Press Characteristics

Terms

• Die Size
• Die Accessibility and Part Handling
• Work to be Performed

Die Size -
This determines the bed area and the right to left and front to back dimensions of the slide, bed, and bolster. The maximum die height determines the shut height, which is normally given as the distance from the bed to the slide with the stroke down and adjustment up (SDAU). This requires the bolster thickness to be added to die height to obtain the shut height. The anticipated range of die heights determines the slide adjustments.
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Die Accessibility and Part Handling -
This determines whether the press will be a straight side or gap frame and whether it will be inclined.
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Work to be Performed -
This determines the capacity, rating, stroke, and strokes per minute (spm) of the press.

Force required to do the work dictates press capacity in tons. The press capacity along with die area dimensions, determines whether the slide is driven by one point, two points, or four points. For crank type presses, one-point and two-point types are referred to as single crank and double crank presses.

Distance through which the dies must operate to produce the part and provide clearance to remove the part, determines length of stroke.

Distance through which work is performed, determines capacity rating which is expressed as distance from bottom of stroke, or the point in the stroke at which the press is rated. Presses rated higher in the stroke require greater torque in the drive and more flywheel energy. Higher torque requirements determine whether a press is a single end drive or twin drive or whether the main shaft is driven from one end or from both ends.

Production requirements along with length of stroke, determines the speed of the press in strokes per minute. Press speed determines whether the press is non-geared (flywheel drive), single geared, or double geared.

For draw work, a method of holding the blank is required. For average draws a pneumatic die cushion is used. In order for any particular die set to perform its task, a certain force must be exerted. Due to the peculiar characteristics of the crank motion, the force available to the dies varies during the stroke of the press. This is due to the fact that while a constant torque is provided by the press drive, the force is transmitted by the rotating members to the reciprocating slide varying from minimum at midstroke to maximum at bottom stroke. It is customary to provide the torque necessary to exert the rated press tonnage at some given point above the bottom of the stroke, commonly 1/4 up in the case of a double geared press. As a corollary to the forgoing discussion of press tonnage, it is necessary to remember at all times that this tonnage must be exerted through a certain distance, thereby using energy, which is a force multiplied by the distance over which the force is applied. Some press operations, such as blanking and piercing, require force be exerted over a comparatively short distance, while other press operations, such as deep draw work, require force to be exerted over a long distance.

The major source of energy (or force capable of being exerted through distance) is provided by the flywheel. The size of the flywheel and the speed at which it rotates determines the energy available. As the energy in the flywheel increases with the square of its speed, it becomes advantageous in cases where large amounts of energy is required to rotate the flywheel as fast as possible. This consideration of the energy requirements, determines to a great extent the types of drives of a press.
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