Journal of Materials Processing Technology xxx (2004) xxxxxx

Knowledge based design of EDM electrodes for mould cavitiespre-machined by high-speed milling

K.R. Mahajan a,b,, G.E. Knoppers a, J.A.J. Oosterling a, C.A. van Luttervelt ba TNO Industrial Technology, De Rondom 1, 5600 HE Eindhoven, The Netherlands

b Faculty of Mechanical Engineering and Marine Technology, Section Production Technology and Organization,Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands

Abstract

Electric discharge machining (EDM) electrode design has always been an important activity in the die and mould making sector.Nowadays, softwares are available to design electrodes. Using software, the EDM specialist has to select the areas in the mould cavitythat are to be EDMed. Once the areas are selected, a typical software designs an electrode along with its holder. If the mould cavity isvery complex, as is the case when the mould cavity is pre-machined by high-speed milling, the EDM specialist has to think of severalpossible electrode combinations/designs and to select the best solution. He does this based on his knowledge of EDM and the knowledgeof the process capabilities of his EDM machine tool. This paper presents the basic principles of designing a knowledge based systemfor automated EDM electrode design. This system works with similar logic, that an experienced EDM specialist would use to designelectrodes. First the overall methodology to design EDM electrodes automatically is described on the highest level. Then the details of thismethodology are explained followed by conclusions. 2004 Elsevier B.V. All rights reserved.

1. Introduction

Today, in order to gain competitive advantage, tool anddie makers make a combined use of conventional technolo-gies like electric discharge machining (EDM) and latesttechnologies like high-speed milling (HSM), to reduce leadtimes for die and mould manufacture. This combined use ofEDM and HSM is done to take advantage of both the pro-cesses in the best possible way. TNO Industrial Technologyin the Netherlands participates in an European Communityproject FASTOOL which aims to integrate an EDM ma-chine tool, a HSM machine tool and a robot for automateddie and mould manufacturing. The idea is that the mouldcavity will first be milled on the HSM machine tool followedby EDMing on the EDM machine tool (if applicable). Oneactivity in this project is the automated generation of EDMelectrode designs given a mould cavity that is partiallymilled, i.e., a pre-milled mould cavity. Besides the FAS-TOOL project, there are two more motives of developingsuch a knowledge based automated system for the designof EDM electrodes. EDM specialists are difficult to find

Corresponding author. Present address: 3/15 White House Society,Golf Club Road, Yesawada, Pune 6, Maharashtra State, India.Tel.: +91-2-06686305.E-mail address: mpreetam@yahoo.com (K.R. Mahajan).

in western countries. Such a system can replace an EDMspecialist by transferring his knowledge to the computer.Secondly, even if a certain electrode design is perfectly mil-lable, it is possible that it is not the technically feasibledesign.

2. Broad idea of the method to automatically designelectrodes

Fig. 1 shows the broad idea of the method to design elec-trodes. The input to the system is the STL file of the mouldcavity obtained after HSM. This file suggests which partsin a mould cavity could not be milled and which need to beEDMed. The areas to be EDMed are then separated intoas many distinct regions as possible. Then these distinct re-gions are treated with similar logic (criteria and rules) thatan EDM specialist would use to combine them with eachother. This combination is referred to as re-grouping in thecoming sections and is nothing but the combination of oneor more regions with each other. Followed by this, we wouldget as an output from this system all possible electrode de-signs. From these electrode designs, an EDM specialist canmake a selection based on the EDM machining times, costs,etc. This methodology of designing electrodes in all possi-ble ways is employed because there exists different opinions

0924-0136/$ see front matter 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2004.02.007

2 K.R. Mahajan et al. / Journal of Materials Processing Technology xxx (2004) xxxxxx

Areas toEDM

Disintegrate areas to EDM into

distinct regions as much as possible

Design electrodes using logic/criteria and

smart combination/re-grouping

EDM electrodes

Fig. 1. General idea of the method to design electrodes automatically.

in industry of which electrode to use for a typical EDMoperation.

In the following sections, the major items as mentionedin the general idea in Fig. 1 are described one by one.

3. Disintegration of regions to EDM

Fig. 2 shows the conceptual scheme, that shows how areasto be EDMed can be disintegrated into as many distinctregions or electrodes as possible. Depending on where EDMneeds to be done, the corresponding areas on the originalmould cavity will be identified using software. The first stepis to make one single big electrode for all such areas. To theentire electrode design, we first apply a (3D) radius detectionalgorithm and find out where the radii are on the edges orsurfaces of the electrode.

