an integrated approach for tool design in ecm

An integrated approach for tool design in ECM V. K. Jain* and K. P. Rajurkar**

In practice the costly trial and error approach of designing and experimentally testing complex-shaped tools in electrochemical machining (ECM) is still fo#owed. This results in low productivity of tooling design in ECM. This, coupled with the initial high cost of ECM set-up, high wages of skilled operators and very high cost of components to be machined are the main obstacles to realizing the full industrial potential of ECM.

The paper reviews different models proposed for the analysis problems (anode shape prediction) and design problems (tool design) in ECM. These models based on the cos ~ method, complex variable approach, empirical and homographic approach, finite difference method, finite element technique, and boundary element method vary in the methodology employed to solve the Laplace equation (the cos 0 method, and empirical and nomographic approaches do not use the Laplace equation), assumptions, appfications, merits and limitations. The interdependence of different parameters, which is the main cause of the low success in tool design for ECM, is illustrated. The finite element formulation for tooling design in electrochemical machining is outlined. Results of two-dimensional tool design for electrochemical drilling are presented. Experiments were conducted using an aqueous solution of NaCI as electrolyte, low alloy steel castings and low alloy steel forgings as work materials, and brass as the too/material. The shape and size of the tools used during experimentation have been found to be in agreement with design.

The paper also proposes an integrated approach for computer-aided tool design in ECM. The scheme emphasizes the need for incorporating the optimization model, decision support system and a computer-aided process planning system in the tool design package. The integration of a simulation system for the computer-based testing and verification of a designed too/is also discussed.

Keywords: tool design, ECM, inter-electrode gap

Electrochemical machining (ECM) is commonly used in various industries such as aerospace, nuclear, automobile etc. It is economical to use ECM to machine complex shapes and di f f icul t- to-machine materials at high material removal rates (MRR) , with good surface finish, wi thout residual stresses and with no damage to the microstructure of the workpiece. Materials that are diff icult to machine by conventional methods are often very hard, tough and heat resistant metals and

* Mechanical Engineering Department, Indian Institute of Technology, Kanpur 208016, India. This work was done when Dr Jain was working as Visiting Professor at the Nontraditional Manufacturing Research Center, Industrial and Management Systems Engineering Department, University of Nebraska-Lincoln, Lincoln, NE 68588, USA. ** Nontraditional Manufacturing Research Center, Industrial and Management Systems, Engineering Department, University of Nebraska-Lincoln, Lincoln, NE 68588, USA

alloys, such as hardened steels, stellite, nimonic and Inconel 1'2. MRRs of the order of 2 5 - 3 0 cm 3 min -1 on chromium nickel-iron alloy has been reported using ECM 3. ECM is preferable for volume production applications so that the high capital cost of ECM set-up and tooling design can be justified.

The f lexibi l i ty of the ECM machines is important when considering the wide range of applications of ECM in many production areas of fine hole dril l ing, deep hole dril l ing, deburring, grinding etc. ECM has been applied for forming three-dimensional complex profiles of turbine blades, stators, a grid of an isotope holder for a nuclear reactor, etc. Further, this process is commonly used for machining parallel-walled cavities and drilling deep round holes for cooling purposes in turbine blades in the diameter range of 1 mm or less with a depth/diameter ratio of up to 200:1. ECM machine tools can be used as machining centres in a real sense. The potential for electrochemical deburring (ECDe) is quite extensive and utilizes the novel concept of a stationary cathode tool, thus

PRECISION ENGINEERING © 1991 Butterworth-Heinemann 111

Jain and Rajurkar--an integrated approach for tool design in ECM

eliminating the need for feed mechanism and control. Fig 1 shows the kinds of operation for which the applications of the principle of anodic dissolution have been reported in the literature 4- 22. ECM is very economical under certain situations. For example, ECDe has cut deburring and inspection times for transmission components by up to 90% 2 . ECM machines are available with current capacities as high as 40000 A or even more, having one, two or even three heads.

About three decades have elapsed since ECM was first confirmed as a viable process. If the process is assessed about its future potential in manufacturing technology, one finds that there are certain areas of potential improvement which may lead to more economic utilization of this process. One of the areas of potential improvement is the search for a universal electrolyte suitable for the majority of engineering materials, with special electrolytes being used for special alloys. Some other areas are as follows: a means of removing metal ions from acid electrolytes, evolving ways for the use of dilute salt electrolytes at say 1-5% concentration rather than those currently used at 10-20% concentration, development of in process inter-electrode gap ( lEG ) measurement and gap control devices, means of hazard reduction and safe disposal of used electrolytes, reduction in ECM equipment costs etc. The advantages of ECM mentioned earlier are offset by the poor geometric tolerances and the limited ability to predict quantitatively the workpiece shape, size and surface finish. A more unfortunate aspect of the application

