The influence of input parametersin case of cold plastic deformation.

Tanasa, Raluca ; Iuras, Emilia ; Carp, Iulia 等

1. INTRODUCTION

The smoothening through plastic deforming using centrifuged balls is considered a simple and economic method of fine processing and also of hardening for the metal surfaces; is a complex phenomenon that includes dynamic interactions between the metallic structure of the tool and the work piece (Stepanenko--Aizenberg M.V., 2007). The method can be successfully applied for cylindrical surfaces (internal and external) and for plane surfaces, as well.

Refering at cold deformation smoothing by mechanical shocks, one regards as an easy and efficient method of the smoothing and hardening surfaces (Shiou F-J. et al., 2003).

Among many advantages of this method (Draghici G., 1984) we could mention the most important ones a) the possibility to be applied on common machine-tools usually used for turning, grinding etc, b) the possibility to be applied by the operators that do not possess a high qualification; c) applying for a large set of external and internal cylindrical surfaces; d) the significant increasing of the fatigue strength etc. As disadvantages, one can consider the lack of the surface continuity, the possibility that the results be influenced by the variation of the initial hardness of the workpiece material etc.

2. THE TECHNOLOGICAL EQUIPMENT

The simplest technological equipment which can be successfully used in superficial plastic deformation processing by percussion, is constituted of a device which can be mounted on a universal machine-tool, lathe or milling-machine, and depending on the shape of the surface to be processed could be revolved or plane, respectively. To process external revolved surfaces, a device as the one from fig.1 can be used, being mounted on a universal lathe's carriage.

The device has an electric motor which, through a cinematic transmission induces the rotary motion [n.sub.s] of the portballs disc. The tool-disc, mounted on the lathe's carriage can be displace along the generatrix of the workpiece, assuring the feed motion [s.sub.1].

The revolved surface is assured by rotating the workpiece with a certain number of rotations per minute [n.sub.p], which is reffered as the circular feed motion [s.sub.c].

During smoothing procedure, the tool-disc is placed very close to the processed surface for the balls to hit the workpiece when it rotates.

[FIGURE 1 OMITTED]

The frequency of the percussions will be dependent on the disc number of rotations per minute and on the number of balls on the disc.

3. WORK HYPOTHESIS

The studies made till now consider that the surface smoothening can be realized in good conditions also by cold plastic deforming. After the collision of some balls with a surface, this is locally deformed and its micro-relief modified. During processing micro-craters appear on the surface, having the shape of spherical calotte. In the theory of the plastic deformations that consider the collision of two bodies, the local deformation is evaluated as the distance between the two bodies after collision, on the direction of their displacement. To observ the modification of the height of micro-irregularities we are using the mathematical relations for local plastic deformations (Ponomariov S.D. et al., 1964) and the equations from the contact theory. We can calculate the elements of the trace left by the deforming ball, i.e. the depth of the micro-crater (the maximum deformation) and its diameter as well. These parameters have been compared with the micro-irregularities of a surface finished through other methods.

3.1. Theoretical considerations

The irregularity assemblies represent the real surface relief. These irregularities appear due to oscillation movements of the tool tip, friction of the tool edge on the surface, high frequencies vibrations of the tool or of the machine-tool. The presence of irregularities on the surface has, in hard work conditions, some disadvantages, as narrowing the effective contact surfaces, worsening the friction and functioning conditions of the part, decrease the resistance to alternate efforts of the material by tensions summing, decreasing the tightness and variation in the effective dimensions of the piece. On the other hand, the absence of these elements makes impossible maintaining the oil film on the contact surface by normal oiling.

The parameters that influence the roughness are: the geometric parts of the tool (by the beam of the pin tool, tool clearance, rake angle, angle of inclination), the cutting edge micro geometric (by usage tool, the roughness of the back edge and front face), the cutting conditions parameters (speed, feed, depth of cut), the physics-mechanic properties of the material piece, cutting oil and the technological system hardness.

4. THE EXPERIMENTAL RESULT

We considered different initial roughness obtained with a lathe tool reinforced with ceramics plate, having radius angle 0, 8 mm, and different working regime for steel OLC45, according to STAS 791-80.

