【正文】 equal to the springback angle with an undercut or relieved punch.Figure8 Methods of reducing or eliminating springbackFor the applications, there are many types of the presses, such as the singleaction straightslide eccentric mechanical press, punch press, hydroformer press, hydraulic press, press brake, tripleaction press, turret press, twopoint press, twindrive press, two point singleaction press, watch press, trimming press, closedtype singleaction crank press, knucklelever press, onepoint singleaction press, openback inclinable press, openside press, fourpoint press, fourcrank press, flywheeltype screw press, friction screw press, straightside singleaction doublecrank press, rockerarm press, screw press and topdrive sheetmetal stamping automatic press and so on. A doubleaction press is used for large, or deep drawing operations on sheet metal parts. This type of press has an outer ram ( blank holder ) and a section inner ram ( punch holder ) . During the operating cycle, the blank holder contacts the material first and applies pressure to allow the punch holder to properly draw the part (Figure 9).Figure9 Typical versatile press A tripleaction press has the same inner and outer ram as the doubleaction press, but a third ram in the press bed moves up allowing a reverse draw to be made in one press cycle. The tripleaction press is not widely used.A knuckle press is used for coining operation. The design of the drive allows for very high pressures at the bottom of the ram stroke. This type uses a crank, which moves a joint consisting of two levers that oscillate to and from dead center and results in a short, powerful movement of the slide with slow travel near the bottom of the stroke. A hydraulic press is used basically for forming operations and has a slower operating cycle time than most mechanical presses. The advantages of hydraulic presses are that the working pressure stroke, and speed of the ram are adjustable (Figure 10). Hydraulic presses belong to the forceconstrained type of forming machines . Their main use is found in those areas of forming technology where the force along the path of the ram must remain constant or under accurate control. The drive mechanism of piston and cylinder acts in a linear manner and is directly connected to the ram. The form of frame construction of hydraulic presses is largely similar to that of mechanical presses. The hydraulic drive units are easily acmodated in the machine frame. Consequently, several hydraulic drives can readily be built into a single machine for plicated forming and cutting operations (drawing, extruding, cutting, swaging, etc.), and the required motions may be easily coordinated. A press brake is essentially same as a gapframe press except for its long bed from 6 to 20 feet (～6m) or more. It is used basically for various bending operations on large sheet metal parts. It can also be used with a series of separate sets of press tools to do light piercing, notching, and forming (Figure 11). This allows parts of a plex design to be accurately made without a highcost press tool by simply breaking the plex part down into several simple operations. This type of operation is used on lowrun or prototype parts.Figure 10 typical hydraulic press Figure 11 Typical press brake Sheet metal can be bent easily with simple fixtures using a press brake. This press brake utilizes long dies in a mechanical or hydraulic press and is suitable for small production runs. The tooli