B

m

m

m

m

m

m

D

w

E Fig

ECB (Electrochemical bor ing) 15

ECB r (Electrochemical broaching) 18

ECB z (Electrochemical bal l iz ing) 13

ECD (Electrochemical d r i l l i ng ) 14

ECD e (Electrochemical debur r i ng ) 9

ECDS (Electrochemical die s ink ing) 7

ECG (Electrochemical g r i nd ing ) 6

ECH (Electrochemical honing) 19

ECM (Electrochemical machining) 16

ECM i (Electrochemical mi l l ing) 11

ECMM (Electrochemical micromachining)

ECS (Electrochemical sawing) 19

ECT (Electrochemical t u rn i ng ) 5

ECT r (Electrochemical t repann ing) 20

ECWC

ESD

STEM

1

17

(Electrochemical wire cu t t ing) 10- 12

(Electrostream d r i l l i ng ) 22

(Shaped tube electromachining) 21

Electrochemical dissolution-based processes

of ECM is the long time needed for design and manufacture of tools (cathodes). This paper addresses the problems of tool design in ECM.

The design of tools for producing complex shapes using ECM continues to be a difficult problem and the biggest challenge to an ECM toot designer due to the complex stochastic and not fully understood lEG phenomena. In actual practice, therefore, the costly trial and error approach for designing and experimental testing of complex shaped tools is still followed. This results in a low productivity of the tooling design. This, coupled with the initial high cost of ECM set-up, high wages of skilled operators and very high cost of components to be machined, are the main obstacles in realizing the full potential of ECM.

This paper proposes a computer integrated approach for tool design based on a finite element technique, in ECM. The proposed approach would further extend the range of applications of ECM and enhance its acceptability.

Tooling design in ECM Tooling design in ECM is classified in two categories, viz. analysis problems (or anode shape prediction), and design problems (or tool design problems). The analysis problems deal with the prediction of work-profile obtainable from a given tool while operating under the specified machining conditions. On the other hand, the design problems deal with the computation of the tool shape and size which under specified machining conditions would produce a workpiece having a prescribed shape, size, accuracy and surface finish.

The work-shape obtainable from a given tool and specified machining conditions in ECM can be drawn with the help of computed lEGs. However, the prediction of lEG is very difficult. The variation in lEG is influenced by a large number of highly interdependent parameters such as the presence of anodic film, electrolyte f low rate, intergranular attack, change in valency of work material during cutting, stray current attack and electrolyte pressure distribution. As an example, Fig 2 shows the interdependence and complexity of relationship in electrolyte circulation and related problems in ECM when viewed with the specific purposes of workpiece accuracy, surface finish and tool damage 12. In some cases, the exact influence of the parameters is not known. Moreover, it becomes extremely difficult, in many cases, to quantify their effects on the desired responses. For example, no mathematical model is available to quantify intergranular attack during ECM. A number of analytical models for anode shape prediction and cathode design problems have been proposed. Fig 3 gives the names of these models, and their source references. A brief review of these models is given below.

112 APRIL 1991 VOL 13 NO 2


More than I one path

l Only one path

Number of electrolyte flow paths

~lW°rkpiece inaccuracyl

; t t . ! . . . . . . . . . . . . . . . . . .

I Long path J /

l,++,++J I ! # I

1Length o~l Sharp change ~n I prow pathsl too, geometry I

,t ,1,1 ~ l Sidef+w I

I Backward flow i

, +w+d,,owl, I

I Poor surface finish I

i i °.°.°°°°o,+°°°.°.,°..I oo ' ° ' : ' ' ° "° ' °°°° ' "° ' ° ' ° °* °°1' °

I V 'ocity n I' I water vap°urJ-- '-"~J

Iv2'iati° I! I

r---[Dead zoneHSeparat ion

L~ Cavitation I

I Tool damage I

I Tempe~tureJ

I Sludge and particles I

' ° °° ' °° . . . . i I Variation in

current density

.............. 1 .... i .........

' I I Electrolyte Direction of I Electrolyte Electrolyte distribution electrolyte flow pressure drop temperature

*Any kind of flow coupled with a sharp change in tool geometry will lead to an inaccuracy in the workpiece

Fig 2 Electrolyte circulation-related problems in ECM

Tooling design in ECMj I I +

--~Workpiece (anode) shape predictionj

Cos e method 23 Conducting paper analogue method 23 PERA 24 Empirical approach 25 Nomographic approach 25,26 Complex variable approach 27 Perturbation analysis 28 Finite difference method 29,30 Finite element method 14,15,31 Boundary element method 32

Fig 3

Tool (cathode) design~

os e method 23 omplex variable approach 33 inite difference method 23,34 inite element method 35,36 oundary element method 37

