Abstract When machining the pocket of a mould in high-speed milling mode, the tool load at a pocket’s narrow area or corner may sharply increase because of the presence of a higher amount of material to be cut. A trochoidal machining method considering milling force, machining tool, and pocket geometry is proposed in this paper. First, a method for the geometric modelling of the engagement angle in trochoidal machining is proposed. Maximum and mean values of the milling force are analysed; meanwhile, the corresponding relationship between the milling force curve and the engagement angle curve during the trochoidal machining process is analysed. Based on fundamental experiments on trochoidal machining, results for the milling force and tool wear are obtained; then, a proper control strategy for cavity trochoidal milling machining is proposed. Based on this control strategy for trochoidal milling machining, two realizations of cavity trochoidal milling machining are proposed. Finally, comparison experiments on cavity machining are conducted. Compared with the feedrate adjustment method, trochoidal machining provides better control over the milling force and tool wear at corners and narrow slots. The milling force and machining vibrations are smaller, and the tool wear is substantially reduced.70710

Keywords: high-speed milling; trochoidal machining; milling force

1. Introduction

High-speed milling provides various benefits in terms of improved production efficiency, machining precision, surface quality, etc. It has been successfully applied in the mould industry and promotes the rapid development thereof. Contour-parallel tool paths are a common high-speed milling method for cavity moulds, where the tool path is generally calculated based on the contour offset and intersection. However, corners and narrow areas such as slots may easily appear between contours. If no special treatments are applied, the following issues can frequently arise in high-speed machining: (1) The engagement angle or engagement arc length between the tool and uncut materials can be greatly increased, resulting in a sharp increase in the contact material. (2) The tool load amount can be much higher at a corner or slot, resulting in a greater fatigue or damage to the tool. These problems are particularly serious in the high-speed milling of harder materials.

The engagement angle and cutting load variations have generated concern by scholars. Various studies have been conducted, including the analysis and modelling of the engagement angle, milling force, and other aspects. Kline et al. (1982) presented a mechanistic model for the force system in end milling and stated that varying the cutting engagement and chip thickness could cause variations in the cutter load. Choy and Chan (2003) reported that the instantaneous cutter sweep angle (CSA) is a suitable parameter for studying chip load in 2.5D pocket milling and proposed a comprehensive modeling method; experiments showed that the model can accurately predict a cutter load pattern at cornering cut. Wei et al. (2010) proposed an effective milling force model for pocket machining and pointed out that the varying feed direction and cutter

engagement could affect the milling force for the entire process. These studies show that the close relationship between tool contact and load changes during machining process. This relationship is used as the basis to establish methods for reducing cutting load, such as the adjustment of the cutting parameters or trajectory.

Two major approaches, namely, the adaptive control and geometry modification of the tool path, to addressing the cutting load variation problem can be identified. The adaptive control approach focuses on controlling the cutting performance by instantaneously adjusting the cutting parameters when milling the work pieces. Spence and Altintas (1994) developed a simulation system based on constructive solid geometry (CSG) and concluded that the feed rate can be automatically scheduled to satisfy force, torque, and part dimensional error constraints. Tarng and Shyur (1993) argued that cutting stability is greatly dependent on the radial depth of cut and proposed a method for identifying the radial depth of cut in pocket machining; they concluded that the feedrate can be adjusted accordingly to maintain a constant material removal rate. Bae et al. (2003) assumed that the cutting force is a function of two major independent variables, namely, 2D chip-load and the feedrate; a simplified cutting model was proposed to adjust feedrate for pocket machining. Liu et al. (2015) presented a feedrate optimization strategy with multiple constrains including relative chip volume, milling force, and cutter deflection. The experimental results demonstrated that the optimized feedrate can satisfy the requirements for a pocket milling process. The