模具专业英语Bending.ppt

Lesson 10 Bending DiesBending is the uniform straining of material,usually flat sheet or strip metal,around a straight axis which lies in the neutral plane and normal to the lengthwise direction of the sheet or strip.Metal flow takes place within the plastic range of the metal,so that the bend retains a permanent set after removal of the applied stress.The inner surface of a bend is in compression;the outer surface is in tension A pure bending action does not reproduce the exact shape of the punch and die in the metal;such a reproduction is one of forming.The neutral axis is the plane area in bent metal where all strains are zero.Bend Radii.Minimum bend radii vary for different metals;generally,different annealed metals can be bent to a radius equal to the thickness of the metal without cracking or weakening Bend Allowances.Since bent metal is longer after bending,its increased length.generally of concern to the product designer,may also have to be considered by the die designer if the length tolerance of the bent part is critical Bending Methods.Two bending methods are commonly madeuse of in press tools.Metal sheet or strip,supported by a V block(Figure 10-la),is forced by a wedge-shaped punch into the block.This method,termed V bending,produces a bend having an included angle which may be acute,obtuse,or of 90 Friction between a spring-loaded knurled pin in the vee of a die and the part will prevent or reduce side creep of the part during its bending.Edge bending(Figure 10-1b)is cantilever loading of a beam.The bending punch 1 forces the metal against the supporting die 2.The bend axis is parallel to the edge of the die.The workpiece is prevent its movement during downward travel of the punch.For U bending(channel bending)pressures will be approximately twice those required for V bending;edge bending requires about one-half those needed for V bending.Springback.After bending pressure on metal is release,the elastic stresses also are released,which causes metal movement resulting in a decrease in the bend angle(as well as an increase in the included angle between the bent portions).Such a metal movement,termed spring back form 10to 15.v-bending dies customarily compensate for springback with V blocks and wedge-shaped punches having included angles Somewhat less than that required in the part The part is bent thought a greater angle than that required but it springs back to the desired angle.Parts produced in other types of bending dies are also overbent through an angle equal to the spring-back angle with an undercut or relieved punch