Electrochemical Honing

Journal of Materials Processing Technology 109 (2001) 270±276

Investigation on non-conventional honing of sculptured surfaces for parts made of alloy steel Robert Dynarowski*, Bogdan Nowicki Warsaw University of Technology, ul. Narbutta 86, 02-524 Warsaw, Poland

Abstract The ®nishing of free-form surface such as press-forming dies, propellers, screw propellers is usually a manual job or is done by robots equipped with special machining heads. The authors have elaborated the non-conventional honing (NH) method which enables to place the pro®ling and the ®nishing on the same machine tool or robots. The abrasive tools have four degrees of freedom, they are elastically pressed against the surface. The minimum allowable curvature radii of the machined parts is 100 mm. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Non-conventional honing; Alloy steel; Robots

Machining of free-form surfaces, particularly large-sized elements such as press-forming dies for car bodies, screw propellers, etc. is one of the outstandingly dif®cult technological and organisational tasks [1±3]. This is due to the unique nature of such production and the following reasons could be distinguished:  high costs of technological devices both for shaping and finishing of such elements,  shortages in high-skilled tool makers needed for the finishing operations,  time-consuming nature of large-sized object finishing (ranging from several up to hundreds of man-hours),  particularly high costs of machining for such elements, and  small production lots. The abrasive ®nishing can be applied with the help of special technological devices or robots of high stiffness. Machining costs are thus considerably enhanced and the number of manufacturing companies capable of meeting such requirements is limited. The required surface roughness (represented by Ra parameter) for the injection moulds is Ra ˆ 0:01ÿ0:1 mm, for the pressing dies Ra ˆ 0:6ÿ1:2 mm, for the screw propellers Ra ˆ 1:2ÿ2:5 mm. These surfaces are usually shaped by milling on the NC machine tools, by tracing, by electrodischarge machining (EDM) or by electrochemical machining (ECM) (Table 1). The required accuracy can be obtained *

Corresponding author.

by milling on the ®ve-axis controlled rigid NC machines. Then, the abrasive ®nish machining is applied only for removing the machining traces and for obtaining the proper surface ®nish. The ®nishing is the most time-consuming operation of the manufacturing process. The machining time depends on the applied tools, on the part complexity, on the nature of the machined material and it varies from few tenths of second up to several hours per square decimetre [2,7,8]. The ®nishing of the large-sized pressing dies and the screw propellers takes from a few days to one or more months. If we assume that the total manufacturing time for largesized pressing dies is 1000±3000 working hours, then the machining time of the ®nishing process accounts for 37± 42% in Japan, USA and Germany [5±8] and up to 50% of the total manufacturing time in Poland [5,6,9,10]. Institute of Manufacturing Engineering, WUT has been searching for universal and ef®cient solutions for this problem and ®nally a new ®nishing method of free-form surfaces has been elaborated and it was called non-conventional honing (NH). The new NH method has been developed by the author [4±9] and it is based on the analysis of the automation circumstances and on the existing methods. The kinematic diagram of this machining method is presented in Fig. 1. The abrasive sticks have three or four degrees of freedom in relation to the machining head. They are able to rotate along A, B, C axis and they can move in Z axis. The contact pressure against the machined surface is ensured by pneumatic system. The abrasive sticks rotate together with the

0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 8 1 0 - 4

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Table 1 Machining methods and devices for free-form surfaces

head and with the spindle while the machined part is moved in X, Y, Z axis. The machining can be done on the milling machine tools with 2.5, 3 or 5 axis control or with the help of robots. The linear movements of the head are controlled by part programs.

For surfaces of large curvature radii, when the differences in the extreme positions of the surface point are smaller than the piston stroke travel of the machining head, the ®nishing can be done on the conventional milling machine tool.

Fig. 1. Schematic drawings of head for non-conventional NH.

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Fig. 2. Trajectory of abrasive grains' traces on the machined surface. Machining parameters: u ˆ 0:262 m=s, Vf ˆ 3:4  10ÿ3 m=s, R ˆ 0:026 m.

The application of several simultaneously working abrasive tools and high rotational speed can result in high metal removal rate. Due to the rotational movement of the abrasive sticks, there are good conditions for self-sharpening. The machining of free-form surfaces using NH method should be considered taking into account speci®c features of this method and particularly the following:  non-uniform density distribution of the abrasive grains' traces on the machined surface, and  machining forces variability due to the effect of the machined surface curvature and the rotational movement of the abrasive tools. The trajectory of the traces of the abrasive grains in relation to the machined surface has been presented in Fig. 2.