Models for tooling design in ECM

Anode shape prediction Since the ability to predict the variation in lEG for any given operating conditions is a prerequisite for proper design of ECM tools, many of these anode shape prediction models (Fig 3) are discussed in terms of the equilibrium gap. Classical ECM theory proposed by Tipton 23'29 is based on the computation of equilibrium gap for the given conditions. However, it does not consider the effects of many important parameters such as the mode of electrolyte flow and change in electrical conductivity of the electrolyte 16. Hence, its scope is limited. It cannot be applied to analyse sharp corners. The exact path of the electric current f low lines within the lEG is difficult to determine analytically. This is one of the reasons responsible for the discrepancy

I Electrolyte contamination I

Electrolyte conductivity

variation

between the analytical and the experimental results. Also, as mentioned above, it is not possible to quantify the effects of many parameters, viz. intergranular attack, passivation etc. Therefore, attempts have been made 25 to derive empirical equations for the evaluation of the lEG. As a step in this direction, K6nig and Paul 25, and Heitman 26 have proposed a nomographic approach for work-profile prediction. But such empirical equations and nomograms are normally valid under the specified working conditions only, which limits their use. Purely analytical methods like the complex variable approach 27 have also been proposed which cannot practically be applied to analyse the real-life problems of complex anode shapes. Analysis for operations like deburring, anodic smoothing or shaping, when the amplitudes of microirregularities on the cathode and the anode are small compared to lEG, has been proposed 28. Keeping in view the limitations of the pure analytical methods, researchers have proposed more useful models based on numerical analysis techniques (also called approximate methods).

The finite difference method has been employed 23'3° for tooling design in ECM. The following Laplace equation (1) has been solved for determining potential ((#) distribution in the lEG.

02~ 02(~ -}- = 0 (1) ~x ~ T~

From the potential distribution, the current density (J) can be evaluated using Eq (2).

J = K O ~ (2) On'

PRECISION ENGINEERING 113


where K is the conductivity of the electrolyte and n' is the normal at a point on the work surface. The initial potentials at the grid points within the lEG region are set by linear interpolation along the vertical gridlines between the tool and the work boundaries. In some cases, some points on the tool and the work boundary may not lie on grid points and the regular stars may not be formed. Such problems have been attempted by the use of the over-relaxation technique. Use of irregular grids along with the regular grids has also been suggested 3°. A polynomial equation instead of a linear interpolation equation is used to describe the potential distribution in the area around a nodal point including its near neighbours. These models are based on simplifying assumptions as mentioned in Ref 16. Also, in the case of a complex-shaped lEG, the too l -work boundaries cannot be matched accurately using square meshes which introduce approximations. It is also not possible to handle by finite difference method (FDM), the non-homogeneity in the domain of interest.

The boundary element method (BEM) using linear and quadratic isoparametric elements has been employed 32 to solve the Laplace equation in the lEG. However, this model cannot address the non-linearities, anisotropy and inhomogeneities of the lEG. Therefore, finite element methods (FEM) seem to be the only better and comprehensive alternative 38.

Anode shape prediction using FEM In FEM, there is a choice of the shape and size of the elements, and it is easy to incorporate different boundary conditions as well as to analyse non-homogeneous situations. Elements of different shapes and sizes are used conveniently at the same time. One- and two-dimensional models have been reported in the literature 14"1s'31. Two-dimensional finite element formulation for the evaluation of potential distribution in the lEG during ECM is discussed here.

In general, the electric field potential distribution within the lEG obeys the Laplace equation (1). The field vector q~ in Eq (1), should be determined in such a manner that it satisfies the boundary conditions and also minimizes the function/(q~) given by Eq (3).

f f E l ~ ( d ~ 2 + ( d # # ) 2 } l /(q~) = 2 [ \ d X / ~ dXdY (3)

Using simplex triangular elements to represent the lEG, the field variable ~(X, Y) can be assumed to vary linearly within the element throughout the solution domain. For such a case, we can write

e (X, Y) = NidPi + NiVPi + Nk@k

= [N]~{~b} ~ (4)

where {~b} ~ is the column vector of nodal potentials for elements e. The interpolation function N e is

defined as follows:

N~ = aB + bBX + ce Y 2A e ( 5 )

where B = i, j, k, A e is the area of an element, and aB, bB and cB are defined as:

a,.= X / Y k - XkYi b ,= Y, . Yk c /= X k ...... X,

Similarly, other terms can be evaluated in cyclic permutation of the subscripts i, / and k.

The element equations, therefore, can be written as

(6 Similarly, equations can be derived for nodes j and k of an element. Then the equations for the nodes i, j and k of an element combined together can be written in a standard form as follows:

'1 = < Ji /:?;: | Los,)

where Km is the stiffness matrix with the following coefficients for element 1.

e,., e,,. e , , / f = (a> L Kk" Kki Kk* J t ~*

and the coefficients of the stiffness matrix are given by

K ~ . = j . ! \ aX OX + ,?Y ~ - / d X d Y (9)

Using the definition of interpolation function, it can be shown that

K~. - (b ib /+ cici) {10) 4Ae • .

