0.025 to 0.125 millimeters midst. After the maskant has hardened, it is cut utilizing a scribing knife and peeled off in the countries of the work surface that are to be etched. The maskant film editing operation is performed by manus, normally steering the knife with a templet. The cut and Peel method is by and large used for big workparts, low production measures, and where truth is non a critical factor. This method can non keep tolerances tighter than A±0.125 millimeter except with utmost attention.
The photographic resist method uses photographic techniques to execute the masking measure. The cover stuffs contain light-sensitive chemicals. They are applied to the work surface and exposed to visible radiation through a negative image of the coveted countries to be etched. These countries of the maskant can so be removed from the surface utilizing photographic developing techniques. This process leaves the coveted surfaces of the portion protected by the maskant and the staying countries unprotected, vulnerable to chemical etching. Photoresists dissembling techniques are usually applied where little parts are produced in high measures and close tolerances are required. Tolerances closer than A±0.0125 millimeter can be held.
The screen resist method applies the maskant by agencies of silk testing methods. In these methods, the maskant is painted onto the workpart surface through a silk or chromium steel steel mesh. Embedded in the mesh is a stencil that protects those countries to be etched from the picture applications. The maskant is therefore painted onto the work countries that are non to be etched. The screen resist method is by and large used in applications that are between the other two dissembling methods in footings of truth, portion size, and production measures. Tolerances of A± 0.075 millimeter can be achieved with this dissembling method.
Choice of the etchant depends on work stuff to be etched, desired deepness and rate of stuff remotion, and surface finish demands. The etchant must besides be matched with the type of maskant that is used to guarantee that the maskant stuff is non chemically attacked by the etchant. Table 4.1 lists some of the work stuffs machine by CHM together with the etchant that are by and large used on these stuffs. Besides included in the tabular array are incursion rates and etch factors.
Material remotion rates in CHM are by and large indicated as incursions rates, mm/min, since rate of chemical onslaught of the work stuff by the etchant is directed into the surface. The incursion rate is unaffected by surface country. Penetration rates listed in Table 4.1 are typical values for the given stuff and etchant and Figure 4.1 shows a chemical machining.
Table 4.1: Common work stuff and etchants in CHM,
with typical incursion rates and etch factors.
Work Material
Etchant
Penetration rates ( mm/min )
Etch Factor
Aluminum and metals
FeCl3, NaOH
0.020
1.75
Copper and metals
FeCl3
0.050
2.75
Magnesium and metals
H2SO4
0.038
1.0
Silicon
HNO3: Hafnium: Water
0.025
Sodium
Mild steel
HCl: HNO3
0.025
2.0
Titanium and metals
Hafnium: HNO3
0.025
1.0
Figure 4.1: Chemical machining ( CHM )
Example 4.1:
What are the three methods of executing the dissembling measure in chemical machining?
Solution:
1. Cut and skin
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 execute the masking measure. The cover stuffs contain light-sensitive chemicals. They are applied to the work surface and exposed to visible radiation through a negative image of the coveted countries to be etched. These countries of the maskant can so be removed from the surface utilizing photographic developing techniques. This process leaves the coveted surfaces of the portion protected by the maskant and the staying countries unprotected, vulnerable to chemical etching.
4.3.2 Chemical MACHINING OPERATIONS AND MACHINES
In this subdivision, chemical machining procedures are divided into 4 types:
1. Chemical milling
2. Chemical blanking
3. Chemical scratching
4. Photochemical machining
Chemical Milling was the first CHM procedure to be commercialized During WORLD WAR II ; a U.S. aircraft company began to utilize chemical milling to take metal from aircraft constituents. Chemical milling is still used mostly in the aircraft industry, to take stuff from aircraft wing and fuselage panels for weight decrease. It is applicable to big parts where significant sums of metal are removed during the procedure. The cut and Peel maskant method is employed. Chemical milling produces a surface coating that varies with different work stuffs. Surface finish depends on deepness of incursion. Metallurgical harm from chemical milling is really little, around 0.005 millimeter into work surface.
