| Summary: | 博士 === 國立中正大學 === 機械系 === 99 === A novel design of follower rest used in ball screw grinding machine, and as well as new double grinding wheel attrition has two plane of symmetry rail processing method are proposed.
Installed on the grinding wheel holder and moving along the ball screw, the follower rest is designed to be a vibration absorber with its driving point attached on the ball screw close to the grinding zone. It is used not only to suppress the ball screw vibration at the position close to the grinding zone but also to avoid the occurrence of chatter. It provides an auxiliary in counteracting the ball screw vibration during a grinding process besides the traditional steady rest fixed on the machine bed. The mechanisms in ball screw vibration reduction as well as in avoiding the chatter using the absorber are formulated and studied using the impedance method. Influences of design parameters of the absorber, i.e. the absorber mass and the absorber damping, on the chatter and the transient response of the ball screw are examined in detail. Results show that increasing the mass of the absorber is the most effective way with respect to vibration reduction at the grinding position as well as avoiding the chatter. On the other hand, the absorber damping has a negative effect both on the vibration reduction of ball screw and avoiding the chatter because it reduces the driving point impedance of absorber at resonance, which is contrary to the structural damping of the ball screw that has a positive effect in avoiding the chatter. Moreover the absorber damping has little influence on the frequency bandwidth in which the ball screw vibration is suppressed by the absorber.
Grinding is the final process of machining a rail. Conventionally, the rail’s surfaces are ground by a single-wheel grinder. The vibrations caused by the grinding process can greatly influence the final surface roughness and dimensional accuracy of the rail. This research investigates performance achieved by using two grinding wheels simultaneously and symmetrically on two opposite surfaces of a rail. In practical terms, the feed force from the two grinding wheels cannot be aligned perfectly, and the imbalance and/or imperfect roundness of the grinding wheels will certainly result in vibrations during the grinding process. This study applies an impedance method to determine rail vibration and the grinding instability, such as chatter caused by feed force misalignment and grinding wheel imbalance. When compared to conventional single-wheel grinding, the results indicate twin-wheel grinding reduces rail vibration, leading to low incidence of grinding chatter and better grinding performance. However, feed force misalignment between the two grinding wheels can lead to increased chatter, and both resonance and chatter may occur at lower grinding speeds as feed force misalignment increases. Results also show that feed force misalignment has a greater effect on rail vibration and chatter than imbalance asynchronization between the two grinding wheels.
Keywords: screw, Rail, grinding, Chatter, impedance, Merritt curve.
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