The matrices for individual elements can be assembled together to give the system of equations. Reactions at the electrodes cause current density dependent overpotential. Their presence at the electrodes would alter the boundary conditions as follows:

q~ = f* (J) at the cathode

= Ev - g* (J) at the anode

where f* (J ) and g* (J) are arbitrary functions for the cathodic and anodic overpotentiats, respectively. For the sake of simplicity, it has been assumed that

114 APRIL 1991 VOL 13 NO 2


the electrode surfaces are equipotential, which means

= 0.0 at the cathode

q~ = Ev at the anode

After substitution of the boundary conditions, a set of simultaneous equations is obtained, which in the present case, has been solved using the Gauss elimination technique.

Fig 4 shows a flowchart for an improved anode shape prediction model. Some of the aspects (viz. data base and animation) shown in Fig 4 have not been incorporated in the earlier models, such as the model in Ref 14. However, these aspects are discussed in this paper. One of the reasons for the failure in the development of an accurate anode shape prediction model is the complexity of interactions among so many parameters. A simple sketch, Fig 5, illustrates the interdependence of different elements of the anode shape prediction model.

Tool design models Tool design for ECM is based on a trial and error method which also involves actual machining. Such procedures are expensive and result in low design productivity. Several attempts have been made to develop a useful and economic tool design model but success has not yet been achieved.

The cos 823,29 method has been developed for predicting cathode shape by erecting normals of length given by Ye/COS 8 (where Ye is the equilibrium gap and e is the angle between the feed and the normal to the work surface) along the work surface, the ends of which define the tool surface. Another graphical technique 34 for tool design is based on the assumption that the current flow between a pair of flux lines is constant and uses a numerical method to solve the field equation. It is a simple technique but yields a low degree of accuracy because of the simplified assumptions made. The complex variable technique 33 is applicable only for simple shapes and is unable to tackle discontinuities of work and tool surfaces. The boundary element method (BEM)37 has also been employed for solution of the tool design problem. The majority of these models are based on simplified assumptions. The limitations of the BEM as mentioned in the earlier section, also apply to tool design using the boundary element method 37.

Correction factor method Recently a correction factor method using the finite element technique has been proposed 35'36 for one- and two-dimensional tool design in ECM. The correction factor concept has been applied to modify in different stages, the tool shapes assumed in first design cycle. The tool-modification process continues until the difference between the predicted work shape and the desired work shape is within the specified tolerances. The tool design procedure

involves the following five major steps:

1. Using the anode shape prediction model based on FEM, the anode profile is estimated for an assumed (or modified) tool shape and specified machining conditions. The initial tool shape is assumed to be approximately complementary to the workpiece shape.

2. The computed and the desired work shapes are compared and the difference between the two is evaluated as an error.

3. If the error is more than the specified tolerance value, the correction factor is calculated.

4. The tool shape is modified by applying the correction factor.

5. Steps 1-4 are repeated until the desired tool shape is achieved.

This concept has been applied for designing one- and two-dimensional tool shapes for plane parallel electrode machining and electrochemical drilling. The flow chart for tool design is shown in Fig 6. This concept of correction factor can be applied to other tool design models employing finite difference or boundary element techniques for anode shape prediction.

Results and discussion Fig 7 shows a comparison of an experimental bare tool and the bare tool designed by using the present model. Fig 8 shows a designed bit type tool and the bit type tool used during electrochemical drilling 31 . The computed work shape and the work shape obtained experimentally are also shown. The difference between the experimental and the computed work shapes at the top of a drilled hole is depicted in Fig 8. It is due to the fact that the effect of stray current attack has not been incorporated in the present model. The disagreement between the designed tool shape and the experimental tool shape has been attributed to inaccuracy in the anode shape prediction model and the inaccurate measurement of the overcut in the transition zone.

Integrated approach for tooling design in ECM

From the literature survey and from the experience of user industries, it is concluded that the present status of tooling design in ECM results in low productivity, very high cost, dependence on trial and error procedures, high response time to the product changes, high lead and delivery times, and high overall cost.

As discussed earlier, some tool design models applicable in real practice have been developed and to a certain extent encouraging results have been achieved. But it would be desirable from the point of view of high productivity, if the different functions needed for any electrochemical machining


lain and Rajurkar--an integrated approach for tool design in ECM

Fig 4

116

Properties

Machining parameters

Define the tool shape and size

Type of tool

Workpiece shape and size

Miscellaneous data

'-- ~ : Yes

No ~r ~ . I Compute number of computational machining cycles, NC I

t I Discretize the domain of interest I

[ Calculate IEG at each node on W/P I

I Initialize the parameters ( V , K , f , T , U } for each node I

__~ Animate the discretized lEG, work shape and tool shape

Compute dependent parameters

Compute coefficients of stiffness matrix

Apply boundary conditions

Ii A . . . . ble the equations

[ i Solve the set of simultaneous equations

l Compute anode profile

I Make other needed calculations

I_ Reset X-coordinates, Reset Y-coordinates

N o ~

I Print the results l

Work ,piece ~ l -

Tool

~ - - - ~ " ' Elect ro, yte ' ~ -

* - - - - t H2 and O2 I~-

* - - - - - - 4 ~" V,r,O,~ I * - --

r . . . . .

-~ Database I and/or

-"1 user

F User -1 L . . . . . .J r 9 1 ~ Mathematical model and/or X, Y, Z coordinates ~ --1._ . . . .

~ bit type or coated ~.

Mathematical model and/or X, Y, Z coordinates

l,ql--~Values o f n etc.[~

~-I Calculate. chemical equivalent ~_

I NC = . . . . . . .

MRR v &t

. . . . 7 User J r . . . . -3 User L. . . . . _J

k . . . . F- User 7 --L . . . . J

~ _ 2se2_ .J

r Database -I

"1,. Dat_abase d --r- . . . . . l User

,nit,o,

I ntermediate , ~ o~?D I modelling J

. Final J" [ L 1

Feed rate I /-fcost)

Current density I J -K(#¢ /~ 'n ' )

* - - t Void fraction I * - 4 1 Conduct iv,ty

Temperature

~--( Ve,oc,ty __1

I ---~Causs elimination methodt

~ - Y - ' / o+ (c" f').&t(seeRef14)

I

I

cLv = A * x * / ( l + A * x * )

I K = K ° ( l + c z & T ) ( 1 - C C v } n ,

I T-~'solve Laplace equation or use simple equations 23

U = Q/wy, or solve Laplace equaWon

Temperature

Electrolyte flow field Field variables

Electric field

If I N C = N C ; machining is over ]

Procedural details of anode shape prediction in ECM, using finite element method

APRIL 1991 VOL 13 NO 2


N o r m a l feed rate

Anode Effective feed rate

gap Angle 0

Over ~ Current voltage density

Effective voltage conductivit,

Applied voltage

Electrolyte " ~ j _ ~ t f E l e c t r o l y t ( f l o w velocity j / ~ k~emperatur

Void fraction

Fig 5 Factors affecting anode profile in ECM

Start )

I Assume the initial tool shape I L

I Predict the w o r k shape I ÷

Compare computed work shape with required work shape. Obtain the deviations between

the two (calculate error at every node)

Fig 6

Yes v

No

( S t o p )

Calculate I correction / factor. I-- Modify t h e / t o o l s h a p e ~

Flow chart for tool design in ECM

activity were integrated by employing the capabilities of high speed computers. This would result in integration of computer-aided design (CAD) with process planning including computer simulation and its testing. Such integration would result in reduced response times to product changes, low lead and delivery times, reduced trial and error and overall costs. An attempt for such integration would lead to enhanced productivity, higher material and equipment utilization, and better consistency and quality.

0 -

- - E x p e r i m e n t a l tool + D e s i g n e d t o o l

Experimental and computed work shape Ra2 Assumed tool corner radius

Overcut, m m

0 1 2 I I I I

f

I I 3 4

E 2 - E

E -

o_ - ~ g _ -t3 c

kE -

:~ 6 -

I I I 0 1 2

m m a

Overcut m m

0 1 2 I I i

0~-

3 I I I 0 ] 2

b mm

Fig 7 Comparison of experimental and designed t o o l profiles used during electrochemical drilling. ( a ) K = 0 . 0 0 5 3 2 ~ - 1 m m - % Ev = 17 .62 V, rt = 5 . 0 3 m m , qc = 2. 1 3 m m , U = 5 . 3 7 m s - 1 ; ( b ) K = 0 . 0 0 6 3 ~ - - 1 m m - 7, Ev = 1 2 . 9 3 V, rt = 4 . 5 4 m m , rtc = 1 . 5 7 m m , U = 5 . 2 6 m s - 1

Process planning While machining complicated components, as shown in Fig 9, it is not always possible to complete the machining process from the state of raw material to the finished component in a single stage. It may have to be done in more than one stage. Fig 10 shows that to achieve a certain minimum MRR (orcurrentdensitysay, 0 .8A mm 2), a large cavity should be machined in more than one stage 11. Sometimes machining may involve altogether different types of operation requiring various kinds of ECM equipment (viz. electrochemical die sinking, electrochemical grinding or electrochemical wire cutting). Certain components may need some operations on ECM, a couple of them on EDM and a few on conventional machine tools. In some industries a turbine blade is machined by ECM while the cooling holes are drilled by EDM. For improving fatigue strength of these electrochemically machined blades, they are subjected to post machining operations. Hence, a



E x p e r i m e n t a l too l b i t - - 0 - - - D e s i g n e d too l b i t

- - x - - C o m p u t e d w o r k s h a p e - - - O - - - R e q u i r e d w o r k s h a p e

