- Email: [email protected]
Speciality of HSC in manufacturing of forging dies
Journal of Materials Processing Technology 157–158 (2004) 536–542
Speciality of HSC in manufacturing of forging dies B. Kecelja,∗ , J. Kopaˇcb , Z. Kampuˇsb , K. Kuzmana,b b
a TECOS, Slovenian Tool and Die Development Centre, Kidriˇ ceva 25, SI-3000 Celje, Slovenia Faculty of Mechanical Engineering, University of Ljubljana, Aˇskerˇceva 6, SI-1000 Ljubljana, Slovenia
Abstract The paper discusses the attributes of the die forging tools manufacturing with high speed milling technology. In the most cases of die forging tools manufacturing high speed milling of hardened material is faster and cheaper manufacturing process than EDM, besides forging tools have better quality and therefore longer tool life. By machining process of the paper presented die forging tools, milling tool length/diameter ratios of extreme values were applied, which have influence on high speed milling strategy and technology. Whole die forging tool design and manufacturing procedure is realised on a method of long distance work. © 2004 Elsevier B.V. All rights reserved. Keywords: High speed milling; Hard milling; High L/D ratio; Forging die
1. Introduction
2. Attributes of HSC and EDM
Nowadays HSC technology is a basic constituent part of every modern tool making company. With the introduction of HSC into tool shops mould making manufacturing technology has been changed, mostly by manufacturing of mould cavities which represent the most pretentious mould parts so that 40% of mould manufacturing time is used for the manufacturing of mould cavity. According to the objective model for cavity manufacturing technology developing, where milling tool, mould and product related parameters are considered, combination of EDM/HSC or entirely HSC manufacturing technology replaced classic die sinking EDM mould cavity machining [1]. For each mould cavity itself has to be ascertain, which technology – rough HSC milling/finish EDM or HSC milling entirely – is the most advantageous by costs and manufacturing time.
In Table 1 [2] the advantages, disadvantages as well as limits of the EDM and HSC milling are presented. For selection of the most appropriate mould cavity machining technology, the energy consumption and ecology are of a great importance too. It is well known that the EDM process has a very high level of energy consumption, therefore it should be used only in cases where, regarding product, milling tool shape or mould related properties does not allow the HSC technology [3]. From ecology point of view HSC technology is prevailing EDM for the following reasons:
∗
Corresponding author. Tel.: +386 3 4264 607; fax: +386 3 4264 611. E-mail addresses: [email protected] (B. Kecelj), [email protected] (J. Kopaˇc), [email protected] (Z. Kampuˇs), [email protected], [email protected] (K. Kuzman). URL: http://www.tecos.si. 0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.07.112
- Technology using less energy is much friendlier to the environment. - Permanent decrease of cutting lubricants and coolants leads to dry machining. - There must be constant monitoring of EDM electrolyte during the process as later on for the waste treatment and disposal [3]. The HSC introduction into tool shops drastically reduces manufacturing times for the mould forming system, however the EDM and HSC technologies are not competitive but they are adjective, since all different forming geometries can
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Table 1 Comparison of manufacturing technologies Criterion
EDM
HSC milling
Materials Geometry Sharpedged inner angles Deep grooves Polished surface Costs of additional finish machining Blur surface Texture modification Geometric accuracy Dispossession effect Premachining Machining tool
All conducting materials Free Radius < 0.1 mm reachable Depend on manufactured electrode Always additional finish machining High Yes Micro cracks + + by large shapes surface dispossession Rough EDM Expensive (milled)
All cutting materials (steel up to 62 HRc) Limited depth, radius Radius at the bottom: > 0.3 mm, radius at the wall: > 1 mm Up to L/D < 10 +, partly without additional finish machining Low No Compressing ++ + by small shapes point dispossession Economy = f (machining costs, volume) Simple, standard product
Reprinted from Ref. [2].
be manufactured with EDM which is not the case for HSC [3].
3. Manufacturing technology of hot forming tool cavities Very important properties of steels in hot-work applications are temperature strength and thermal shock resistance, which help to determine the material selected for the tool, due to the high operating temperatures existing at the area of the contact between die and workpiece. But one of the most important properties, if not the single most important property, is the toughness. The toughness serves as an indication of the ability to resist the formation and growth of cracks [4]. Combination of HSC technology and milling tools with new coatings (TiAlN) is a ground for successful cost and time reduced hard milling, in range of 46–62 HRc of already hardened tool cavities for hot forging, and therefore reduction of EDM. Tools for forming processes as forging or deep drawing can be almost entirely milled because sharp inner angles basically do not appear. Reachable dispossession effect by hard milling is comparative or even better as by EDM, electrode is unnecessary and surface quality is better [2]. 3.1. Hard milling with high L/D ratio Forging part geometry defines parting plane and therefore required milling tool length/diameter (L/D) ratio. The latter have great influence on machining accuracy and surface quality. High spindle speeds increase the severity of vibration at the tool tip. To protect tool life and surface quality, more rigid tools have to be favoured. For end mills this means using of shortest tool as possible and favouring a tool with shorter flutes, which brings a larger and more rigid central core [5]. In Figs. 1 and 2, the main two criteria that influence on HSC manufacturing are presented. The adequate depth of cut ap depends on material hardness and milling tool slenderness, where the latter is defined as a ratio between length
Fig. 1. The influence of workpiece hardness on depth of cut [6].
