Jeong Hoon Ko

Development of a virtual machining system, part 1: approximation of the size effect for cutting force prediction

Abstract In this three-part paper, components of a virtual machining system for evaluating and optimizing cutting performance in 2 1 2 -axis NC machining are presented. Part 1 describes a new method of calculating cutting-condition-independent coefficient and its application to the prediction of cutting forces over a wide range of cutting conditions. The prediction of the surface form error and transient cutting simulations, described in Parts 2 and 3, respectively, can be effectively performed based on the cutting force model with the improved size effect model that is presented in Part 1. The relationship between the instantaneous uncut chip thickness and the cutting coefficients is calculated by following the movement of the center position of the cutter, which varies with nominal feed, cutter deflection and runout. The salient feature of the presented method is that it determines the cutting-condition-independent coefficients using experimental data processed for one cutting condition. The direct application of instantaneous cutting coefficient with size effects provides more accurate predictions of the cutting forces. A systematic comparison of the predicted and measured cutting forces over a wide range of cutting conditions confirms the validity of the proposed mechanistic cutting force and size effect models.

3D Ball-End Milling Force Model Using Instantaneous Cutting Force Coefficients

Application of a ball-end milling process model to a CAD/CAM or CAPP system requires a generalized methodology to determine the cutting force coefficients for different cutting conditions. In this paper, we propose a mechanistic cutting force model for 3D ball-end milling using instantaneous cutting force coefficients that are independent of the cutting conditions. The uncut chip thickness model for three-dimensional machining considers cutter deflection and runout. An in-depth analysis of the characteristics of these cutting force coefficients, which can be determined from only a few test cuts, is provided. For more accurate cutting force predictions, the size effect is also modeled using the cutter edge length of the ball-end mill and is incorporated into the cutting force model. This method of estimating the 3D ball-end milling force coefficients has been tested experimentally for various cutting conditions.

Off-line feed rate scheduling using virtual CNC based on an evaluation of cutting performance

Abstract This paper presents an advanced technology to automatically determine optimum feed rates for 2 1 2 axis CNC machining, without requiring the expertise of a machinist or the information contained in a machining data handbook. Present CAM technology does not consider important physical properties such as cutting forces and machined surface errors. However, the virtual machining system developed in this study can simulate real machining for a given set of NC codes. An analytical model for off-line feed rate scheduling is formulated to improve productivity and machining accuracy. Using this model, it is possible to regulate the cutting force, which drastically improves the overall form accuracy of the machined surface.

Development of a virtual machining system, part 2: prediction and analysis of a machined surface error

In part 2 of this three-part paper, a newly developed method that predicts the three-dimensional machined surface errors generated during the peripheral end milling process is presented. From the cutting force prediction system of Part 1, since the uncut chip thickness is calculated by tracing the movement of the cutter, the positions at which the cutting edges pass over the workpiece surface can readily be obtained. In this part of the paper these positions are used to construct surface error maps. In addition, by using the estimated locations of the peak and valley values of the cutting force component normal to the machined surface, a quantitative analysis of the machined surface error is given and followed by theoretical explanations. A series of machining tests on aluminum workpieces were conducted to validate the effectiveness of the model. The predicted cutting forces and surface errors were found in good agreement with their measured counterparts.

Chatter stability of plunge milling

Plunge milling operations are used to remove excess material in boring cylinders, roughing pockets, dies and mold cavities. This paper presents a frequency domain, chatter stability prediction theory for plunge milling. The regenerative chip thickness is modeled as a function of lateral, axial and torsional vibrations. The stability of the plunge milling is formulated as a fourth order eigenvalue problem by relating the regenerative chip thickness, cutting forces and torque, and the structural modes of the cutter. The stability lobes are predicted analytically from the eigenvalue solution. The stability lobes are experimentally proven by conducting over one hundred plunge milling tests.

