Effects of the friction coefficient on the minimum cutting thickness in micro cutting

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Abstract

In the ultra precision diamond cutting process, the rake angle of the tool is likely to become negative because the edge radius of tool is considerably large compared to the sub-micrometer depth of cut. The round edge of the tool might sometimes cause plowing results in a poor surface, or burnishing which results in a shiny surface depending on the depth of cut. This study deals with the relationship between the friction of a tool-workpiece and the minimum cutting thickness in micro cutting. Proposed is an ultra precision cutting model in which the tool edge radius and the friction coefficient are the principal factors determining the minimum cutting thickness with a continuous chip. According to the model, a smaller edge radius and a higher friction coefficient make the cutting depth thinner. The experimental results verify the proposed model and provide various supporting evidence.

Introduction

Ultra precision diamond cutting is an efficient manufacturing method of precision parts in various fields of the high-tech industry such as electrics, electronics, information and communication technology, biotechnology, precision machinery, and others. Since the accuracy of diamond cut parts is determined by the relative motion between the tool and the workpiece, an understanding of the physical phenomena of the micro cutting process is necessary. Especially, an investigation of the minimum cutting thickness is very important in order to achieve more accurate machining. In conventional cutting, the tool edge radius is of no concern because it is so small compared to the depth of cut of a few millimeters. In ultra precision cutting, the rake angle is always negative because just a portion of the tool edge is occupied. This might cause plowing and a poor surface, or sometimes burnishing and a shiny surface, depending on the depth of cut.

The minimum cutting thickness in precision machining has been studied in the different ways by several researchers. Basuray et al. defined the blunt tool as a tiny cylindrical surface and analyzed the transition point from pure plowing to cutting. An approximate analysis predicted the neutral point angle corresponding to the minimum cutting thickness to be 37.6° [1]. Ikawa et al. estimated the edge radius of a diamond tool using SEM and proved experimentally that a continuous chip could be generated at nanometer order depth of cut [2]. Lucca et al. reported that the effects of plowing, due to a large effective negative rake angle resulting from the tool edge radius, have become important in micro machining. Furthermore, the cutting thickness without a continuous chip was estimated from the observation of the cutting force in a study on energy dissipation in ultra precision machining [3]. Yuan et al. presented a simple analytical expression for the minimum cutting thickness derived from relationships between tool sharpness, cutting force and friction coefficient [4].

This study focus on the relationship between the friction of a tool-workpiece and the minimum cutting thickness for the purpose of producing higher quality machined parts. In this study, a micro cutting model is proposed, which is that the tool edge radius and the friction coefficient between a tool and a workpiece govern the minimum cutting thickness which can produce a continuous chip. The theoretical model indicates that a smaller edge radius and a higher friction coefficient result in a thinner cut. The characteristics of a cutting near the minimum cutting thickness are investigated from the viewpoints of cutting force, resultant force direction, surface roughness, micro hardness, and chip shape.

Section snippets

Modeling of minimum cutting thickness

In ultra precision diamond cutting, the minimum cutting thickness depends on the tool edge radius and the physical relationship between a tool and a workpiece. Fig. 1 shows the material behavior of a sub-micrometers precision diamond cutting. In the case of a relatively small cutting depth compared to the tool edge radius, some material may be deformed, uncut, underneath the tool. This is called plowing, and the force associated with this is defined as the plowing force. This force is

Experimental equipment

Fig. 8 shows a photograph of the shaping machine used in this study. The machine is composed of three translational axes, X, Y, and Z. The X-axis is guided by an air bearing with no friction and driven by a steel band whereas the Y and Z axes are actuated by ballscrews. The resolution of the Z axis having a ball reducer is 0.02 μm. The shaping machine has a semi-closed loop control system that controls the position as well as the federate with encoder signals.

Square-type mono crystal diamond

Friction coefficient

A friction test was conducted to measure the friction coefficient, which is calculated as the ratio of the tangential force (FT) and the normal force (FN), between the workpiece and a diamond tool. Fig. 10 shows the results of the friction test. The friction coefficient between a diamond tool and materials such as Al, Brass, and OFHC (Oxygen Free High Conductive copper) used in this study, are measured to 0.3, 0.2, and 0.4, respectively. Therefore, the minimum cutting thickness can be predicted

Conclusions

In ultra precision diamond cutting, the minimum cutting thickness was investigated theoretically as well as experimentally. The conclusions are as follows.

  • (1)

    The minimum cutting thickness was determined by the tool edge radius and the friction coefficient of a workpiece-tool.

  • (2)

    From the theoretical model, the minimum cutting thicknesses were 0.09–0.12 μm for the three materials, and those were almost equivalent to the experimental results.

  • (3)

    When cutting at the minimum cutting thickness, a continuous

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