Chemical Blanking uses chemical eroding to cut really thin sheet metal parts down to 0.025 millimeters thick and /or for intricate film editing forms. In both cases, conventional clout and decease methods make non work because the stamping forces damage the sheet metal or the tooling cost would be prohibitory or both. Chemical blanking green goodss parts that are burr free, an advantage over conventional shearing operations. Methods used for using the maskant in chemical blanking are either the photoresists method or the screen resist method. For little and intricate film editing forms and close tolerances, the photoresist method is used ; otherwise, the screen resist method is used.
The little size of the work in chemical blanking excludes the cut and Peel maskant method. Application of chemical blanking is by and large limited to thin materiala and intricate forms. Maximum stock thickness is about 0.75 millimeter. Besides, hardened and brickle stuffs can be processed by chemical blanking where mechanical methods would certainly fracture the work.
Tolerances every bit near as A±0.0025 millimeter can be held on 0.025 millimeter midst stock when the photoresist method of cover is used. As stock thickness additions, more generous tolerances must be allowed. Screen resist dissembling methods are non about so accurate as photoresist. Consequently, when close tolerances on the portion are required, the photoresist method should be used to execute the masking measure.
Chemical Engraving is a chemical machining procedure for doing name home bases and other level panels that have lettering and graphics on one side. These home bases and panels would otherwise be made utilizing a conventional engraving machine or similar procedure. Chemical engraving can be used to do panels with either deep-set inscription or raised inscription, merely by change by reversaling the parts 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 procedures, except that a filling operation follows etching. The intent of filling is to use pigment or other surfacing into the deep-set countries that have been created by etching. Then, the panel is immersed in a solution which dissolves the resist but does non assail the coating stuff.
Photochemical machining ( PCM ) is chemical machining in which the photoresist method of cover is used. The term can hence be applied right 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 forms are required on level parts. Photochemical procedures are besides used in electronic industry to bring forth intricate circuit designs on semiconducting material wafers. Figure 4.2 shows a material remotion in Chemical Machining
Figure 4.2: Material remotion in chemical machining ( CHM )
Example 4.3:
What is different between Chemical Engraving and Photochemical Machining?
Solution:
Chemical Engraving is a chemical machining procedure for doing name home bases and other level panels that have lettering and graphics on one side. These home bases and panels would otherwise be made utilizing a conventional engraving machine or similar procedure. Chemical engraving can be used to do panels with either deep-set inscription or raised inscription, merely by change by reversaling the parts 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 cover is used. The term can hence be applied right to chemical blanking and chemical engraving when these methods use the photographic resist method.
Example 4.4:
What is the restriction of Chemical blanking procedure?
Solution:
Application of chemical blanking is by and large limited to thin stuffs. Maximal stock thickness is about 0.75 millimeter. Besides, hardened and brickle stuffs can be processed by chemical blanking where mechanical methods would certainly fracture the work.
Exercise 4.1
1. What is the intent of make fulling operation in Chemical scratching procedure?
2. What is the advantage of utilizing 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 procedure in this group is electrochemical machining ( ECM ) . Electrochemical machining removes metal from and electrically conductive workpiece by anodal disintegration, in which the form of the workpiece is obtained by a formed electrode tool in close propinquity to, but separated from, the work by a quickly streamlined electrolyte. ECM is fundamentally a deplating operation.
As illustrated in the Figure 4.3, the workpiece is the anode, and the tool is the cathode. The rule underlying the procedure is that stuff 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 quickly between the two poles to transport off the deplated stuff, so that it does non go plated onto the tool.
Figure 4.3: Electrochemical machining diagram ( ECM )
The electrode tool, normally made of Cu, brass or chromium steel steel, is designed to treat about the opposite of the coveted concluding form of the portion. An allowance in the tool size must be provided for the spread that exists between the tool and the work. To carry through metal remotion, the electrode is fed into the work at a rate equal to the rate of metal remotion from the work. Metallic remotion rate is determined by Faraday ‘s First Law, which states that the sum of chemical alteration produced by an electric current ( i.e. , the sum of metal dissolved ) is relative to the measure of electricity passed ( current ten clip ) .