and diameter of milling tool: ap = RK2 K3
(1)
Thus the depth of cut ap can be defined as a product of milling tool radius R, material hardness coefficient K2 and milling tool slenderness coefficient K3 values. By material hardness up to 52 HRc the value of material hardness coefficient K2 is between 0.1 and 0.12, at higher material hardnesses coefficient K2 values decreases to the range from 0.04 to 0.05 by 63 HRc [6]. Up to the L/D ratio = 5 tool slenderness coefficient K3 has value 1, after which rapidly decrease to the value of 0.1 at L/D = 10. Therefore at L/D > 10 high speed milling in hard material is possible only with special measures [6]. On the other side, the ability to take lights cuts quickly makes the milling machine more efficient with delicate tools that are very long relative to their diameter. This helps in at
Fig. 2. The influence of slenderness on depth of cut [6].
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least two cases: - milling deep cavities or deep slots, - milling fine details with very small tools [5]. At high L/D ratios milling tool deflection increases, which is consequence of large run out of milling tool and low tool rigidity. Tool deflection and milling strategy affect on dimension accuracy and surface roughness of machined tool cavity. HSM – dry cutting is by milling of 50 HRc materials very successfully too [7].
Table 2 Chemical structure of applied hot die forging tool steel Vidar Supreme— WNr. 1.2343 (%) C Si Mn Cr Mo V
0.38 1.0 0.4 5.0 1.3 0.4
4. Industrial example of hard milling of deep cavities and fine details 4.1. Attributes of industrial example forging die In Fig. 3 lower die of die forging tool is presented, which was made of steel Vidar Supreme (WNr. 1.2343) hardened on 50 HRc and applied for hot die forging of product fixing plate M16 made of steel WNr. 1.0570. In Table 2 chemical structure of applied hot-work tool steel is presented. All alloyed elements values are in range of standardizable values of typical hot-work tool steel WNr. 1.2343. Forging process was based on two phases forging. In Figs. 4 and 5, cavities of both forging phases with further discussed details are presented. 4.2. Machining technology In Table 3 parameters for machining of specific section of cavities are presented.
Fig. 4. Cavity of first forging phase with maximum depth.
For finishing cavity pocket (shown in Fig. 4) solid carbide ball nose end milling tool Fraisa U5286 D4R2 mm was applied, which is presented in Fig. 6. Smaller shank diameter, which follows milling tool cutting part, enables milling of vertical surfaces. Otherwise milling tool shank rubs against already machined surfaces, which can cause a serious damage for manufactured cavity and milling tool. Existent length of reduced shank can be enlarged by additional shank grinding on required length.
Fig. 3. Lower die of forging tool for hot forging of steel.
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Fig. 7. Ball nose end mill OSG FX-RB-EBD D0.6R0.3 mm [9].
Fig. 5. Cavity of second forging phase with small inner radii at maximum depth.
Fig. 8. Ball nose end mill Sandvik R216.42-01030-AP10G 1610 D1R0.5 mm.
Table 3 Machining parameters
Tool Coating L/D ratio Shank diameter (mm) RPM ap (mm) ae (mm) Strategy
Fig. 4
Fig. 5
Ball nose end mill D4R2 mm TiAlN 8 4 20000 0.08 0.13 Semifinish + finish of whole cavity
Ball nose end mill D0.6R0.3 mm TiAlN Over 30 6 20000 0.04 0.05 Semifinish + finish of fine details Fig. 9. Definition of radial ae and ap axial depth of cut [10].
In Table 4 cutting geometry is presented and represents typical cutting geometry required for hard milling. Free angle α has minimum still acceptable value. Rake angle γ is negative, what results in good cutting stability and higher rake surface wear on the other side. For successful surface milling very sharp milling tool is required. Applied L/D ratio represents boundary value, which is still acceptable to mostly nonproblematical hard milling. In Fig. 7 solid carbide ball nose end milling tool OSG FXRB-EBD D0.6R0.3 mm, which was applied for restmilling of details shown in Fig. 5, is presented. Value of applied L/D ratio is extreme high, over 30. Milling tool is an example of shank reduced milling tool of small cutting diameter, where shank diameter has to be large enough to ensure appropriate
attachment into collet and holder. Better example of reduced milling tool is presented in Fig. 8, where reduction is achieved with small angle transition, which ensures more rigid milling tool. In this case L/D ratio cannot be established accurately. Base of hard milling technology is high frequency repetition of light cuts, which are defined by axial and radial depth of cut and feed. In Table 3 presented axial and radial depth of cut are suitable for particular milling tool according to the supplier recommendations. In Fig. 9 the definition of radial and axial depth of cut is presented. Tool life of this HSC milled forging tool was approximately 30% longer than the same EDM machined forging tool (customer feedback information).