In order to establish an empirical model of the surface roughnesses obtained during the plastic deformation process the experiments were made at different working parameters, presented in table 1. The parameters are: [n.sub.p]-piece revolution, [f.sub.1]-longitudinal feed, [d.sub.b]-balls diameter, [R.sub.ai] [[micro]m]--initial surface roughness parameter. In figure 2 we indicate the profile of the initial roughness, and figure 3 (a and b) presents the final surface roughness surface after machining using different working conditions. The influence of the initial surface roughness on the final surface roughness parameter is shown in figure 4; one can observe the influence of the initial roughness; if the initial roughness (marked II in graph) is important after manufacture the roughness surface improvs as indicated by dependence (I).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The analysis of the experimental data on the surfaces roughness, obtained after the manufacturing process was realized using home-made computer software written in GWBASIC language.

Using an alghoritm based on the least square method (Cretu, Gh. 1992), from experimental result we have determined the following empirical relation, that describe in the most adequate way the studied model:

[R.sub.af] = 1,008391 + 0,005464[n.sub.p] - 0,14367f + 0,02795[d.sub.ball] = 0,2869167 (1)

[FIGURE 4 OMITTED]

In order to determine what element has the highest influence on the [R.sub.a] surfaces roughness parameters, we have established a function type power:

[R.sub.a] = 5,953731 - 17,93975/[n.sub.p] - 0,05896/f - 3,791355/ [d.sub.ball] - 13,15997/[R.sub.ain] s(5) = 0,1735373 (2)

From the empirical model obtained for the [R.sub.a] - roughness parameter, and studying the size of the exponents (2), it can be observed that the initial roughness of the workpiece [R.sub.ainit] [[micro]m has the most influence on the [R.sub.afinal] parameter followed by the [n.sub.p] - piece revolution [rot/min], [f.sub.1]-longitudinal feed [mm/s] and [d.sub.ball] - ball diameter [mm].

When the ball diameter and the speed of work piece are increased, the surface roughness value decreases. These experimental observations confirm our initial hypothesis.

5. CONCLUSION

In general, it is indicated to have an initial high roughness, obtained with a low cost procedure in order to obtain in the end a material with excelent roughness properties.

We could remark that taking into consideration the absolute sizes of the exponents belonging to different factors; we can put in order these factors on the base of their importance. Some simplifying hypothesises were accepted to develop theoretical models for the surface roughness parameter Rz which characterizes the finished surface. In the future, are planned to use autocorellation equation to verify the validity of the theoretical models.

6. REFERENCES

Cretu, Gh. (1992). Fundamentals of the experimental research. Handbook for the laboratory activities (in Romanian). Institutul Politehnic, Iasi

Draghici G. (1984), Manufacturing machines technologies (in Romanian), Editura Didactica si Pedagogica, Bucuresti

Ponomariov S.D. et al. (1964) Resistance calculation in machine manufacturing (in Romanian), Editura Tehnica, Bucuresti

Shiou F-J. & Chen C-H. (2003), Freeform surface finish of plastic injection mold by using ball-burnishing process, Journal of Materials Processing Technology, Vol. 140, pp. 248-254

Stepanenko-Aizenberg M.V. (2007), Model of high velocity penetration of an indenter into a medium. Journal of Minning Science, Vol. 43, pp. 254-272

Tab. 1. Input parameters values

 [n.sub.p] [R.sub.a] [R.sub.a]
Nr. [rot/ f [d.sub.ball] init final
Crt. min] [mm/s] [mm] [[micro]m] [[micro]m]

 1 20 0,05 6,78 7.73 1.77
 2 160 0,05 6,78 4.23 1,4
 3 20 0,224 6,78 4.94 1,8
 4 160 0,224 6,78 5.45 2,13
 5 20 0,05 10 5.46 1,0
 6 160 0,05 10 4.15 0,98
 7 20 0,224 10 3.83 1.02
 8 160 0,224 10 4.82 3,10