. . . . B i t h o l d e r Ra 2 A s s u m e d tool c o r n e r r a d i u s

O v e r c u t , mm 0 1

II .. II

II II

- II II

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8 i

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r- t l p I j

_ I I I I I I

- I I I I I I

- I1 I I

g - _[i I

°i I I 2 3

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Fig 8 Comparison of experimental and designed tool profiles used during electrochemical bit drilling, f = O.037mm rain - I , K = 0 .0007~ - I r a m - l : (a) Ev = 6.54 V, rt = 6.03 ram, rtc = 2.39 mm, bb = 5.22 mm; (b) Ev = 7.64 V, r t = 5.50 mm, rtc = 1.50 mm, b b = 3.80 mm

need would arise to properly sequence these operations. This paper addresses process planning and simulation for only the group of ECM based processes. Fig 1 1 (a) shows that if sequencing is not given due importance, high velocity impingement of electrolyte on the machined surface could result in corrosion and possibly final rejection. Fig 11 (b) shows the ECM in proper sequence so that corrosion of the machined surfaces may be avoided 11.

Process planning 39 in ECM possesses the basic structure of any conventional system, but the approach to each of the phases is radically different. A user friendly system needs to be developed considering the experience of the planner. A schematic diagram of the proposed integrated system is given in Fig 12. The whole system can be divided into three modules: system input module, the process planning module (including optimization) and tool design and process simulation module.

The system input module provides all the relevant data and initial information necessary to

20 .0 to 20 .2 I,Jo<,,I

\, / i l i i

o.o to 2 0 ± 0 . 1 2 5

30±0~125

I-",T'<-- ~ . . . . . 3Q ° to A i

I F-- I J ____~

. . . . 125 ...............................

Fig 9 A proposed component 3~ for electrochemica/ machining

T o o l s

r ~ 3

Fig 10 Electrochemical machining of a large cavity in several stages

design the tool, plan the machining of components and validate the designed tool. It can be done either interact/rely or from a common database. The database manager would permit the user to add new records, list, delete or edit previously entered data. The items about which the information should be stored in the database are the properties of commonly used workpiece material, tool material and electrolytes. Table 1 shows the kind of information related to metals, alloys and electrolytes that should be stored in the database. Machine tool

118 APRIL 1991 VOL 13 NO 2


Finished surface

8

Electrolyte Tool bit holder

Tool bit holder Tool bit (non conducting material) E l e c t r o l y t e ~

b Fig 1 1 (o) Improper sequencing of operations would result in corrosion: (b) proper sequencing helps in avoiding corrosion

specifications (Table 2) should also be stored in the database so that the system can select the most economical machine tool.

The process planning system should be able to support as many operations as possible. A partial list of such operations can be prepared using Fig 1. When a large amount of material is to be machined or very high accuracy is needed, the machining should be planned in two or more stages as done in conventional machining, ie, rough machining and finish machining. Then rules have to be developed for the quantity of material left for finish machining so that the desired accuracy can be achieved with minimum possible COSt 11'39

A minimum current density criterion (about 0.8 A mm -2) can be used to calculate the maximum machinable area and also to decide whether it needs multistage machining. It will also ease the tooling design problems especially in the case of complex shaped workpieces (Fig 9). This information can also be used to obtain a logical combination of operations. The sequencing of the selected machining areas should be done carefully by accounting for various types of constraint such as dimensional, geometrical, stray corrosion and electrolyte flow arrangement. Table 3 39 gives the sequencing of operations and the values of optimum machining parameters for the component shown in

Fig 9. The table also gives the different kinds of constraint and formation of an anteriority matrix to determine the appropriate sequence of operations.

Optimization Optimal values of machining parameters (electrolyte flow velocity, voltage and tool feed rate) should be found before designing a tool. This can be done by considering multicriteria optimization 4° rather than single objective optimization. In this case, a linear goal programming technique has been employed. Geometrical accuracy, material removal rate and tool-life are used as objective functions. Temperature, passivation and choking are used as constraints. Voltage, feed rate and electrolyte flow velocity are considered as design variables. According to rough or finish machining, objective functions are given priorities and weightings, ie, in finish machining, accuracy would have first priority and MRR would be second. This would be reversed in the case of rough machining.

For selection of the most economical ECM machine tool for manufacturing a component, various costs (ie, machine tool depreciation, tool costs, labour costs, electricity costs, overhead costs and miscellaneous costs) should be considered.

Simulation By using a two- or three-dimensional solid modeller and finite element modelling tools, coupled with analysis simulation tools to predict the transient anode profiles, electrolyte flow patterns and heat transfer trends, the electrochemical manufacturing engineer will be able to explore alternative process plans and evaluate trade offs. Process simulation is a technique to support the manufacturing engineer's experience for reduced lead time, lower cost, increased product quality and better understanding of the process 41.