5. Experimental work
Fig. 6. Ball nose end mill Fraisa U5286 D4R2 mm [8]. Table 4 Ball nose end mill Fraisa U5286 D4R2 mm cutting geometry (◦ )[8] α β γ λ
4 96 −10 30
On the basis of industry experiences with the intention of optimization of high speed milling and hard cutting respectively by manufacturing of forming tools representative system of experiences was created. The main purpose of experiences is to ascertain convenience of milling with high milling tool L/D ratio grades by finishing of forming tool cavity surfaces. For preparation of a representative experimental system already known industry practice facts were considered. For representation of an average hot die forging tool type and a hardness of typical material, a geometry and requested sur-
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B. Kecelj et al. / Journal of Materials Processing Technology 157–158 (2004) 536–542 Table 5 Machining parameters Surface inclination (◦ )
RPM vf (mm/min) ap (mm) ae (mm) Cutting fluid Strategy
0
45
86
20000 2000 0.1 0.1 Air/dry cutting Parallel down milling
20000 4000 0.1 0.1 Air/dry cutting Parallel down milling
17190 3440 0.1 0.1 Air/dry cutting Parallel down milling
5.2. Machining technology
Fig. 10. Draft of experimental part.
face quality of average cavity and manufacturing technology have to be considered. The aim of experiments is on the basis of taken snapshots to determine deflections of milling tool during milling of representative surfaces with different L/D ratio grades. 5.1. Preparing of experimental part
For three axes high speed milling a solid carbide ball nose end mill coated with TiAlN was selected, which assure successful hard milling. Selection of a milling tool diameter for manufacturing of cavity generally depends on its details. For experiments a milling tool with diameter of 4 mm was selected, therefore it represents mostly applied milling tool for finishing of cavities. A diameter of the shank is the same as a diameter of the cutting part, which simplifies and makes more accurate calculation of milling tool L/D ratio. At the same time more intensive deflection of milling tool can be expected as in case of greater shank diameter, which increase milling tool stability during milling process. As already mentioned the aim of experiments is observing of conditions during surface finishing, therefore the machining technology is adjusted to this specific type of cutting process. All cutting parameters are applied from selected milling tool producer recommendations for individual experimental surface inclination [11]. Parameters ae and ap for milling of each of three different surface inclination are selected thus the theoretical thickness of chip is in all cases the same, what eliminates the influence of varying specific cutting force (Table 5). By all machining operations strategy of parallel down dry milling was applied. In Fig. 11 up and down milling is presented. When the cutting edge goes into cut in down milling, the chip thickness
In Fig. 10 the experimental part draft is presented. Material of the experimental part is a tool steel for hot-work WNr. 1.2344 hardened on 50 HRc. On the part three experimental surfaces of different inclination are placed, which represent different kind of surfaces that consist forging tool cavity: - nearly horizontal surfaces, - surfaces of transition and - nearly vertical surfaces. All surfaces are ground, which assure very accurate positioning of experimental part and the same milling parameters by all experiments with different combination of surface inclination and milling tool L/D ratio.
Fig. 11. Up and down milling [12].
B. Kecelj et al. / Journal of Materials Processing Technology 157–158 (2004) 536–542
has its maximum value, on the contrary in up milling it has its minimum value. In up milling is considerably more heat generated than in down milling, because of higher friction on cutting edge. In down milling the cutting edge is mainly exposed to compressive stresses, which are much more favourable for the properties of solid carbide compared with the tensile stresses developed in up milling. Therefore in modern HSC milling down milling is in use. It assures low milling tool wear although cutting process is more pretentious because of greater cutting forces. Modern machine tools are more rigid, that is why allow use of down milling. 5.3. Realisation of measurements All experiments were realised on vertical milling center Mori Seiki Frontier-M. In Fig. 12 testing part of measurement chain is presented. With the camera all phenomena during milling of selected surfaces and L/D ratio were filmed. The camera was connected with PC by imaging source, which allowed to software-based frame grabber to scoop picturebased information. After remaking received picture information deflections of milling tool were analysed. After finished experimental manufacturing roughness of manufactured surfaces was measured. 5.4. Results In Table 6 values of measured deflections are presented. Value of specific deflection is average value of three measured deflections obtained during manufacturing of same combination of inclination and L/D ratio. In these calculations deflections of first cuts are not included, which are greater because first cut chips are much thicker and different shaped than all other that are matter of experiments.
