Design and Analysis of the Critical Components Of a Vertical Machining Centre

Abstract

The performance of a CNC machine tool solely depends on the functioning of its critical components, whose design thereby plays the crucial role in bringing out optimized performance. Thus the design and analysis of each and every critical component, such as the spindle, the ball screws, the slide ways, etc., needs to be performed and optimized to arrive at final design decisions. Vertical Machining Centre (VMC) counts as one of the prominent CNC machines in which milling operation is carried primarily. VMC innovations and developments have brought about quantum improvements in its salient features like improved flexibility and reliability, increased feeds, speeds and overall machining construction and rigidity, reduced loading, tool changing and other non-cutting time, greater Machine Control Unit (MCU) capability and compatibility with systems, reduced operator involvement and lesser noise with advanced safety features. The present paper deals with the methodology of the design and analysis of the most critical components of a Vertical Machining Center (VMC), i.e. the spindle, the ball screws and the guide ways. The methodology of the design of the spindle unit is discussed based on the loading conditions, the torque transmitted and the deflection produced. An algorithm is formulated for the design of the spindle for rigidity as well as strength taking into consideration the maximum force induced in it during milling operation from the inputs of the radial and axial depths of cut and the feed. The deflection of the spindle is restricted to permissible limits for a VMC while considering its rigidity. The output from the program for the dimensions of the spindle is taken for a particular VMC model having a power output of 3.7 KW and rated rotational speed of 400 revolutions per minute. The spindle designed from these dimensions is then preliminary analyzed in ANSYS first with a line model to optimize the dimensions. The solid model of the spindle is then created with these optimized dimensions and is then analyzed in ANSYS once again with the critical loading conditions to find out and analyze the stresses at critical locations and possibilities of failure. The deflection of the spindle is also minimized and kept within allowable limits to keep the accuracy unimpaired. An algorithm is also formulated for the selection of the ball screws for a VMC based on loading, rigidity, dynamic, maximum rotation and fatigue life considerations, from manufacturer’s catalogue and selection of the ball screws is done for the model being worked upon in this paper. A methodology for the design and selection of the guide ways for VMC from manufacturer’s catalogue based on the maximum loading conditions and the rigidity required is devised. Conclusions are drawn for comparison of the dimensions of the critical components of the VMC arrived at with the existing models.