Simulation of the ECM process will significantly reduce and, in some cases, eliminate the iterative process of full scale testing of tools before releasing for actual production on the shop floor. This is essential to meet the challenge of designing, producing and introducing tools for totally new products in much shorter times with little help from experience. This would improve the productivity of the tool designer and manufacturing engineer. The traditional procedure of designing and testing of a tool for ECM is shown in Fig 13(a). However, use of simulation of the process and computer testing of the designed tool (Fig 13(b)) will enhance the productivity of a designer by satisfactory validation of the performance of the designed tool on the computer before releasing it for full scale shop floor testing. This will reduce the number of full scale shop floor tests for evolving a satisfactory shape and size of the tool for testing the ECM process. Hence, a tailored simulator should be developed for ECM process and tool performance validation.



I S~rt I

.. "4 T echn,ca, drawing input I I Anode shape prediction model~

~'~I~ I Anode specification ]

r - - - 7 Electrolyte selection I

L _ _ _ . ~ Feature identification ]

~ Yes specifications needed?

I Database F -- -- --I~t Select f irst machine tool I

~ ~ ........... No

I Calculate maximum permissible area and flow length I

I Combine features I

I Sequence the operations I

lOpt,m,zat,oo mode,ldatabose]-- ' I Ca,cu,ate opt,mum mach,n,og parameters and cost I

I Econom,cs ana,ysis mode, I - -- - - ~ No

Yes

! I i

• ~I Select next machine t o o l ] ~

J Select most economical machine tool J +

I Too, de,lgn mode, I- . . . . "1 Design cathode I t " -]

J Anode shape prediction mode, J-- ~ Simo,ate the process. Predict anode shape. J i . . . . . . . . . . i Display comparison with desired work shape I iKedesign tne critical area onwl~l-- ~

.~.~d nJrr m n a j n ° ~ ~ ~ ~ - o t---o empirilcal i

No ~ ~ - - - . ~ . Yes improvement in

IUse the tool shape in preceeding cycle I

I Print the process plan and designed tool shape ]

stop

Fig 12 Flowchart for the proposed integrated approach for tool design in ECM

120 APRIL 1991 VOL 13 NO 2


T a b l e 1 P r o p e r t i e s o f m e t a l s and a l l o y s

Metals

Specif ic Thermal Electrical Density, heat, conduct iv i ty , conduc t i v i t y A tomic Valency

SN Code Name kg m 3 J k g - 1 K -1 W m K -1 #D -1 cm -1 we igh t ( 1 ) ( 2 ) ( 3 ) Remarks

1 010 Fe 7897 452.0 + 73 + 1 /9 .7 55.85 2 3 +At 20 °C 2 015 Ni 8906 445.9 + 90.0 + 1 /6 .85 58.71 2 3

Alloys

Specif ic Thermal Chemical Density, heat, conduct iv i ty , Electrical equivalent

SN Code Name kg m -3 J kg -1 K -1 W m 1 K-1 conduc t iv i t y (1 ) (2 ) (3 Remarks

1 11 5 Ni-Cr 8522 460.0 17.0 * * 80 Ni, 1 5 Cr

Thermal and electrical properties of electrolytes

Electrical conduct iv i ty , Specif ic Thermal

SN Electrolyte Code Concentrat ion ~ - 1 cm-1 heat conduc t iv i t y

+ + 0 . 0 2 / o c 1 NaCI 001 50 g 1-1 0.1 + +

* Can be calculated after knowing the remaining constituents of the alloy. ++ To be calculated.

T a b l e 2 ECM machine tools

Number of m/c Range of Electrolyte Type of Electrolyte tools Current, voltage, Feed rate, f low rate, electrolyte pressure,

SN Make Code available A V IPM Table size GPM handled PSI

1 Anocut 101 2 0 -40000 0-30 0.000-0.040 40" x 36" x 40" 0-300 NaOH 0-250

Table 339

Anter ior i t ies Operat ion Stray Component Economics code Dimensional Geometrical Technolog ica l corrosion distort ion of too l ing

2 R + 3 F . . . . . . 5 R + 6 F + 7 R + 8 F (2R + 3 F ) - - - - (2F + 3F), 1F - - - - 1F - - - - - - ( 2 R + 3 F ) - - ( 2 R + 3 F ) 4F - - - - - - ( 2 R + 3 F ) , I F , 9F - - - - 9F - - - - - - ( 2 R + 3 F ) , I F - - - - 1OF - - - - (2R + 3F), 1F - - ( 5 R + 6 F +

7 R + 8 F )

Sequence of operat ions 2 R + 3 F , 1F, 5 R + 6 F + 7 R + 8 F , IOF, 9F, 4F


Join and Rajurkar--an integrated approach for tool design in ECM

W/P specs.