Development of a virtual machining system, part 3: cutting process simulation in transient cuts

Abstract In Parts 1 and 2 of this three-part paper, a mechanistic cutting force model was developed and machined surface errors for steady cuts under fixed cutting conditions were predicted. The virtual machining system aims to simulate and analyze the machining and the machined states in a general flat end-milling process. This frequently involves transient as well as steady cuts. Therefore, a method for simulating the cutting process of transient cuts needs to be developed to realize the virtual machining system concept. For this purpose, this paper presents a moving edge-node (ME) Z-map model for the cutting configuration calculation. The simulation results of four representative transient cuts in two-dimensional pocket milling and an application of off-line feed-rate scheduling are also given. In transient cuts, the cutting configurations that are used to predict the cutting force vary during the machining operation. The cutting force model (Part 1) and surface error prediction method (Part 2) were developed for steady cuts; these are extended to transient situations using the ME Z-map model to calculate the varying cutting configurations efficiently. The cutting force and surface errors are then predicted. To validate the feasibility of the proposed scheme, the measured and predicted cutting forces for transient test cuts were compared. The predicted surface error maps for transient cuts were constructed using a computer simulation. Also, off-line feed-rate scheduling is shown to be more accurately performed by applying the instantaneous cutting coefficients that were defined in Part I.

Dynamics and Stability of Plunge Milling Operations

Plunge milling operations are used to remove excess material rapidly in roughing operations. The cutter is fed in the direction of the spindle axis which has the highest structural rigidity. This paper presents a comprehensive model of plunge milling process by considering rigid body motion of the cutter, and three translational and torsional vibrations of the structure. The time domain simulation model allows prediction of cutting forces, torque, and vibrations while considering tool setting errors and time varying process parameters. The stability law is formulated as a four-dimensional eigenvalue problem, and the stability lobes are predicted directly with analytical solution in frequency domain. Time domain prediction of cutting forces and vibrations, as well as the frequency domain and chatter stability solution are verified with a series of plunge milling experiments.

A Feasibility Study on Laser-Mechanical Drilling of SiC Ceramics

Structural SiC (α-type) is believed to be widely applied in hostile environments such as high-temperature, high-corrosive applications in the semiconductor industries due to its superior thermo-physical and mechanical properties. However, the extremely high hardness and brittleness of SiC makes hole drilling difficult by the conventional mechanical drilling (CMD) technique. Laser can be used to drill SiC; but the resultant holes are often tapered and uneven, with tendency for microcracks and thermal damage to occur at the hole entry due to the high thermal shock from the laser. This paper reports on the experimental results of a sequential laser-mechanical drilling (LMD) technique for drilling α-SiC. At first, an Nd:YAG laser was used to drill a series of pilot holes on a 3 mm thick SiC plate. Then a diamond-coated carbide drill was sequentially applied to these holes to obtain desired hole diameter of 0.5 mm. A number of through holes on SiC (aspect ratio: 6) were successfully obtained using this approach. The quality of the drilled holes were assessed in terms of the entrance and exit sizes and conditions, hole taper angle, hole edge shapes, and microcracks. Finally, comparisons of the LMD performances were also made against the holes predrilled by the laser itself and holes of the similar size drilled separately with the CMD technique. The experiment results show that the proposed drilling approach can effectively drill α-SiC ceramics.

The Effect of One Directional Ultrasonic Vibration Assistance in High Speed Meso-Scale Milling Process

One-directional ultrasonic vibration assisted milling system is designed and its performance is investigated in terms of machined surface quality under 135,000 rpm. The ultrasonic vibration generator excites the workpiece with a frequency around 40 kHz and amplitude of a few micro meters. The milling tool’s cutting speed is controlled by an air-bearing spindle system. Both feed-directional and cross-feed directional ultrasonic vibration assistance are considered in order to understand the mechanism of ultrasonic vibration assistance for surface roughness generation. A comparison is done on a milled surface which is generated with and without ultrasonic vibration assistance. The experimental results show that ultrasonic vibration assistance can improve the machined surface quality which depended on the cutting edge radius and the feed per tooth.

Application of ultrasonic vibration assistance into tooling axis direction in meso-scale milling

So far, the industrial application of ultrasonic vibration assistance has been successful in continuous machining process such as turning process where ultrasonic vibration velocity is much higher than cutting velocity. Recently, the application of ultrasonic vibration assistance in milling process has been experimentally investigated for feed and cross-feed directions. This paper focuses on the effect of ultrasonic vibration assistance in spindle axial direction for improvement of machined surface. With the designed ultrasonic vibration assisted milling process with 39.7 kHz and a few micro-metre amplitudes, workpiece vibrates along spindle axial direction while different RPMs and feed rates are applied. The axial directional vibration assistance acts as additional cutting motion which further reduces the leftover surface error. Experimental results validate that surface roughness can be improved from 20% to 65% for the tested conditions. The simulation results have been proposed to validate the surface ...