V = CIt ( 1 )
Where V = volume of metal removed, mm3 ; C = a invariable called the specific remotion rate which depends on atomic weight ; valency, and denseness of the work stuff, mm3/amp-s ; I= current, As ; and t = clip, s ( min ) . Based on Ohm ‘s jurisprudence, current I = E/R, where E = electromotive force and R = opposition.
Under the conditions of the ECM operation, opposition is given by
grARoentgen= ( 2 )
Where g = spread between electrode and work ( mm ) ; R = electric resistance of electrolyte, ohm-mm ; and A = surface country between work and tool in the on the job frontal spread, mm2. Substituting this look for R into Ohm ‘s Law, we have
EagrI=( 3 )
And replacing this equation back into the equation specifying Faraday ‘s Law
C ( Eat )grVolt=( 4 )
It is convenient to change over this equation into an look for provender rate, the rate at which the electrode ( tool ) can be advanced into the work. This conversation can be accomplished in two stairss. The first 1 is to change over volume of metal removed into a additive travel rate:
C ( Eat )grVolAt=Fr=( 5 )
Where Fr = provender rate, mm/s. Second, allow us replace I/A in topographic point of E/ ( gr ) , as
provided by equation 3. Therefore, provender rate in ECM is=CurieAFr( 6 )
Where A = the frontal country of the electrode, mm2. This is the jutting country of the tool in the way of the provender into the work. We should observe that this equation assumes 100 % efficiency of metal remotion. The existent efficiency is in the scope 90 % to 100 % and depends on tool form, electromotive force and current denseness, and other factors.
The predating equations indicate the of import procedure parametric quantities for finding metal remotion rate and provender rate in electrochemical machining: gape distance g, electrolyte electric resistance R, current I, and electrode frontal country A. Gap distance demands to be controlled closely. If g becomes excessively big, the electrochemical procedure shows down. However, if the electrode touches the work, a short circuit occurs, which stops the procedure wholly. As a practical affair, spread distance is normally maintained within a scope 0.0075 to 0.75 millimeters.
Waters is used as the base for the electrolyte in ECM. To cut down electrolyte electric resistance, salts such as NaCl or NaNO3 are added in solution. In add-on to transporting off the stuff that has been removed from the workpiece, the following electrolyte besides serves the map of taking heat and H bubbles created in the chemical reactions of the procedure. The removed work stuff is in the signifier of microscopic atoms, which must be separated from the electrolyte through extractor, deposit or other agencies. The detached atoms form a thick sludge whose disposal is an environmental job associated with ECM. Large sum of electrical power are required to execute ECM.
As the equations indicate, rate of metal remotion is determined by electrical power, specially the current denseness that can be supplied to the operation. The electromotive force in ECM is kept comparatively low to minimise curving across the spread.
Electrochemical machining is by and large used in applications where the work metal is really difficult or hard to machine, or where the workpart geometry is hard ( or impossible ) to carry through by conventional machining methods. Work hardness makes difference in ECM, because the metal remotion is non mechanical. Typical ECM applications include:
1 ) Die sinking, which involves the machining of irregular forms and contours into hammering dies, plastic casts and other defining tools.
2 ) Multiple hole boring, where many holes can be drilled at the same time with ECM and conventional boring would likely necessitate the holes to be made consecutive.
3 ) Holes that are non round since ECM does non utilize a rotating drill.
4 ) Deburring.