Followed by radii detection, the surfaces or edges withcommon surface radii/edge radii, will be highlighted andseparated as distinct electrodes. The reason to do this specif-ically is to take advantage of the fact that most moulds anddies have surfaces or edges with similar geometric character-istics, like for example, common values of edge radii, com-mon surface radii, etc. This separation of electrodes basedon common radii will give the best result in terms of indi-vidual options. Someone might even argue what happens ifthe mould has a free form shape without existence of com-mon/distinct edge/surface radii? In such cases, the systemwill not separate the regions, but continue to keep the wholegeometry as one distinct region. On each of these separatedregions, certain rules are applied to still separate the regionsinto smaller regions. These rules are derived from the crite-ria that are considered by an EDM specialist when he de-

signs electrodes. These criteria and the associated rules arediscussed in more details in the coming sections. The sepa-ration of electrodes into still smaller electrodes based on therules is done because although a region of the electrode mayhave common surface or edge radii, it may still violate oneor more other rules. After this step, the regions that violateany one of the rules will be stored as individual options. Itis wise to have these individual options because these mighthave advantages over EDMing with combined electrodesor EDMing with pure deep sinking in terms of gain in ma-chining times.

4. Re-grouping/combination of regions to EDM

Fig. 3 shows the details of the electrode re-grouping/combination procedure.

The input to this system is the saved individual optionsobtained and as shown in Fig. 2 by the box store ALLoptions. Then any option is selected and combined with asecond option. When the rules are again applied, it becomesclear if they should remain as a single electrode (the rulesand the method of how to apply these rules and the criteriathey are based on, are explained in the next section). If theycan, then this combination is saved in a database. Then tothis combination, the third option is added and the feasibilityis checked. If the rules are satisfied, this combination issaved as a second result in the database and so on. If anyof the option violated the rules, the next option is taken.A counter is kept for the very first option that is selected,such that when all the options are over, it takes the secondoption and the process is repeated over again. The final resultfrom this system is EDM electrode designs in all possibleways.

The minimum number of combinations (electrode de-signs) one has to perform (can obtain) using this iterativesystem can be calculated by the following mathematical ex-pression (which can be found in standard text on appliedmathematics):

CanMin =n!

a!((n a)!)where, C is the total number of combinations one has toperform using the above procedure, n the number of storedelectrode options, a the size of each individual combination.

Note that the size of the combination has always to be2, in order to calculate the minimum number of combina-tions. To test the above equation consider that we have fourelectrode options (named 14) which are stored and whichare to be combined amongst them using the above system.Now, if we go through the above system (and always gothrough the No loop), we will have the following possi-ble combinations, namely, 12, 13, 14, 23, 24 and 34(in total six combinations). Note, that we will not have thecombinations 21, 31 and 32, because the system is de-signed such, that the same combinations (like 12 and 21)

K.R. Mahajan et al. / Journal of Materials Processing Technology xxx (2004) xxxxxx 3

Fig. 2. Disintegration of areas to EDM into distinct regions.

are not considered again the second time. In actual practice,this will be done by placing history checks in the softwareto avoid re-combinations and subsequently reduced process-ing speed. Substituting the number of saved options n = 4,in the equation, we get the result as:

CanMin =4!

2!(4 2)! =4 3 2 1

2 2 = 6

which matches with the number of combinations obtainedearlier.

The maximum number of combinations (electrode design)one can perform (obtain) can also be calculated as:CMax = Can + Ca+1n + + Cn1n + Cnnwhere Can is the minimum number of combinations obtainedearlier, a the size of each individual combination, and nthe number of stored options being treated. Depending onthe result of applying the rules in this re-group procedure,

the actual number of possible electrode designs can varybetween the minimum and the maximum. This can be ex-pressed mathematically as:

CMax N CanMinwhere N is the final number of electrode designs obtainedafter actually applying the rules.

5. What are the criteria that an EDM specialist uses todesign EDM electrodes?

In this section, the criteria that an EDM specialist consid-ers to design electrodes are discussed followed by convert-ing the criteria to rules, that can be used for disintegratingand re-grouping of electrodes as seen in Figs. 2 and 3. Thesecriteria have been identified based on inputs from die andmould makers and from inside knowledge within TNO.

4 K.R. Mahajan et al. / Journal of Materials Processing Technology xxx (2004) xxxxxx

No

Yes

Figure 1 again re-producedhere to show relationship withFigure 3

Areas toEDM

Disintegrate areas to EDM into distinctregions as much as

possible

Design electrodes using logic/criteria and

smart combination/re-grouping

EDM electrodes

Store ALL the options

(1), (2),(n)

Select option (1)

Combine withnext option

Possible to

combine ?

Save thisoption in a database

EDM electrodes

Select the nextoption

Apply rules

Counter (1),(n)

(2), (3), (4)

Fig. 3. Details of the re-group procedure to re-group different options.

