The press working process is to apply the large forces by press tools for a short time interval in order to produce the shearing of deformation of the work materials. The time of producing a finished part is generally less than one second. Press working forces are setting up ,guided and controlled in a machine referred to as a press. Energy stored in the rotating flywheel of a mechanical press or supplied by a hydraulic system in a hydraulic press is transferred to the ram for its linear movements.
For the applications, there are many types of the presses and the following are the basic types used in industry:
(1) An open-back inclinable press ,also called a gap-frame press ,has a C-shaped frame which can be inclined an angle to the base ,allowing for the disposal of the finished parts by gravity .The open back allows the feeding and unloading of stock ,workpieces and finished parts from front to back.
(2) A straight side press has columns at ends of the bed, with rectangular windows to allow the feeding and unloading of stock workpieces.
(3) A hydraulic press is extensively used in open-side forging, extrusion, and sheet metal forging .In addition, it may be used in a wire drawing, powder metallurgy, plastic forging and special forging. The ram of a hydraulic press is driven by hydraulic cylinders and pistons， which are parts of high pressure hydorlic or hydropneumatic system.After a rapid approach speed ,the ram with upper die attached moves at a slow speed while exerting a squeezing force on the work metal .Pressing speed can be accurately controlled to permit control of metal-flow velocities; this is particularly advantages advantageous in producing closed –tolerance forgings. And that the working pressure and the stroke can be adjusted is another advantage of a hydraulic press.
(4) A double-action press is usually used for deep drawing on sheet and draw slide . The blank holder slide is hollow rectangular and is guided on gibs located on the press uprights .The draw slide fits into and moves up and down along the gibs of the hollow blank holder. Both slides are driven from a centralized drive including a single clutch and brake for actuation . In a typical press cycle ,when the clutch is actuated the blank holder descends faster than the draw slide . The blank holder contacts the stamping at about the time when the draw slide is halfway down on its stroke. The blank holder slide then dwells, or remains stationary under pressure, while the draw slide completes its down stroke .The blank holder holds the blank to prevent it from wrinkling.
(5) A triple-action press has the same inner and outer ram as the double-action press, but a third
Ram in the press bed moves up allowing a reverse draw to be made in one press circle.
(6) A knuckle lever press is used for coining . The drive design allows for very high pressures at the bottom of the ram stroke .This type use 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.
(7) A press brake is essentially the same as a gap-frame press except for its long bed from 1.8 to
2 m or more .It is used mainly for 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 forging. This allows parts of a complex design to be accurately made without a high-cost press tool by simply breaking the complex part down into several simple operations . This type of operation is used on low-run or prototype parts .The tooling cost is usually very low ,but the labor cost is high as the parts are manually located and transferred in each station.
Press operator safety must be a primary concern for everyone in the press area .While working under the ram the press control must be locked in the “off” position and safety blocks placed under the ram to prevent it from coating down. While the press is running ,the use of proper guards and safety procedures must always be followed.