Theoretical and experimental researches on 3-axis ball nose end milling.

Cosma, Marius ; Daraba, Dinu ; Alexandrescu, Ioan Marius 等

1. INTRODUCTION

In automobile industry there are many small pieces (clamping devices, gripers etc.), which have a generally simple geometry but at least one small surface which is sculptured and must follow the lines of the vehicle components design.

In the manufacturing process of these parts, the most commonly used are 3-axis CNC milling centers and sculptured surfaces can have different orientations in relationship with tool axis.The influence of the surface orientation (inclination) however was not considered in the machined strategy, starting with tool path program in CAM software, which can allow the management of various modes of tool paths generation, but cannot decide which one is the best.

The objective of these theoretical and experimental researches is to develop a methodology of machining directions according to the surface inclination.

In 3-axis ball nose end milling, the big problem is to avoid cutting at tool tip that is advancing in linear motion at a zero cutting speed. The first condition in this way is to provide a minimal inclination angle for the workpiece surface. The tool path orientations and relative position between tool axis and normal to the machined surface have a big influence on the cutting process, surface quality and chatter stability of the dynamic system (Cosma, 2007) (Pay & Cosma, 2007).

Considering these issues, it is necessary to study the influence of tool path orientations and relative position and optimize the cutting process.

There is a wide range of published information on cutting tools and related data (cutting speeds, feed rates, depths of cut, etc.), however, relatively little information has been published on the evaluation of tool paths and cutter entrance in uncut chip for this application.

In the paper (Iwabe et al., 2004), the FEM model of a ball end mill is made out and the cutter deflection triggered by cutting force is calculated using the model and cutting area calculated by 3D-CAD only for vertical position of tool (which is not proper for cutting in reality).

Other studies (Terai et. al., 2004)--worked out the thickness of the undeformed chip and the influence of that tool orientation on the ball nose end mill, without considering the impact of cutter path selection, with proper account of the machining parameters, such as cutting forces, vibration analysis and workpiece surface quality.

2. THEORETICAL RESEARCHES

2.1 Geometrical model and tool path definitions

The geometrical method used in this study is available if boundary surfaces are generated, first by simplifying the motion of the cutting edge, only in the revolution of the tool, when the reference point of the cutting edge moves along a closed circle trail (in reality it is a looped orthocycloidal track), secondly, the surface machined by the preceding path is constructed by the surface of sphere, and third, the initial surface can be considered to be flat for a very small area. As a result of these pre-conditions it is easy to determine the boundary surfaces [Cosma, 2006]: initial surface--plane; first revolution--sphere; second revolution--sphere; surface machined by preceding path--circular cylinder.

The 3D-CAD study of uncut chip, cross sections and cutting area is made using the scheme from figure 1. On inclined surface, the tool path orientations are determined by feed and according as step-over directions, can be (Fig. 2): 1--vertical upward (V. U.), 2--horizontal upward (H. U.), 3--vertical downward (V. D.), 4--horizontal downward (H. D.).

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2.2 Theoretical study results

Theoretical study was made for 150, 300, 450 and 600 workpiece inclinations. The uncut chips projections and cross sections are presented in figure 3, where it is clearly shown that the best cutting edge entrance (to high thickness and minimum area) is for H.U. 450 to 600, low conditions for H.U. 15[degrees] (high thickness and big area) and in generally for H.D. (for 15[degrees] to 30[degrees] the tool tip is still in contact with cutting surface--the black area--and the cutting edge entrance is at the point of minimum chip thickness and sizeable length of contact between cutting edge and workpiece surface).

3. EXPERIMENTAL RESEARCHES

3.1 Tooling, equipment and cutting conditions employed

A new indexable solid carbide ball nose end head was used in this experiment, type SECO TOOLS Minimaster B120P with coating code F30M, with 2-flute, 4 mm radius, helix angle and radial rake angle of 0[degrees]. The cutting experiments were carried out in down milling on a workpiece (steel OL 37 type, STAS 500/1;2-80) with four little surfaces for different tool path orientations (Fig. 2), for each angle inclination [theta] on values 15[degrees], 30[degrees], 45[degrees] and 60[degrees]. The cutting tests were performed on a vertical CNC 3-axis milling machine Microcut Challenger 2412 with a continuous variable speed up to 10 000 rpm and a maximum spindle power of 25 kW. Cutting speed used was constant for all experiments 60 m/min. The axial and radial depth of cut used was 0.8 mm and feed per tooth 0.1 mm/tooth.

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3.2 Experimental results

The surface quality, as a result of ball nose end milling is determined by topography and can be evaluated using the arithmetical mean deviation of the profile Ra, measured in feed direction for a sampling length of 4.0 mm and cut-off length of 0.8 mm, represented in figure 4.

Chart from figure 4 shows significant differences in surface quality depending on the tool path orientations and tool tilting surface.

These influences can be explained, by differences in input of cutting edge, such as HU to large chip thickness and contact length at the minimum value and for HD input at minimum chip thickness and maximum length of contact.

The different aspects of surface topography (only for 45[degrees]) for tool path orientations, in HD and HU are shown in figure 5.

4. CONCLUSION

The geometric model for ball nose end milling is acknowledged by experimental results and can be evaluated for different cutting parameters. Use always vertical upward tool path orientations for inclined surface around 15[degrees] and horizontal upward for 45[degrees]. Avoid using horizontal upward in case of surface inclinations around a 15[degrees] angle, horizontal downward for 30[degrees] and vertical downward for 45[degrees]. It is clear that for 45[degrees] in horizontal upward are better cutting conditions that in horizontal downward. Generally, the vertical downward renders the most difficult cutting conditions.

Future research should be extended to other materials and in 5-axis milling

5. REFERENCES

Cosma, M. (2006). Geometric Method of Undeformed Chip Study in Ball Nose End Milling, 6th INTERNATIONAL MULTIDISCIPLINARY CONFERENCE, North University of Baia Mare, Scientific Bulletin Series C, Vol. XXI, May, 2006, Romania, pp. 49-54, ISSN-1224-3264

Cosma, M. (2007). Horizontal Path Strategy for 3D-CAD Analysis of Chip Area in 3-Axes Ball Nose End Milling, 7th INTERNATIONAL MULTIDISCIPLINARY

CONFERENCE, North University of Baia Mare, Scientific Bulletin Series C, Vol. XXI, May, 2007, Romania, pp. 115-120, ISSN-1224-3264

Iwabe, H.; Natorri, S.; Masuda, M., & Miyaguchi, T. (2004).

Analysis of Surface Generating Mechanism of Ball End Mill Based on Deflection by FEM, JSME International Journal, Serie C, Vol. 47, No. 1, 2004, pp. 8-13, ISSN 1344-7653

Pay, E. & Cosma, M. (2007). Vertical Path Strategy for 3D CAD Analysis of Chip Area in 3-Axes Ball Nose End Milling, 7th INTERNATIONAL MULTIDISCIPLINARY CONFERENCE, North University of Baia Mare, Scientific Bulletin Series C, Vol. XXI, May, 2007, Romania, pp. 585-590, ISSN-1224-3264

Terai, H.; Hao, M.; Kikkawa, K., & Mizugaki, Y. (2004). Geometric Analysis of Undeformed chip Thickness in Ball-Nosed end Milling, JSME International Journal, Serie C, Vol. 47, No. 1, 2004, pp. 2-7, ISSN-1344-7653