4.4.1 Advantages of ECM include:
1 ) Small surface harm to the workpart
2 ) No burrs as in conventional machining
3 ) Low tool wear ( the merely tool wear consequences from the fluxing electrolyte )
4 ) Relatively high metal remotion rates for difficult and hard 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 important 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 version of ECM designed to take burrs or to round crisp corners on metal workparts by anodal disintegration. The hole in the workpart has a crisp burr of the type that is produced in a conventional through hole boring operation. The electrode tool is designed to concentrate the metal removal action on burr. Parts of the tool non being used for machining are insulated. The electrolyte flows through the hole to transport away the burr atoms. The same ECM rules of operation besides use to ECD. However, since much less stuff is removed in electrochemical deburring, rhythm clip are much shorter. A typical rhythm clip in ECD is less than a minute. The clip can be increased if it is desired to round the corner in add-on to taking the burr.
4.4.4 ELECTROCHEMICAL Grinding
Electrochemical grinding ( ECG ) is a particular signifier of ECM in which a revolving grinding wheel with a conductive bond stuff is used to augment the anodal disintegration of the metal workpart surface. Abrasives used in ECG include aluminium oxide and diamond. The bond stuff is either metallic ( for diamond abradants ) or resin bond impregnated with metal atoms to do it electrically conductive ( for aluminum oxide ) . The scratchy grits stick outing from the crunching wheel at the contact with the workpart set up the spread distance in ECG. The electrolyte flows through the spread between the grains to play its function in electrolysis.
Deplating is responsible for 95 % or more of the metal remotion in ECG and the scratchy action of the crunching wheel take the staying 5 % or less, largely in the signifier of salt movies that have been formed during the electrochemical reactions at the work crunching wheel in ECG lasts much longer than a wheel in conventional grinding. The consequence is a much higher crunching ratio. In add-on, dressing of the grinding wheel is required much less often. These are the important advantages of the procedure. Applications of ECG include sharpening of cemented carbide tools and grinding of surgical acerate leafs, other thin wall tubings and delicate parts.
Example 4.6:
What is the intent of utilizing electrochemical deburring?
Solution:
Electrochemical deburring is used to take burrs or to round crisp corners on metal workparts by anodal disintegration. The electrode tool is designed to concentrate the metal removal action on burr. Parts of the tool non being used for machining are insulated. The electrolyte flows through the hole to transport away the burr atoms.
Example 4.7:
What is the important advantage of the electrochemical grinding?
Solution:
The crunching wheel in ECG lasts much longer than a wheel in conventional grinding. The consequence is a much higher crunching ratio. In add-on, dressing of the grinding wheel is required much less often.
Exercise 4.2
1. A square hole is to be cut utilizing ECM through a home base of pure Cu ( valency = 1 ) that is 20 millimeter midst. The hole is 25 millimeter on each side, but the electrode that is used to cut the hole is somewhat less than 25 millimeter on its sides to let for overcut, and its form includes a hole in its centre to allow the flow of electrolyte and to cut down the country of the cut. This tool design consequences in a frontal country of 200 mm2. The applied current = 1000 A. Using an efficiency of 95 % , find how long it will take to cut the hole.
2. In a certain chemical blanking operation, a sulphuric acid etchant is used to take stuff from a sheet of magnesium metal. The sheet is 0.25 millimeter midst. The screen resist method of dissembling licenses high production rates to be achieved. As it turns out, the procedure is bring forthing a big proportion of bit. Specified tolerances of A±0.025 millimeters are non being achieved. The chief in the CHM section ailments that there must be something incorrect with the sulphuric acid. “ Possibly the concentration is wrong, ” he suggests. Analyze the job and urge a solution.
Summary
In this chapter we have studied the chemical and electrochemical machining procedure that involves in untraditional procedure. The untraditional procedures are by and large characterized by low stuff remotion rates and high specific energies relative to conventional machining operations. The capablenesss for dimensional control and surface coating of the untraditional procedures vary widely, with some of the procedures supplying high truths and good coatings, and others giving hapless truths and coatings. Surface harm is besides a consideration. Some of these procedures produce really small metallurgical harm at and instantly below the work surface, while others ( largely the thermal based procedures ) do considerable harm to the surface.