541
Table 6 Deflections of milling tool (mm) Surface inclination (◦ )
0 45 86
L/D ratio 7
10
0.0175 0.014 0.015
0.065 0.045 0.051
Deflections of smaller L/D ratio are smaller, also the differences between different inclined surfaces are smaller. By both L/D ratios deflections on surfaces, which represent nearly horizontal surfaces, are the greatest. Those results are consequence of bad cutting conditions in the centre of ball nose end milling tool, specially on such small depth of cut, which appears by finish milling of cavity. Results of the other two inclined surfaces, where deflections are not so high, proves the rule of milling with flank part of ball nose end milling tool. Obtained data can be explained with inclination angle β = 10–20◦ of milling tool by which the best cutting results appear [13]. Situation of inclined ball nose end milling tool with constant inclination is possible to achieve with technology of five axes milling. For all machined surfaces roughness Ra in transversal and longitudinal direction regarding milling toolpath has been measured. In Figs. 13 and 14 roughness measurement results for L/D = 7 are presented. Same type of results for L/D = 10 in Figs. 15 and 16 are presented. Generally, surface roughness Ra is higher by L/D = 10 and also by transversal direction of particular applied milling tool L/D ratio. For each combination of L/D ratio and measuring direction increasing of
Fig. 13. Transversal surface roughness of different inclined surfaces by L/D = 7.
Fig. 12. Testing part of measuring chain.
Fig. 14. Longitudinal surface roughness of different inclined surfaces by L/D = 7.
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Fig. 15. Transversal surface roughness of different inclined surfaces by L/D = 10.
Still applicable L/D ratio value depends on material hardness, required dimension accuracy, required surface roughness and required minimal inside corner radius. At this stage a combination of both procedures is convenient for tool manufacturing for products with low and middle surface roughness and dimension accuracy requests. The main problem for finish milling of cavity with high L/D ratios regarding surface roughness represents nearly vertical surfaces where milling tool deflection influence on surface roughness and dimension accuracy. Regarding milling tool wear horizontal surfaces, where during three axes milling appear very bad cutting conditions in centre of ball nose end milling tool, are the most problematic. Five axes milling of simple forming cavities is questionable because of higher costs and in some cases collision problems.
References
Fig. 16. Longitudinal surface roughness of different inclined surfaces by L/D = 10.
surface roughness regarding to surface inclination increasing can be ascertain. The greatest milling tool deflection by horizontal surface does not result in higher surface roughness. On the contrary, surface roughness is the lowest and is result of improved cutting conditions in the area near the ball nose end milling tool center. Dimension accuracy is result of deflections. By L/D = 10 dimension accuracy is still acceptable for average forging tool. 6. Conclusion and perspectives Generally, the reduced need for EDM in tool manufacturing is increasing whenever it is possible. Thus HSC milling technology with two extreme applications, hard milling and milling at high length/diameter (L/D) ratios, increasingly replaces EDM by manufacturing of all those exacting tool cavities details, where EDM would typically be used. Low cutting forces by HSM ensure successfully product [14].
[1] B. Nardin, K. Kuzman, Determination of manufacturing technologies in mould manufacturing, in: Proceedings of the AMME 2000, Gliwice-Sopot-Gda´nsk, Poland, 2000, pp. 391–394. [2] J. Schock, P. von Meiss, HSC-Fr¨asen im Werkzeug- und Formbau, Werkzeug und Formenbau: Tagung Aachen, VDI Verlag GmbH, D¨usseldorf, 1998, pp. 105–115. [3] K. Kuzman, B. Nardin, J. Kopaˇc, Comparison of HSC and EDM from energy consumption and ecology point of view, in: Proceedings of the ICEM 99, N¨urnberg, Germany, 1999, pp. 115–120. [4] H. Schweiger, H. Lenger, H.-P. Fauland, K. Fisher, A new generation of toughest hot-work tool steels for highest requirements, in: Proceedings of the Fifth International Conference on Tooling, Leoben, Austria, 1999, pp. 285–294. [5] http://www.mmsonline.com/hsm/hsmevent/diemold.html. [6] K. Fritsche, N. Glatthor, HSC-Fr¨asen im Formenbau: Rationalisierung durch Komplettbearbeitung auf Bearbeitungszentren, Moderne Industrie, Landsberg, 1997. ˇ s, Calculation of optimum variables for dry machining, Int. J. [7] F. Cuˇ Flex. Autom. Integ. Manuf. 7 (3/4) (1999) 393–406. [8] Fraisa, Fraisa 2000, Edition 1.4.2000, Bellach 2000. [9] OSG Ultra FX Carbide End Mills, vol. 4, OSG Corporation. [10] T. Roblek, Optimization of high speed machine tools performances, Master Thesis, Ljubljana, 2002. [11] Hanita, Solid Carbide Vision, Recommended Working Details, Hanita Metal Works Ltd., 2001. [12] Die & Mould Making, Application Guide, Sandvik Coromant, Sandviken, 1999. [13] H. Schulz, Hochgeschwindigkeitsbearbeitung (High Speed Machining), Carl Hanser Verlag, M¨unchen, Wien, 1996. [14] J. Kopaˇc, Cutting forces and their influence on the economics of machining, J. Mech. Eng. 48 (2002) 121–133.