As seen in Fig. 4, the criteria can be listed as:

1. EDM machining depths.2. Horizontal distances between regions to EDM.3. Surface roughness required on the mould cavity.4. EDM machining strategy used.5. Machining times and costs obtained by using a certain

electrode design.6. Manufacturability of the electrode.

From these criteria, the EDM machining times and costsand the manufacturability of the electrodes have not beenconsidered to convert to rules because, these two criteria are

EDM machining

strategy

Distances between

regions to EDM

Surface roughness required

Manufacturability of the electrode

Machining times and

costs

Criteria for grouping EDM

electrodes

EDM machining

depths

Fig. 4. Criteria for grouping/designing electrodes.

already considered elsewhere in the FASTOOL project. Allthe other criteria listed above have been investigated andconverted to rules. The criteria and the conversion to rulesare discussed next one by one.

5.1. EDM machining depth criterion

EDM specialists in practice, use this criterion to limitthe complexity of the designed electrode from the EDMmachining point of view. When differences in machiningdepths are beyond a certain limit, uneven wear occurs onelectrodes. This also means that for the next finishing EDMoperation, the number of electrodes needed for region withlonger machined depth and machining time will be higherthan for region with lower machined depth and machiningtime.

5.1.1. Converting this criterion to a rule to apply it whendesigning electrodes

This section describes how this criterion is converted toa rule. Normally in practice, and based on experience a dif-ference in machined depth of about 2530% is allowed.This is also because it is difficult to predict wear of EDMelectrodes. Hence, to begin with a factor of 0.3 has beentaken. Much further research is required to predict electrodewear. According to this discussion, the rule can be writtenas hmax 0.7 = hmin, where hmax is the depth of region 1

K.R. Mahajan et al. / Journal of Materials Processing Technology xxx (2004) xxxxxx 5

while hmin is the depth of region 2 which are to be combinedwith each other. The values of the machined depths can befound out by performing geometric analysis using software.The rule will remain valid (consequently combining thetwo regions) when the above equation is satisfied and viceversa.

5.2. Horizontal distances between regions toEDM criterion

In practice, there are two factors the EDM specialist willuse when combining electrodes as far as horizontal distancesare considered. The first one is the maximum allowable over-all dimension of the electrode that can be safely clamped inthe EDM machine tools automatic tool changer (ATC) sys-tem and secondly the size of the pallet system on which theelectrodes are placed (which are used to transport electrodesfor measurement, etc.). The pallet system has been addedbecause nowadays even small to medium sized tool and diemaking companies shift to standardization practices.

5.2.1. Converting this criterion to a rule to apply it whendesigning electrodes

In this, one electrode is designed (by combining two re-gions) and the overall size (L (mm) B (mm) H (mm))is saved. Then, from the pallet size and the maximum al-lowable electrode dimensions in the ATC system, we selectthe smallest overall dimension. We select the smallest onebecause we can then compare this smallest one with theoverall size of the combined electrode. If the combinedsize of the designed electrode is smaller than the selectedsmallest, then this rule is valid and vice versa.

5.3. Difference in surface roughness criterion

There are several examples of mould cavities in industrywhere different parts of a cavity have different surface rough-ness requirements. In such cases one electrode can never bedesigned for two regions with different surface roughnessrequirements. This is due to the fact that different surfaceroughness requirements call for different process parametersduring the EDM operation.

5.3.1. Converting this criterion to a rule to apply it whendesigning electrodes

To convert this criterion to a rule, regions in a mouldwhich have such special requirements, have to be selectedon the highest level of the system (i.e., when the mould cav-ity is input in the CAM system for HSM), where regionswith special surface requirements will be given attributeslike special surface roughness. This information will then becarried throughout the system up to this level of EDM elec-trode design where the system will try to combine regionswith each other. When two regions with different surfaceroughness values are combined the rule will be violated,consequently separating such regions.

5.4. EDM machining strategy criterion

Different EDM machine tool makes have different capa-bilities in terms of machining strategies available. However,the most common strategies available on most machinetools and the ones most commonly used are deep sinking,conical orbiting movement and the star-like orbiting move-ment. Some new EDM machine tools have the 3D-orbitingmovement as an extended machining strategy. This cri-terion has been added to this system of designing EDMelectrodes because some of the strategies create geometricerrors during/after EDMing. These errors can be com-pensated/eliminated by the use of alternative machiningstrategies and solutions. However alternative machiningstrategies and solutions (may) call for additional number ofEDM electrodes during EDMing. For instance pure deepsinking calls for a higher number of electrodes increasingthe EDM time and overall costs while orbiting EDM re-quires less number of electrodes while resulting in lowerEDM machining times and costs.