The cross-section of the machined area is shown in Fig. 3. The width of the area corresponds to the double distance between the head axis and the abrasive stick axis. The differences in depth of the material removal result from the density of probabilitydistributionof the traces occurringonthe machined surface and from the in¯uence of the lay direction angle upon the removal rate. Assuming that the successive machining paths (strips) are tangent to each other, the surface shape errors may be up to 5±10 mm for the machining depth of 100 mm. If we allow the overriding of the machining strips, the surface shape errors can be considerably smaller. The macro pro®le of the concave surface shaped by milling prior to machining and after machining has been presented in Fig. 3. The milling traces have been removed without any change of the surface shape.

Fig. 3. Macro pro®le of concave surfaces prior to machining (upper shape) and after machining (lower shape).

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generatrix of the machined surface (Fig. 5). In case of the concave surface, the abrasive stick radius is equal to the curvature radius of the machined surface. The contact is also quasi-linear and the direction is perpendicular to the surface generatrix. The contact is of quasi-point nature in case of free-form surfaces, while the abrasive stick is in full planar contact with the machined surface for ¯at surface machining. Actually, the heads of relatively large diameters are used for large-sized surfaces (Fig. 1a), while for small-sized surfaces heads shown in Fig. 1b are recommended. The machining of the corners and the hardly accessible places is impossible. The machining of those places as well as of the edges and the corners is accomplished by traditional ®nish machining methods. The heads of 100 mm diameter and of 25 mm piston stroke travel enable machining of surfaces with the curvature radii R > 100 mm. The machining result is in¯uenced by several factors. The most important ones are listed below (Fig. 6):

Fig. 4. Hone-material contact.

The forces exerted by the abrasive stick on the machined surface are in¯uenced by the temporary curvature radius, pressure of the compressed air, friction force of the piston moving in cylinder, spring reaction, etc. Fig. 4 illustrates the contact areas of sticks and machined surfaces for cases of ¯at, concave and convex surface types. The contact of the abrasive stick and the convex surface is quasi-linear and the direction of the contact line is parallel to

 kinematic parameters, i.e. rotational speed (u), and machining feed-rate (Vf), load ( p) applied to the machining surface by the abrasive sticks,  abrasive tools (type of abrasive grains (Al2O3), bond, grit size (Z), hardness (h) and structure of abrasive sticks), and  type of working liquid and methods of its introduction into the machining area and amount of liquid. The machining results can be evaluated using the following criteria:  removal rate which is measured by the volume of the material removed per minute Q (mm3/min) and the surface machined per minute Q1 (dm2/min),

Fig. 5. The plot of the changes for normal force exerted by the abrasive stick on the convex surface when machining with speed o ˆ 51:31 sÿ1 , pressure p ˆ 245162:5 Pa and for machined surface curvature k ˆ 400 mm.

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Fig. 6. Schematic of NH machining being an investigation object.

 surface roughness (Ra, Rm, Dq, Sm, tp), level of residual stress and of strain-hardening for surface layer,  geometry of surface defined as the macro profile deviation after machining relative to the deviation prior to machining, and  abrasive sticks wear q (mm3/min).

The numerical values of the independent variables have been summarised in Table 2. The regression equations have been determined based on 35 experiments carried out for each surface type (¯at, concave and convex). The analysis of the investigated factors has shown the following:

The investigations have been performed for ¯at, concave and convex surface, according to rules of experimental design (®ve-level composite design for four independent variables, extended by ®ve extra experiments). The following parameters have been assumed as constants:

 the volumetric removal rate Q ˆ 44ÿ1800 mm3 =min and it is influenced mainly by grit size (the greater the grit size, the greater is the Q),  honing sticks wear q for typical machining conditions is q ˆ 200 mm3 =min and it is influenced mainly by stick hardness,  surface roughness, represented by Ra parameter; Ra ˆ 0:3ÿ4:0 mm. It is influenced mainly by grit size.

 material subject to machining: alloy steel NC6 Ð typical for punch dies,  abrasive tools: honing sticks made of alumina A in ceramic bond,  working liquid: 25% solution of compounded oil in kerosene,  feed rate: 85 mm/min, and  number of machining passes: 2.