Speciality of HSC in manufacturing of forging dies B. Kecelja,∗ , J. Kopaˇcb , Z. Kampuˇsb , K. Kuzmana,b b
a TECOS, Slovenian Tool and Die Development Centre, Kidriˇ ceva 25, SI-3000 Celje, Slovenia Faculty of Mechanical Engineering, University of Ljubljana, Aˇskerˇceva 6, SI-1000 Ljubljana, Slovenia
Abstract The paper discusses the attributes of the die forging tools manufacturing with high speed milling technology. In the most cases of die forging tools manufacturing high speed milling of hardened material is faster and cheaper manufacturing process than EDM, besides forging tools have better quality and therefore longer tool life. By machining process of the paper presented die forging tools, milling tool length/diameter ratios of extreme values were applied, which have influence on high speed milling strategy and technology. Whole die forging tool design and manufacturing procedure is realised on a method of long distance work. © 2004 Elsevier B.V. All rights reserved. Keywords: High speed milling; Hard milling; High L/D ratio; Forging die
1. Introduction
2. Attributes of HSC and EDM
Nowadays HSC technology is a basic constituent part of every modern tool making company. With the introduction of HSC into tool shops mould making manufacturing technology has been changed, mostly by manufacturing of mould cavities which represent the most pretentious mould parts so that 40% of mould manufacturing time is used for the manufacturing of mould cavity. According to the objective model for cavity manufacturing technology developing, where milling tool, mould and product related parameters are considered, combination of EDM/HSC or entirely HSC manufacturing technology replaced classic die sinking EDM mould cavity machining [1]. For each mould cavity itself has to be ascertain, which technology – rough HSC milling/finish EDM or HSC milling entirely – is the most advantageous by costs and manufacturing time.
In Table 1 [2] the advantages, disadvantages as well as limits of the EDM and HSC milling are presented. For selection of the most appropriate mould cavity machining technology, the energy consumption and ecology are of a great importance too. It is well known that the EDM process has a very high level of energy consumption, therefore it should be used only in cases where, regarding product, milling tool shape or mould related properties does not allow the HSC technology [3]. From ecology point of view HSC technology is prevailing EDM for the following reasons:
∗
Corresponding author. Tel.: +386 3 4264 607; fax: +386 3 4264 611. E-mail addresses: [email protected] (B. Kecelj), [email protected] (J. Kopaˇc), [email protected] (Z. Kampuˇs), [email protected], [email protected] (K. Kuzman). URL: http://www.tecos.si. 0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.07.112
- Technology using less energy is much friendlier to the environment. - Permanent decrease of cutting lubricants and coolants leads to dry machining. - There must be constant monitoring of EDM electrolyte during the process as later on for the waste treatment and disposal [3]. The HSC introduction into tool shops drastically reduces manufacturing times for the mould forming system, however the EDM and HSC technologies are not competitive but they are adjective, since all different forming geometries can
B. Kecelj et al. / Journal of Materials Processing Technology 157–158 (2004) 536–542
537
Table 1 Comparison of manufacturing technologies Criterion
EDM
HSC milling
Materials Geometry Sharpedged inner angles Deep grooves Polished surface Costs of additional finish machining Blur surface Texture modification Geometric accuracy Dispossession effect Premachining Machining tool
All conducting materials Free Radius < 0.1 mm reachable Depend on manufactured electrode Always additional finish machining High Yes Micro cracks + + by large shapes surface dispossession Rough EDM Expensive (milled)
All cutting materials (steel up to 62 HRc) Limited depth, radius Radius at the bottom: > 0.3 mm, radius at the wall: > 1 mm Up to L/D < 10 +, partly without additional finish machining Low No Compressing ++ + by small shapes point dispossession Economy = f (machining costs, volume) Simple, standard product
Reprinted from Ref. [2].
be manufactured with EDM which is not the case for HSC [3].
3. Manufacturing technology of hot forming tool cavities Very important properties of steels in hot-work applications are temperature strength and thermal shock resistance, which help to determine the material selected for the tool, due to the high operating temperatures existing at the area of the contact between die and workpiece. But one of the most important properties, if not the single most important property, is the toughness. The toughness serves as an indication of the ability to resist the formation and growth of cracks [4]. Combination of HSC technology and milling tools with new coatings (TiAlN) is a ground for successful cost and time reduced hard milling, in range of 46–62 HRc of already hardened tool cavities for hot forging, and therefore reduction of EDM. Tools for forming processes as forging or deep drawing can be almost entirely milled because sharp inner angles basically do not appear. Reachable dispossession effect by hard milling is comparative or even better as by EDM, electrode is unnecessary and surface quality is better [2]. 3.1. Hard milling with high L/D ratio Forging part geometry defines parting plane and therefore required milling tool length/diameter (L/D) ratio. The latter have great influence on machining accuracy and surface quality. High spindle speeds increase the severity of vibration at the tool tip. To protect tool life and surface quality, more rigid tools have to be favoured. For end mills this means using of shortest tool as possible and favouring a tool with shorter flutes, which brings a larger and more rigid central core [5]. In Figs. 1 and 2, the main two criteria that influence on HSC manufacturing are presented. The adequate depth of cut ap depends on material hardness and milling tool slenderness, where the latter is defined as a ratio between length
Fig. 1. The influence of workpiece hardness on depth of cut [6].