5.4.1. Converting this criterion to a rule to apply it whendesigning electrodes

The system has been designed to take into considerationdifferences in the capabilities of EDM machine tools interms of machining strategies and the user choices in termsof quality requirements of the mould cavity. The first inputsto this system are the two regions which are to be designedas one electrode (e.g. regions 1 and 2). These regions willhave undergone a geometry analysis on the highest level.The user on the highest level will have selected the regionswhere he wishes to have special quality requirements fromthe mould cavity (examples are near sharp corners andlimits on the values of the allowable geometric errors) andthe EDM machine tool he wishes to use. This informationwill be brought from the highest level to this level of EDMelectrode design. Details are avoided here and are availablefrom the author on request. In brief, the general idea isthat depending on the mould quality requirements and theselected EDM machine tool, the system will search for so-lutions and automatically assign additional number of EDMelectrodes and the machining strategies to use.

6. How to use the rules to design EDM electrodes?

So far we have discussed the conversion to rules of thefollowing criteria:

1. The EDM machining depths.2. The horizontal distances between regions to EDM.3. The existence of special surface roughness and texture

on mould surfaces.4. The EDM machining strategy.

These rules are applied during disintegrating and re-grouping of regions to finally obtain the designed electrodes.

6 K.R. Mahajan et al. / Journal of Materials Processing Technology xxx (2004) xxxxxx

During disintegrating the regions, the rules do not haveto be applied in a specific sequence. However, during there-grouping procedure, these rules are applied in a specificorder of importance. This order of importance is actuallythe order of the weight each rule carries when allowing acombination. For example, the surface roughness rule willnot allow a combination of two regions in any case. Henceit is wise to always keep such regions separate from eachother no matter what the result of applying other rules is.Hence, during the re-group procedure the rules are appliedin the following order one by one:

(1) Surface roughness rule.(2) EDM machining depth rule.(3) Horizontal distances between regions to EDM rule.(4) EDM machining strategy rule.

The re-grouping will take place according to the procedureexplained in Fig. 3.

7. Conclusions

In this paper the basic principles of designing a knowl-edge based automatic EDM electrode design system wereexplained. The success of this system depends to some extentto the radius detection software algorithm. Although diffi-cult to implement, this software algorithm is not impossibleto realize. In principle the system can also accommodate asection to predict EDM machining times and the amount ofelectrode wear. This would enable to determine the numberof subsequent electrodes needed in view of required geomet-ric accuracy of the cavity. Some preliminary work in thesedirections has been performed but many particularities ofEDM machining have to be researched. Outside the FAS-

TOOL project, this knowledge based system can be expectedto work in a semi-automatic fashion in a very good way. Theuser (by then the EDM specialist would not be required) willonly have to separate the areas he wants to EDM. The systemwill then by itself be able to design feasible electrodes. Thesystem has been designed to take care of differences in EDMmachine tools, that makes it quite unique. This system provesto be an important development in modern tool manufacture.

Further readingElectrode design software Power Shape or commonly

known as PS electrode, 2003. http://www.delcam.com.Electrode design software Quick electrode, 2003.

http://www.cimatron.com.X.M. Ding, J.Y.H. Fuh, K.S. Lee, Computer-aided EDM

electrode design, Comput. Ind. Eng. 42 (2002) 259269.K.R. Mahajan, Towards knowledge-based design of EDM

electrodes for pre-milled mould cavities, Masters ResearchThesis, PT 03.082, Section Production Technology and In-dustrial Organization, Delft University of Technology, TheNetherlands, 2003.

FASTOOL Project Proposal, TNO Industrial Technology,The Netherlands, 2002.

B. Lauwers, Computer-aided process planning and man-ufacturing for electrical discharge machining, Ph.D. Thesis,93 D5, Department of Mechanical Engineering, KatholiekeUniversiteit Leuven, Belgium, 1993.

F. Staelens, Overall on-line optimization of planetary elec-tro discharge machining, Ph.D. Thesis, 90 D2, Departmentof Mechanical Engineering, Katholieke Universiteit Leuven,Belgium, 1990.

T. Altan, B.W. Lilly, J.-P. Kruth, W. Knig, H.K. Tnshoff,C.A. Luttervelt, A.B. van Khairy, Advanced techniques fordie and mould manufacturing, Ann. CIRP 42 (2) (1993) 707.

Knowledge based design of EDM electrodes for mould cavities pre-machined by high-speed millingIntroductionBroad idea of the method to automatically design electrodesDisintegration of regions to EDMRe-grouping/combination of regions to EDMWhat are the criteria that an EDM specialist uses to design EDM electrodes?EDM machining depth criterionConverting this criterion to a rule to apply it when designing electrodes

Horizontal distances between regions to EDM criterionConverting this criterion to a rule to apply it when designing electrodes

Difference in surface roughness criterionConverting this criterion to a rule to apply it when designing electrodes

EDM machining strategy criterionConverting this criterion to a rule to apply it when designing electrodes

How to use the rules to design EDM electrodes?Conclusions