8

t If any discrepancy l~ I modify the designed tool [~

~l Design ~-i~1 Manufacture H Full scale ~ 1 ~ ~l the tool the tool test of tool

Compare machined workpiece with desired

specifications Release the tool for actual product:ion

W/P specs. ~] Design L ~l Simulate Manufacture ~l the tool F ~ [ the process the tool

1 If any discrepancy modify the designed tool

~__~ Compare the ~ L ~ predicted W/P profile with the required one

b [ If any discrepancy l~ [ redesign the tool ]~

Full scale test of tool ease the

tool for actual I productlorl i

J

Fig 13(a) Traditional procedure for designing and testing a tool for ECM; (b) proposed procedure for designing and testing a tool for ECM

The problems to be faced will be large in number if an attempt is made to make it as a general simulator which can deal with most of the kinds of operations listed in Fig 1. Hence, the tailored simulation could be made up of generic modules which simulate the fundamental mechanism present in the process. The preprocessor of the simulation would generate a finite element model from the stored graphic description of the part. The finite element nodes, elements, boundary conditions, etc., are submitted to the simulation analysis program. A post processor or graphic output display of the results would exhibit the expected and desired workpiece profile and the designed tool profile. The process plan will display the machining parameters, electrolyte and its concentration, the code of the selected machine tool, and the name and sequence of operations. Fig 14 shows an electrochemical machining process simulation program.

Decision support system Coherent systems of computer based technology are used by managers as an aid to their decision making in semistructured or unstructured decision tasks. Such systems are used to support rather than replace managerial judgement 42. With the help of a decision support system (DSS), say, for electrolyte selection, it will be possible to represent the variation in electrolyte conductivity with temperature and void fraction, and other properties in terms of graphs and tables. Representation of the information in this fashion will improve the effectiveness and efficiency of a decision maker eg designer or process planner. There are other situations where tools like a DSS can be used during tool design for ECM. For example, feature identification (Fig 15) and coding of parts could be done by using a DSS. Fig 16 shows a simple flow chart for a DSS for electrolyte selection.

l IECM process simulationl

'- .1 Expe.menta, progra I Process design rl- #[Common databaser I i I J L _ ~

I Process slmulationl -- tAr.ficia` intel,genq 1 IMode, input parameters I [Model va,dati----- q

Pre-processor Analysis • Electrolyte flow • Heat transfer • Potential distribution • Anode profile

Post processor

• Design rules • Experience

• Part geometry • Part specs • W/P material properties • Tool material properties • Electrolyte properties • M/C tool specs • Cost details

• Electrolyte conc •Electroiyte flow/velocity patter:~ • Voltage •Temperature distribution • Feed rate, etc. • Interelectrode gap

• Anode profiles

Fig 14 Simulation program for ECM

122 APRIL 1991 VOL 13 NO 2

Jain and Ra ju rka r - -an integrated approach for too l design in ECM

Tool feed

Tool feed YL Electrolyte v o ut

~, Electrolyte ~llf ~ ff ~ ( in

I i I t-; I Ii i Ji I

I i I I I

~q , FL-~ Shaped F--]I ~, I t ----3/, , I [ ~ I - [ I I I

Hoe is cav,ty (2) Ill, II II,,ll I I I IF embedded~l ,Ik\\\\~ I II, II II, ILl

in the ~I~ llLX~\\\\~ Drilled I ~II ii II H !! II'~ I .... k\\\\\'ql! l ~ h o l e (I) I/'ILLUI ILLUI \J

N\\\\\~ILL-UN\\\. '. ' .I Two similar holes machined using a single tool

Feed a g b

( - ) L ~ O n e set of combinations Tool Elect r°lytelbL ~ . ~ /

Block s i z i n g ~ r ~ ~ - i , - Out

W o r ~ ( ( + ) l ~ D i e cavity

J

C Fig 15 Examples o f features ident i f icat ion in ECM39. (a ) embedded features, ( b ) similar features, (c) combining features with block sizing

Start )

I Type of work material to be machined I

LDisplay list of electrolytes that can machine this W/PI

I Select one of them I

I Check for MRR and stray cutting effects I

l lnput the code of electrolyte chosen I

C ) Fig 16 F lowchar t for decision suppor t system ( D S S ) for electrolyte selection

With the proposed integrated system, the ECM tool designer and manufacturing engineer wi l l be able to make critical and early process planning decisions wi th better knowledge and understanding of the desired products and processes wi thout many surprises on the shop floor.

Concluding remarks In this paper, a two-d imensional f inite element formulat ion of anode shape prediction model and procedural details of a correction factor method for tool design in ECM have been discussed. The designed tool profile and the profi le of the tool used during experimentat ion agreed well . The proposed integrated system would eventual ly enhance the

qual i ty and product iv i ty of the tool designer for ECM processes.

Acknowledgement The authors are grateful to the Nebraska Research Init iat ive Funds for f inancial support for this work. They are also grateful to Ms Karla Jay of the Industrial and Management Systems Engineering Department of the University of Nebraska-Lincoln for her help in preparing this manuscript.

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Ja in and Ra ju rka r - -an in tegrated approach for too l design in ECM

24 Report No. 145, Production Engineering Research Association, Melton Mowbray, 1968

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28 McGeough, J. A. Principles of Electrochemical Machining, Chapman and Hall, London, 1974

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an integrated approach for tool design in ecm

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