and diameter of milling tool: ap = RK2 K3
(1)
Thus the depth of cut ap can be defined as a product of milling tool radius R, material hardness coefficient K2 and milling tool slenderness coefficient K3 values. By material hardness up to 52 HRc the value of material hardness coefficient K2 is between 0.1 and 0.12, at higher material hardnesses coefficient K2 values decreases to the range from 0.04 to 0.05 by 63 HRc [6]. Up to the L/D ratio = 5 tool slenderness coefficient K3 has value 1, after which rapidly decrease to the value of 0.1 at L/D = 10. Therefore at L/D > 10 high speed milling in hard material is possible only with special measures [6]. On the other side, the ability to take lights cuts quickly makes the milling machine more efficient with delicate tools that are very long relative to their diameter. This helps in at
Fig. 2. The influence of slenderness on depth of cut [6].
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least two cases: - milling deep cavities or deep slots, - milling fine details with very small tools [5]. At high L/D ratios milling tool deflection increases, which is consequence of large run out of milling tool and low tool rigidity. Tool deflection and milling strategy affect on dimension accuracy and surface roughness of machined tool cavity. HSM – dry cutting is by milling of 50 HRc materials very successfully too [7].
Table 2 Chemical structure of applied hot die forging tool steel Vidar Supreme— WNr. 1.2343 (%) C Si Mn Cr Mo V
0.38 1.0 0.4 5.0 1.3 0.4
4. Industrial example of hard milling of deep cavities and fine details 4.1. Attributes of industrial example forging die In Fig. 3 lower die of die forging tool is presented, which was made of steel Vidar Supreme (WNr. 1.2343) hardened on 50 HRc and applied for hot die forging of product fixing plate M16 made of steel WNr. 1.0570. In Table 2 chemical structure of applied hot-work tool steel is presented. All alloyed elements values are in range of standardizable values of typical hot-work tool steel WNr. 1.2343. Forging process was based on two phases forging. In Figs. 4 and 5, cavities of both forging phases with further discussed details are presented. 4.2. Machining technology In Table 3 parameters for machining of specific section of cavities are presented.
Fig. 4. Cavity of first forging phase with maximum depth.
For finishing cavity pocket (shown in Fig. 4) solid carbide ball nose end milling tool Fraisa U5286 D4R2 mm was applied, which is presented in Fig. 6. Smaller shank diameter, which follows milling tool cutting part, enables milling of vertical surfaces. Otherwise milling tool shank rubs against already machined surfaces, which can cause a serious damage for manufactured cavity and milling tool. Existent length of reduced shank can be enlarged by additional shank grinding on required length.
Fig. 3. Lower die of forging tool for hot forging of steel.
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Fig. 7. Ball nose end mill OSG FX-RB-EBD D0.6R0.3 mm [9].
Fig. 5. Cavity of second forging phase with small inner radii at maximum depth.
Fig. 8. Ball nose end mill Sandvik R216.42-01030-AP10G 1610 D1R0.5 mm.
Table 3 Machining parameters
Tool Coating L/D ratio Shank diameter (mm) RPM ap (mm) ae (mm) Strategy
Fig. 4
Fig. 5
Ball nose end mill D4R2 mm TiAlN 8 4 20000 0.08 0.13 Semifinish + finish of whole cavity
Ball nose end mill D0.6R0.3 mm TiAlN Over 30 6 20000 0.04 0.05 Semifinish + finish of fine details Fig. 9. Definition of radial ae and ap axial depth of cut [10].
In Table 4 cutting geometry is presented and represents typical cutting geometry required for hard milling. Free angle α has minimum still acceptable value. Rake angle γ is negative, what results in good cutting stability and higher rake surface wear on the other side. For successful surface milling very sharp milling tool is required. Applied L/D ratio represents boundary value, which is still acceptable to mostly nonproblematical hard milling. In Fig. 7 solid carbide ball nose end milling tool OSG FXRB-EBD D0.6R0.3 mm, which was applied for restmilling of details shown in Fig. 5, is presented. Value of applied L/D ratio is extreme high, over 30. Milling tool is an example of shank reduced milling tool of small cutting diameter, where shank diameter has to be large enough to ensure appropriate
attachment into collet and holder. Better example of reduced milling tool is presented in Fig. 8, where reduction is achieved with small angle transition, which ensures more rigid milling tool. In this case L/D ratio cannot be established accurately. Base of hard milling technology is high frequency repetition of light cuts, which are defined by axial and radial depth of cut and feed. In Table 3 presented axial and radial depth of cut are suitable for particular milling tool according to the supplier recommendations. In Fig. 9 the definition of radial and axial depth of cut is presented. Tool life of this HSC milled forging tool was approximately 30% longer than the same EDM machined forging tool (customer feedback information).
