Research On Servo-controlled Bar Cropping

Multi-station cold forming machine is an equipment specially used to produce fasteners like bolts and nuts in mass production.The first process is bar cropping,which obtains the billet in ready for next extruding processes.So the cutting surface quality of the billet has great influence on forming process planning,product accuracy and tool life.Two materials 20 Cr and SCM435 were studied,and the bar cropping experiments were carried out using H1F200 servo press.Full factorial experimental scheme that one parameter(length-diameter ratio)changed from 0.5:1,1:1 to 2:1 and the other parameter(cropping speed)changed from 200mm/s,400mm/s,600mm/s to 800mm/s was designed.In order to evaluate the cutting surface quality quantitatively,a cutting surface quality evaluation system consisting of ovality,surface flatness and angle was used.Experiments were conducted using 20 Cr and SCM435.By analyzing the cutting surfaces,following conclusions could be drawn: as the cropping speed increases,cutting surface quality also gets better and the improvement is mainly reflected in the fact that ovality becomes smaller and surface flatness becomes better.However,the angle variation displays no obvious rule.A novel bar cropping process called pre-shearing cropping process was proposed in this paper.The bar would be pre-sheared at a low cropping speed and was then cropped at a high cropping speed which made the billet crack along the pre-cropping direction.Under the conditions that bar diameter was 15 mm,pre-shearing speed was 10mm/s,high cropping speed was 600mm/s,pre-shearing values changed from 1mm,2mm,3mm to 4mm,experiments with 20 Cr and SCM435 materials were conducted.By analyzing the cutting surfaces,following conclusions could be drawn: when the pre-shearing value was 3mm which equaled to 20% of the diameter,cutting surface with best surface flatness and least angle can be obtained.Pre-shearing cropping process of 20% of bar diameter pre-shearing value was compared with conventional cropping process at 600mm/s cropping speed,it is found that as for 20 Cr material,pre-shearing cropping process can improve surface flatness and as for SCM435 material,pre-shearing cropping process can reduce angle.Gathering the force-time and the displacement-time data of these two types of processes,results show that the pre-shearing cropping process can reduce the biggest cropping force to some extent.