Recently activities for shortening the development time of machine tools and demands for high-value products are consistently being increased. The machine tool manufacturers have been applying advanced analysis techniques to shorten development time. As a result, many studies for predicting the performance in design stage have been conducted to solve these issues. So far, most manufacturers of machine tools have verified designs excluding the cutting process, even though the main performance is the machining quality of the workpiece. Also, conventional development process takes a lot of time for design changes to solve ma-chining problems, because physical prototypes are evaluated through cutting tests. Therefore, the evaluations under the virtual environment reflecting actual dynamic behaviors are needed for improving design quality and shortening development time, and it has been required to consider together the cutting process model for analyzing the machining quality before the manufacturing of the prototype. First of all, the configuration of machine tools such as axis arrangements and feed drive mechanisms must be evaluated completely in design stage, because it has decisive effects on the machine performances, which are productivity and machining quality. Especially, in high feed milling, inertia forces in addition to cutting forces, friction, backlash, and ser-vo response depending on drive mechanisms become the main causes of dynamic motion errors. Therefore, it is necessary to predict and analyze how the dynamic motion errors due to configurations are transferred onto machined surfaces in design stage.
This paper presents an integrated dynamic model considering the all interaction of the machine structure, the control system, and the cutting process. The modal truncation technique using the steady-state dynamic analysis in a selected location and frequency range of interest is applied to increase the efficiency of mecha-tronics simulations including the cutting process, which is described as the flat end-milling and ball end-milling. The interaction between and the tool and the workpiece results in the determination of undeformed chip thickness, and changes in the undeformed chip thickness are directly transferred onto the machined sur-face. Thus, the prediction of the machining quality depends on the extent to which the undeformed chip thickness is defined. The total undeformed chip thickness is introduced by using the kinematics of milling, the tool run-out, the vibration of the tooling and structural system as well as the vibration and tracking error of the feed drive system simultaneously. We considered two different machine configurations, which are the C-frame type machine with the ballscrew drive system and the gantry type machine with the linear motor drive system, and selected two configurations having different machine characteristics in terms of speed and accu-racy. Evaluation of surface quality due to machine configuration such as axis arrangements and feed drive mechanism is carried out by using the proposed model.
Also, the surface generation model based on the simulated dynamic errors such as vibrations of tool and workpiece and tracking errors by servo bandwidth, friction, and backlash was introduced to predict and ana-lyze the surface profiles and form accuracies according to machining conditions and machine configurations. The Fast Fourier Transform (FFT) method was utilized to clarify the cause of dynamic errors through the simulated and measured surface profiles in various cutting conditions. In particular, this proposed prediction model allows accurate design verifications for high-speed machines because it reflects the influences of the high inertia forces and the contour errors caused by high feed rate. Thus, high feed peripheral milling and sculptured milling applications were demonstrated in order to show the feasibility of the proposed prediction model, and we showed the superiority of prediction by comparing the simulated and measured results in high feed milling.
Finally, it was confirmed that the proposed method could be applied effectively for identifying the causes of dynamic errors on the machined surface depending on machine configurations, control parameters, and cutting process parameters without cutting tests, and the developed process actually makes it possible to ap-ply an inverse analysis, which is to analyze the simulated finished surface at each step of the shape genera-tion process, for solving the various machining problems. This approach is completely general, could be ap-plied to the New Product Development (NPD) process for machine tools.
공작 기계의 다기능화가 요구됨에 따라 복잡한 기계 구성의 high-end 공작 기계 개발이 증가 추세에 있으며, 사용 조건이 확대 됨에 따라 다양한 하중 조건과 경계 조건을 고려한 공작 기계의 설계가 요구되고 있다. 또한, 복잡해지는 요구 조건에 반해, 적기에 제품 개발을 통한 시장 선점이 무엇보다 중요시 되고 있다. 최근 들어 공작 기계 제조사들은 제품 개발에 소요되는 시간과 비용을 절감하기 위해 선진화된 해석 기술을 도입하여 설계 단계에서 사전에 문제를 점검하고 해결하려는 프런트 로딩(front loading)에 많은 노력을 기울이고 있다. 제품의 개념 설계 단계에서 구조 형상을 결정한 후 주요 구성품들의 시방을 결정하며, 상세 해석을 통해 이들 도면화 시키는 과정에 이르기까지 시뮬레이션을 통한 예측 설계가 설계 품질의 향상을 가져 왔다. 그러나, 공작 기계의 성능은 가공 대상물의 가공 품질로 평가 되는데 반해, 지금까지 단순한 가공 정보만을 활용하거나 가공 공정을 배제한 채, 정확한 부하를 고려하지 않은 해석적 평가가 이루어져 왔다. 공작 기계는 다른 기계 시스템과 달리 기계 구조, 제어 시스템과 함께 가공 공정이 서로 상호 작용을 하면서 동적 거동을 하기 때문에 이를 통합하여 고려할 필요가 있다.
본 연구는 공작 기계의 기계 구조, 제어 시스템, 그리고 가공 공정의 상호 작용을 모두 고려하는 통합 동적 예측 모델을 제시하고자 한다. 가공 공정을 고려한 통합 동적 예측 모델은 가상 환경에서 요구되는 가공을 수행하게 되며, 가공 표면 생성 모델을 이용하여 가공 중 발생되는 다양한 공작 기계의 동적 오차들이 가공 표면에 미치는 영향들을 예측 가능하게 한다. 제안된 통합 동적 예측 모델은 수직형 머시닝 센터의 대표 구조인 볼스크류 구동 메커니즘의 C-frame형 공작 기계와 리니어 모터 구동의 gantry형 공작 기계의 구조를 대상으로 적용하였으며, 장비 구조 및 가공 공정에 따른 가공 특성을 예측하고 이를 분석하여 제안한 설계 방법론이 설계 단계에서 장비의 성능을 평가하고 분석할 수 있는 중요한 수단이 될 수 있음을 검증하였다.
