Metal Forming Operations

Stamping presses utilize the force of a moving ram to transmit force, or an amount of tonnage, to specific tooling in order to achieve a product’s final shape, often with little or no scrap and, whenever possible, with minimal operator intervention.
A wide variety of parts are formed in a similarly wide variety of press sizes and types. The moving forces of the press are generated by either mechanical or hydraulic mechanisms.
Simple, single dimension stampings can be accomplished with small bench top presses generating as little as five tons. Larger, more geometric shapes are formed with very large presses rated in thousands of tons. Press speed may also vary from 10 to 18 strokes per minute to as fast as 1800 strokes per minute.
Both the hydraulic and the mechanical press are classified by the type of frame upon which the moving elements of the press are mounted. The two most common frame types are the gap-frame, or “C” frame, and the straight side press. Each have their own advantages and disadvantages.
The “C” type press allows easy access to three sides of the die area. They require less floor space and in ranges from 35 to 60 tons they may cost half as much as a similar sized straight side press. However, the “C” press, by its design, is prone to angular misalignment as the open frame deflects as force is brought to bear on the die or tooling. Although such misalignment is not always a problem, it is overcome by using heavier and thus more expensive presses.
A popular variation of the gap frame press is the open back inclinable or “OBI” press. By pivoting the frame, usually backwards, on its base, finished part or scrap discharge is more easily facilitated. However, it’s the open back stationary (OBS) which is the more popular of the two. OBS presses use timed blasts of air, mechanical devices, or conveyor system to discharge parts or scrap.
Straight side presses are so named because of the vertical columns or uprights on either side of the machine. This design eliminates the problem of angular deflection. Also, die life and part accuracy are enhanced. The four principle components of the straight side press are:
• The crown which supports the motor, flywheel, and other driving mechanisms. The columns which support the crown and are fitted with adjustable guides gibs) which insure parallelism, squareness, and proper movement of press components.
• The bed or foundation of the press.
• The bolster which mounts on the press bed and accommodates the die while strengthening the bed.
Mechanical presses have an electric motor that turns a flywheel. The flywheel revolves around a crankshaft until engaged by a clutch device. Then, through a series of drive train components, the energy of the rotating flywheel is transmitted to the vertical movement of the slide or ram.
There are three basic drive train variations. In the direct drive arrangement, the drive motor, through a belt arrangement, rotates the flywheel. This method provides the highest speeds, are more easily maintained while losing less mechanical energy. However, to gain their maximum force, which only occurs near the bottom of the ram stroke, the press must always be operated at its maximum speed. There is also some angular misalignment as torque is applied to only one end of the crankshaft. In other drive arrangements using single and double gear reductions along with eccentric gear drives, the misalignment problems are eliminated and more power for the forming and the deep drawing of larger parts is possible. Other important components of the mechanical press are the clutch and braking systems. While the clutch allows the energy of the revolving flywheel to be transmitted to the crankshaft, the braking system holds the ram in position when the clutch is disengaged.
The major consideration in the selection of a mechanical press is the force capacity or force that can be exerted at a specified distance above the bottom of the stroke. This is expressed in tons or metrically in kilonewtons. Force capacity is determined by flywheel energy (speed) and torque capacity which is the press’ ability to transmit energy through the drive train and ram into the die. Geared presses do not increase force capacity. The gear ratio that is present is primarily a means to obtain the most efficient flywheel speed, and thereby enhancing torque capacity.
The definition of a “high speed mechanical press” is generally accepted as being one capable of 300 strokes per minute and higher. The press speed for small high volume parts can be as fast as 1400 strokes per minute. While the mechanical press remains the most common, the hydraulic press is gaining popularity, having some distinct advantage. The full force of the press can be delivered at any point of the stroke. Deep drawing and forming require strong forces high in the stroke. Also, the stroke of the hydraulic press can be adjusted to facilitate part clearance between cycles. The ability to pre-set the working hydraulic pressure allows the use of many different tool and die heights and the forming of varying work thickness’.
Press Selection
There is no single universal press that can provide productive and cost efficient operation. Compromises must be made in order to use the press for more than just one type of stamping. Such compromises include consideration of the following primary factors:
• Press size
• Force available
• Energy
• Speed
Other considerations can include:
• Size and geometry of the work
• Number of operations to be performed
• Quantities and production rates
• Accuracy
• Finish requirements
• Equipment costs
Such costs would also include the control systems for the press. As stamping becomes more automated, the use of CNC systems and various electro-mechanical and solid state control devices become necessary. Additionally, devices and systems to feed material to the press must also be factored in. There are mechanical blank handling systems where manual handling is not practical due to speed and size. High volume feeding is done with coil stock which also requires an investment in coil feeding equipment.