5. Experimental work
Fig. 6. Ball nose end mill Fraisa U5286 D4R2 mm [8]. Table 4 Ball nose end mill Fraisa U5286 D4R2 mm cutting geometry (◦ )[8] α β γ λ
4 96 −10 30
On the basis of industry experiences with the intention of optimization of high speed milling and hard cutting respectively by manufacturing of forming tools representative system of experiences was created. The main purpose of experiences is to ascertain convenience of milling with high milling tool L/D ratio grades by finishing of forming tool cavity surfaces. For preparation of a representative experimental system already known industry practice facts were considered. For representation of an average hot die forging tool type and a hardness of typical material, a geometry and requested sur-
540
B. Kecelj et al. / Journal of Materials Processing Technology 157–158 (2004) 536–542 Table 5 Machining parameters Surface inclination (◦ )
RPM vf (mm/min) ap (mm) ae (mm) Cutting fluid Strategy
0
45
86
20000 2000 0.1 0.1 Air/dry cutting Parallel down milling
20000 4000 0.1 0.1 Air/dry cutting Parallel down milling
17190 3440 0.1 0.1 Air/dry cutting Parallel down milling
5.2. Machining technology
Fig. 10. Draft of experimental part.
face quality of average cavity and manufacturing technology have to be considered. The aim of experiments is on the basis of taken snapshots to determine deflections of milling tool during milling of representative surfaces with different L/D ratio grades. 5.1. Preparing of experimental part
For three axes high speed milling a solid carbide ball nose end mill coated with TiAlN was selected, which assure successful hard milling. Selection of a milling tool diameter for manufacturing of cavity generally depends on its details. For experiments a milling tool with diameter of 4 mm was selected, therefore it represents mostly applied milling tool for finishing of cavities. A diameter of the shank is the same as a diameter of the cutting part, which simplifies and makes more accurate calculation of milling tool L/D ratio. At the same time more intensive deflection of milling tool can be expected as in case of greater shank diameter, which increase milling tool stability during milling process. As already mentioned the aim of experiments is observing of conditions during surface finishing, therefore the machining technology is adjusted to this specific type of cutting process. All cutting parameters are applied from selected milling tool producer recommendations for individual experimental surface inclination [11]. Parameters ae and ap for milling of each of three different surface inclination are selected thus the theoretical thickness of chip is in all cases the same, what eliminates the influence of varying specific cutting force (Table 5). By all machining operations strategy of parallel down dry milling was applied. In Fig. 11 up and down milling is presented. When the cutting edge goes into cut in down milling, the chip thickness
In Fig. 10 the experimental part draft is presented. Material of the experimental part is a tool steel for hot-work WNr. 1.2344 hardened on 50 HRc. On the part three experimental surfaces of different inclination are placed, which represent different kind of surfaces that consist forging tool cavity: - nearly horizontal surfaces, - surfaces of transition and - nearly vertical surfaces. All surfaces are ground, which assure very accurate positioning of experimental part and the same milling parameters by all experiments with different combination of surface inclination and milling tool L/D ratio.
Fig. 11. Up and down milling [12].
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has its maximum value, on the contrary in up milling it has its minimum value. In up milling is considerably more heat generated than in down milling, because of higher friction on cutting edge. In down milling the cutting edge is mainly exposed to compressive stresses, which are much more favourable for the properties of solid carbide compared with the tensile stresses developed in up milling. Therefore in modern HSC milling down milling is in use. It assures low milling tool wear although cutting process is more pretentious because of greater cutting forces. Modern machine tools are more rigid, that is why allow use of down milling. 5.3. Realisation of measurements All experiments were realised on vertical milling center Mori Seiki Frontier-M. In Fig. 12 testing part of measurement chain is presented. With the camera all phenomena during milling of selected surfaces and L/D ratio were filmed. The camera was connected with PC by imaging source, which allowed to software-based frame grabber to scoop picturebased information. After remaking received picture information deflections of milling tool were analysed. After finished experimental manufacturing roughness of manufactured surfaces was measured. 5.4. Results In Table 6 values of measured deflections are presented. Value of specific deflection is average value of three measured deflections obtained during manufacturing of same combination of inclination and L/D ratio. In these calculations deflections of first cuts are not included, which are greater because first cut chips are much thicker and different shaped than all other that are matter of experiments.
