Error Factors in Vertical Positioning System

Machine tools are improved capacity remarkably during the 20th century. Improving the precision of machine tools are related with precision of products and accurate processing is always associated with the subject of interest. There are a lot of the elements that determine the precision of the machine, as guides, motors, structure, control, etc. In this paper we focused on the phenomenon that vertical movement system has worse precision than horizontal movement system even they were made up with same components. The vertical movement system needs to be studied differently from the horizontal movement system to develop its precision. The vertical movement system has load on its transfer direction and it makes the movement system weak in precision than the horizontal one. Some machines have mechanical counter balance, hydraulic or pneumatic counter balance to compensate the weight of the machine head. And there is several type of compensating the weight. It can push the machine head and also can use chain or wire lope to transfer the compensating force from counter balance to machine head. According to the type of compensating, there could be error from friction, pressure error of hydraulic or pressure control error. Also according to what to use for transferring the compensating force, transfer error of compensating force could be occur.. Keywords—Chain chordal action, Counter balance, Setup error, Vertical positioning system.. I. VERTICAL MOVEMENT SYSTEM HE vertical movement system has load on its transfer direction than the horizontal one. The gravity force which effects on the horizontal movement system does not affect at thrust force directly, but load at ball screw of vertical movement system are different when it moves upper side and down side [1],[2]. Fig. 1 Horizontal and vertical movement system Hyun-Gwang Cho and Wan-Seok Yang are with the Department of Mechanical Engineering, Gyeongsang National University, 900 Gajwa, Jinju, 660-701, Korea (e-mail: [email protected]). Su-Jin Kim is with the Department of Mechanical Engineering, Gyeongsang National University, 900 Gajwa, Jinju, 660-701, Korea (e-mail: [email protected]). Jeong-Seok Oh and Chun-Hong Park are with the Korea Institute of Machinery & Materials, Daejeon, South Korea, 305-343 (e-mail: [email protected]) The counter balance to compensate the weight of vertical table is used in heavy machine tools. There are mechanical counter weight and hydraulic counter weight. The advantage of counter weight is reducing the weight and moment of vertical table and the disadvantage of it is the poor dynamic characteristic. And the chain connecting vertical table and counter weight causes counter force transmission error [3]. Vertical table is supported by guide such as slide guide, rolling guide and hydro static guide. The traction force moving vertical table up and down is made by ball screw or linear motor. If the table is too heavy to be sustained by motor, counter balance is used to compensate the weight of the table. The mechanical counter weight is indirectly connected by chain that transmits counter force [4]. II.FORCE AND MOMENT IN VERTICAL GUIDE Fig. 3 shows guide, ball screw, machine head, counter balance and force, moment that works on it. Machine tool has setup error of it and also has assembly errors in it. So the gravity force direction of vertical table is not always same to counter force direction. Fig. 2 Compensation system in vertical table Fig. 3 Force and moment in vertical guide Hyun-Gwang Cho, Wan-Seok Yang, Su-Jin Kim, Jeong-Seok Oh, Chun-Hong Park Error Factors in Vertical Positioning System T World Academy of Science, Engineering and Technology International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:5, No:11, 2011 2230 International Scholarly and Scientific Research & Innovation 5(11) 2011 scholar.waset.org/1999.8/15233 In te rn at io na l S ci en ce I nd ex , M ec ha ni ca l a nd M ec ha tr on ic s E ng in ee ri ng V ol :5 , N o: 11 , 2 01 1 w as et .o rg /P ub lic at io n/ 15 23 3 If we define the moving direction x and normal direction y and z, the gravity force and counter weight force vector has not only x value but also y and z values. The acting point of gravity force is at the center of weight of vertical table and acting point of counter weight is at the connected point of head. In ideal case the acting line of counter force is parallel with gravity force passing the center of vertical table. But in general two force vectors are not parallel which cause the moment force. The net force and moment acting the vertical table of machine tool are sustained by guide and ball screw.The net force along x axis is sustained by ball screw and y and z axis are sustained by guide. III. COUNTER BALANCE The table weight in vertical motion causes large axial load on ball screw. The counterweight is used to compensate the gravity force on vertical table of machine tools. Although vertical movement system is assembled with counterweight to increase their capacity and reliability, become less precise due to vibration [5],[6]. After the vertical table decelerates and stop at the desired point the remained vibration causes wavy marks on the work piece that reduce the surface quality. There are three kinds of counterweight: mechanical, hydraulic and pneumatic counterweight, which can be divided into two vibration models. A. Vibration model Counterweight is used to compensate the gravity force of vertical table. Although it is effective at static valance, the vibration is increased at dynamic case. Adding one more component to the machine structure means dynamic problem needs to be reconsidered. And vibration analysis is coming up. By simplifying machine center as Lin's work [1], vibration model can be carried out. Ball nut is considered as a source getting acceleration a(t) and connected to machine head m¬2. Machine head is connected to counterweight system m1 by chain or steel wire with stiffness k1 and damping c1. Connection between the spindle and ball nut have stiffness k2 and damping c2. Fig. 4 (a) shows the mechanical counterweight model and vibration equation is (2). Fig. 4 (b) presents the hydraulic or pneumatic counterweight model. Vibration equation in this case can be expressed as (2) with m1 << m2. B. Vibration simulation by Matlab Fig. 5 introduces a schematic model built in matlab simulink. Initial parameters of this study case are Table I. One complete move of machine head to the target is normally divided in 3 stages: acceleration, constant velocity and deceleration. Fig. 6 expresses velocity and acceleration through time during one move of the ball nut, meanwhile a(t) in (3) is input as: