A process-design approach to error compensation in the end milling of pockets

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Abstract

In the end milling of pockets, the tool deflection varies over the entire machining cycle, which includes straight segments as well as corners. The accuracy in corner cutting is strongly influenced by the deflection of the end mill caused by the variation of the cutting forces. The way to improve accuracy in corner cutting is by decreasing the radial depths of cut to reduce the cutting forces and thereby the end mill deflection errors. By process design it is possible to achieve gradual reduction in the radial widths of the cut during corner cutting. Thus, there is a need to identify the cutting conditions in order to control the process such that tool deflection errors are minimized and compensated for. The paper presents a process design methodology to achieve a significant reduction in the cutting forces and consequently in the tool deflection errors. The experimental results of cutting force measurement and the estimation of tool deflection errors are discussed in detail. The results indicate that significant improvement in the accuracy of end milled pockets can be obtained by error compensation.

Introduction

End milling is a common machining process for the manufacture of dies and moulds, as well as high precision aerospace components. The milling process causes machining errors due to the tool deflection, machine tool geometric errors, thermal effects, tool wear, etc. Amongst the numerous error sources, tool deflection is the most dominant factor causing errors. Whilst it is difficult to eliminate all of the machining process errors, estimating and compensating the tool deflection errors is important to improve the accuracy of the machined parts. Hence, the prediction of errors due to tool deflection with the cutting forces modeling in the end milling process has been the subject of many investigations [1], [2], [3], [4], [5], [6].

The error sources attributed to the machining process are closely related to the workpiece, the tool or the interaction between the two, and include errors due to tool deflection, workpiece deflection, tool parameters (including material, geometry, wear), cutting parameters (including speed, feed, depth of cut, coolant), chucking and fixturing (including workpiece location and clamping errors), and materials stability, including residual stresses due to cutting. Dimensional accuracy is mainly a matter of preventing errors of longitudinal and circumferential form [7]. Errors of longitudinal form result from static deflection of the spindle and the workpiece under the cutting forces and thermally induced stresses in the machine. Errors of circumferential form result from run-out of the spindle and from vibration of the tool or workpiece.

The instantaneous deflection is dependent upon the static stiffness of the cutting tool and the instantaneous cutting force [8]. In the end-milling process, which is carried out with slender helical cutters, the cutting forces vary periodically. The slender helical end mill experiences both static and dynamic deformations, which manifest as dimensional and surface-finish errors on the machined part. The tool deflection is believed to be closely related to the cutting tool stiffness.

For milling a complex shape or a simple rectangular pocket, the accuracy in corner cutting is mainly influenced by the deflection of the end mill caused by variation in the cutting forces [9], [10]. This must be due to the variable radial depth of cut in corner cutting between the straight part and the corner resulting in variable accuracy. The way to improve accuracy in corner cutting is by decreasing the depths of cut to reduce the cutting forces and, consequently, the end mill deflection errors. This can be achieved by performing a roughing cut to reduce the material to be cut at the corners, followed by a finishing cut. This method makes the radial widths of cut during corner cutting smaller and eliminates sudden increase of radial widths. The results of Iwabe et al. [11] also point out that the radial depths of the cut and the chip areas increase rapidly at the inside corners causing rapid increase in the cutting forces, which result in deterioration of the accuracy and tool wear.

It is evident that appropriate process design is the key to achieve machining accuracy by making a gradual reduction in the radial widths of the cut during the corner cutting of pockets in end milling. It is important to distinguish the requirements of process design for milling conditions in straight cutting and corner cutting in pocket milling applications in order to control the process conditions such that tool deflection errors are minimized and compensated for. The paper presents a methodology to predict the cutting forces and the tool deflection errors. The experimental results of cutting force measurement and the estimation of tool deflection errors are discussed in detail. The results indicate that process design has a significant influence on the accuracy of end milled pockets.

Section snippets

Cutting forces

Estimation of cutting forces is essential for process monitoring and control. The cutting forces vary with tool geometry and machining conditions. There are other fundamental factors affecting the cutting forces such as the material hardness, friction and stresses at the tool–chip interface. Without the consideration of cutter geometry, frictional conditions, and the workpiece material strength characteristics and tangential stresses on the tool–chip interface, the data on cutting forces would

Cutting modes

Different cutting modes that are commonly used in pocket milling are illustrated in Fig. 6. Slot cutting can be defined as the milling process performed by the cutter with the radial width of cut equal to its diameter, which means that the cutter is vertically immersed into the workpiece. Immersion milling is normally used in the finishing process, in which the cutter is radially immersed into the workpiece material and the cutting process mainly occurs at the periphery. The cutting involves

Conclusions

The end milling force model based on input machining data (tool geometry, cutting conditions, workpiece material properties) has proven to be applicable for generating accurate and reliable cutting force data. However, more results are required to establish the validity of the model for a wide range of cutting parameters and a variety of materials.

Experiments with different cutting modes show that the deflection errors vary considerably with the type of cutting mode, particularly in corner

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1

Visiting Professor, 1995–1997 (on leave from Kiev Polytechnic Institute, Ukraine).

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