541
Table 6 Deflections of milling tool (mm) Surface inclination (◦ )
0 45 86
L/D ratio 7
10
0.0175 0.014 0.015
0.065 0.045 0.051
Deflections of smaller L/D ratio are smaller, also the differences between different inclined surfaces are smaller. By both L/D ratios deflections on surfaces, which represent nearly horizontal surfaces, are the greatest. Those results are consequence of bad cutting conditions in the centre of ball nose end milling tool, specially on such small depth of cut, which appears by finish milling of cavity. Results of the other two inclined surfaces, where deflections are not so high, proves the rule of milling with flank part of ball nose end milling tool. Obtained data can be explained with inclination angle β = 10–20◦ of milling tool by which the best cutting results appear [13]. Situation of inclined ball nose end milling tool with constant inclination is possible to achieve with technology of five axes milling. For all machined surfaces roughness Ra in transversal and longitudinal direction regarding milling toolpath has been measured. In Figs. 13 and 14 roughness measurement results for L/D = 7 are presented. Same type of results for L/D = 10 in Figs. 15 and 16 are presented. Generally, surface roughness Ra is higher by L/D = 10 and also by transversal direction of particular applied milling tool L/D ratio. For each combination of L/D ratio and measuring direction increasing of
Fig. 13. Transversal surface roughness of different inclined surfaces by L/D = 7.
Fig. 12. Testing part of measuring chain.
Fig. 14. Longitudinal surface roughness of different inclined surfaces by L/D = 7.
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Fig. 15. Transversal surface roughness of different inclined surfaces by L/D = 10.
Still applicable L/D ratio value depends on material hardness, required dimension accuracy, required surface roughness and required minimal inside corner radius. At this stage a combination of both procedures is convenient for tool manufacturing for products with low and middle surface roughness and dimension accuracy requests. The main problem for finish milling of cavity with high L/D ratios regarding surface roughness represents nearly vertical surfaces where milling tool deflection influence on surface roughness and dimension accuracy. Regarding milling tool wear horizontal surfaces, where during three axes milling appear very bad cutting conditions in centre of ball nose end milling tool, are the most problematic. Five axes milling of simple forming cavities is questionable because of higher costs and in some cases collision problems.
References
Fig. 16. Longitudinal surface roughness of different inclined surfaces by L/D = 10.
surface roughness regarding to surface inclination increasing can be ascertain. The greatest milling tool deflection by horizontal surface does not result in higher surface roughness. On the contrary, surface roughness is the lowest and is result of improved cutting conditions in the area near the ball nose end milling tool center. Dimension accuracy is result of deflections. By L/D = 10 dimension accuracy is still acceptable for average forging tool. 6. Conclusion and perspectives Generally, the reduced need for EDM in tool manufacturing is increasing whenever it is possible. Thus HSC milling technology with two extreme applications, hard milling and milling at high length/diameter (L/D) ratios, increasingly replaces EDM by manufacturing of all those exacting tool cavities details, where EDM would typically be used. Low cutting forces by HSM ensure successfully product [14].
[1] B. Nardin, K. Kuzman, Determination of manufacturing technologies in mould manufacturing, in: Proceedings of the AMME 2000, Gliwice-Sopot-Gda´nsk, Poland, 2000, pp. 391–394. [2] J. Schock, P. von Meiss, HSC-Fr¨asen im Werkzeug- und Formbau, Werkzeug und Formenbau: Tagung Aachen, VDI Verlag GmbH, D¨usseldorf, 1998, pp. 105–115. [3] K. Kuzman, B. Nardin, J. Kopaˇc, Comparison of HSC and EDM from energy consumption and ecology point of view, in: Proceedings of the ICEM 99, N¨urnberg, Germany, 1999, pp. 115–120. [4] H. Schweiger, H. Lenger, H.-P. Fauland, K. Fisher, A new generation of toughest hot-work tool steels for highest requirements, in: Proceedings of the Fifth International Conference on Tooling, Leoben, Austria, 1999, pp. 285–294. [5] http://www.mmsonline.com/hsm/hsmevent/diemold.html. [6] K. Fritsche, N. Glatthor, HSC-Fr¨asen im Formenbau: Rationalisierung durch Komplettbearbeitung auf Bearbeitungszentren, Moderne Industrie, Landsberg, 1997. ˇ s, Calculation of optimum variables for dry machining, Int. J. [7] F. Cuˇ Flex. Autom. Integ. Manuf. 7 (3/4) (1999) 393–406. [8] Fraisa, Fraisa 2000, Edition 1.4.2000, Bellach 2000. [9] OSG Ultra FX Carbide End Mills, vol. 4, OSG Corporation. [10] T. Roblek, Optimization of high speed machine tools performances, Master Thesis, Ljubljana, 2002. [11] Hanita, Solid Carbide Vision, Recommended Working Details, Hanita Metal Works Ltd., 2001. [12] Die & Mould Making, Application Guide, Sandvik Coromant, Sandviken, 1999. [13] H. Schulz, Hochgeschwindigkeitsbearbeitung (High Speed Machining), Carl Hanser Verlag, M¨unchen, Wien, 1996. [14] J. Kopaˇc, Cutting forces and their influence on the economics of machining, J. Mech. Eng. 48 (2002) 121–133.