Press Fundamentals

BASIC DESIGN AND PRESS CHARACTERISTICS

A mechanical press is a machine with the space to contain and a mechanism to actuate metal working dies at the proper speed, and with mechanically developed energy to allow the dies to perform specific cutting, blanking, and forming work for which they are intended.

The physical characteristics of any particular press are determined by the following:

DIE SIZE – This determines the bed area, the right to left and the 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.

DIE ACCESSIBILITY AND PART HANDLING – This determines whether the press will be a straight side or gap frame construction, and whether it will be inclined.

WORK TO BE PERFORMED – This determines the capacity, rating, stroke, and the strokes per minute of the press.

Force required to do the work determines press capacity in tons. The capacity of the press along with die area dimensions, determine 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 can determine whether a press is a single end drive or twin drive. This is 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, according to the laws of harmonic motion. 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 forces X distance. 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, will require the force to be exerted over a considerable 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.

FLYWHEEL TYPE PRESS – The flywheel is mounted directly on the main shaft. This type of press is primarily for blanking and piercing operations where energy requirements are comparatively small and where the press operates at a fairly high speed in strokes per minute. This speed varies from about 90 spm to over several hundred spm.

SINGLE GEARED TYPE – The flywheel is mounted on a separate shaft and a single gear reduction is used to transmit the power to the main shaft. This type of press is used for shallow draw work, forming, and similar operations requiring considerably more energy than is available on the flywheel type press. In this type of press the strokes per minute, is generally in the range of 30 to 90 spm.

DOUBLE GEARED TYPE – The flywheel is mounted on a separate shaft and a double gear reduction is used to transmit power to the main shaft. This type of press is used on deeper draw work where considerable amount of energy is required due to the longer distance as well as where the maximum drawing speed of various metals limits the speed of the slide. Double-geared presses generally operate in a speed range of 10 to 30 spm.

PRESS TYPES:

OPEN BACK INCLINABLE PRESS (OBI) – the most versatile and most widely used of mechanical presses. It is a single crank press of either the flywheel or single geared type. It has a gap frame that is arranged so that the press may be inclined. This frame provides unobstructed access to the die space from three sides of the press and the inclinable allows the press to be arranged so that the finished parts may be removed from the die by gravity, and directed through an opening in the back of the press. OBI presses are built in a wide range of sizes with nominal tonnage ratings from 15 to 200 tons, and built to standard specifications. The stroke, shut height and spm may be varied within limits. The OBI has been put to countless different uses and is an ideal general purpose machine for work involving blanking, bending, and shallow drawing.

SINGLE CRANK GAP FRAME PRESS (OBS) – Similar to the OBI, this press lacks the inclining feature and is permanently in the vertical position.

SINGLE CRANK HORN PRESS is very similar to the single crank gap frame. The horn press frame is constructed without a bed and is arranged for mounting a horn. This press may also be furnished with a swing away table or adjustable bed allowing the horn press to be used as a gap frame press. The single crank horn press is used for work that must be performed over a projecting die, such as piercing holes in the walls of cylinders or rings etc.

SINGLE CRANK STRAIGHT SIDE PRESS – this press is generally furnished with a single geared drive and is used for more accurate work requiring greater rigidity of the press frame than it is possible to obtain from the gap frame press.

DOUBLE CRANK PRESS – Furnished in either the straight side or gap frame construction, this press type usually has a single geared drive. It is used for blanking, piercing, forming, bending, or shallow draw work where the dies are wide and the slide is held in better alignment by having two points of suspension at which the force is applied. The torsional capacity may be increased by the use of a twin drive (a gear on each end of the main shaft). Both the single crank and double crank press can be furnished with double gearing to reduce the number of strokes per minute. This machine is built in a wide variety of capacities and sizes.

ONE TWO AND FOUR POINT ECCENTRIC GEAR STRAIGHT SIDE PRESSES – These presses usually have a double geared drive and are used a great deal for medium and heavy blanking, and draw work where long strokes and fairly slow speeds are required.

PROGRESSIVE DIE PRESSES – These presses are commonly used for producing finished parts from coil, and are also referred to as high speed presses or high production presses. They may be single or double crank, eccentric shaft, straight side presses with press mounted double roll feeds and scrap choppers. The design of this class press takes into consideration the additional accuracy and rigidity required by progressive die operations.

ECCENTRIC SHAFT PRESSES – These presses are recommended for short stroke work or for high speed operation. Basically the same as the crank press, these presses have an eccentric shaft instead of a crankshaft. The use of the eccentric shaft reduces the effect of bending on the main shaft thus making the shaft stiffer.

FOOTNOTE: (Mechanical Press Fundamentals – A Guide to Efficient Press Selection and Use. An American Metal Stamping Association Publication)