The h parameter present in Tables 2 and 3 is describing stick hardness (usually speci®ed using letters instead of numerical values). According to Polish Standard (PN-86/ M59 119), h parameter denotes the depth of pit being a result of standard test performed on special machine using

Table 2 The actual and coded values of the independent variables i

1 2 3 4

Input variable

p Z T u

Variable notation

x1 x2 x3 x4

Actual/coded value

Units

ÿ2

ÿ1

0

1

2

98.065 40 G 0.262

137.29 60 J 0.403

196.13 100 M 0.645

274.58 150 P 1.209

392.26 220 S 1.975

kPa ± ± m/s

R. Dynarowski, B. Nowicki / Journal of Materials Processing Technology 109 (2001) 270±276

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Table 3 Results of regression calculations for convex surfaces Parameter Qmat Ra1 Ra2 Dq1 Dq2 Sm1 Sm2 q Mz Q/q

Regression equations 2.41

1.46

ÿ0.88

0.7

0.33

e p Z u h e0.98 Zÿ0.69 u0.39 h0.36 p0.47 e1.91 Zÿ0.74 u0.35 h0.32 p0.4 e1.24 u0.27 p0.29 Zÿ0.19 h0.17 e3.55 Zÿ0.45 h0.27 p0.16 u0.09 e6.53 Zÿ0.47 hÿ0.11 p0.12 e3.69 Zÿ0.15 p0.31 u0.17 hÿ0.11 e3.48 h2.45 Zÿ1.95 u1.15 p1.7 eÿ0.59 p0.54 Zÿ0.22 hÿ0.2 uÿ0.14 eÿ1.06 hÿ2.12 Z1.07 uÿ0.46 pÿ0.24

R

F

T1

T2

T3

T4

0.85 0.88 0.89 0.72 0.71 0.75 0.48 0.94 0.84 0.83

19.05 25.55 28.74 8.28 7.89 13.5 2.25 58.05 18.21 16.2

5.77 ÿ7.9 ÿ8.9 4.08 ÿ5.3 ÿ4.8 ÿ1.1 11.6 5.61 ÿ7.7

ÿ4.9 4.99 4.72 2.86 3.03 ÿ1.1 1.71 ÿ9.6 ÿ3.2 4.08

4.32 3.94 3.66 ÿ2.7 1.38 0.92 1.48 6.36 ÿ2.9 ÿ1.9

1.78 3.87 3.47 2.2 1.17 ± ÿ0.8 5.99 ÿ2.2 ÿ0.6

predetermined amount of sand driven into stick by jet under given pressure. The graphical illustration of the most descriptive regression equations is presented in the form of 3D graphs (Figs. 7±9).

The images explain dependencies between machining conditions (input parameters) and the corresponding machining results (output parameters) for convex surfaces (Figs. 7±9). The investigations on the material removal considered in direction perpendicular to stick feed rate have shown that it is not uniform and it is dependent on the machined surface shape so that resultant accuracy is conditioned by the proper machining strategy. The average uniformity error is 0.02 mm when removing 0.2 mm thick layers. The investigations on the dependence between the machining conditions and machining productivity as well as surface roughness have shown that applying the socalled ``hard'' machining parameters have resulted in extra high productivity (1800 mm3/min), applying proper grit size, loads and machining speed have resulted in extra low surface roughness (Ra ˆ 0:3 mm). Wear of abrasive sticks is at least 10-fold lower than material removal rate and for typical machining conditions it is equal to 200 mm3/min.

Fig. 7. Surface roughness height vs. stick grit size Z and peripheral speed u.

Fig. 8. Material removal rate vs. stick grit size Z and pressure p.

Fig. 9. Material removal rate to wear of abrasive sticks vs. stick grit size Z and hardness h.

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References [1] T. Aoyama, I. Inasaki, Development of a Robot System for Metal Molds Polishing, Vol. XXI, Department of Mechanical Engineering, Keio University, Yokohama, 1993, pp. 295±301. [2] J. Brevick, Automated die and mold ®nishing: an experimental study of short stroke honing, in: Proceedings of the 20th NAMARA International Conference, USA, 1992. [3] I. Inasaki, K. Tonshoff, T. Howes, Abrasive machining in the future, Ann. CIRP 42 (2) (1993). [4] B. Nowicki, The new method of free form surface honing, Ann. CIRP 42 (1) (1993).

[5] B. Nowicki, Wniosek Patentowy, P-292530, SposoÂb obroÂbki i gøowica do gøadzenia powierzchni zwøaszcza o zarysie krzywoliniowym. [6] B. Nowicki, Machining method and head for honing free form surfaces, Patent. [7] B. Nowicki, Automatyzacja obroÂbki wykanÂczajaÎcej powierzchni krzywoliniowych, Mechanik Nr-1 1995 rok. [8] B. Nowicki, Automation of free form surface ®nishing. [9] K. Saito, Finishing and polishing of free-form surfaces, Bull. Jpn. Soc. Prec. Eng. 18 (2) (1984) 621±923. [10] H. Weule, S. Timmerman, Automation of the surface ®nishing in the manufacturing of dies and molds, Ann. CIRP 39 (1990) 299.