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Sheet Metal Presses

Classification of presses

Types of presses for sheet metal working can be classified by one or a combination of characteristics, such as source of power, number of slides, type of frame and construction, type of drive, and intended applications.

Classification on the basis of source of power

• Manual Presses: These are either hand or foot operated through levers, screws or gears. A common press of this type is the arbour press used for assembly operations.

• Mechanical presses: These presses utilize flywheel energy which is transferred to the work piece by gears, cranks, eccentrics, or levers.

• Hydraulic Presses: These presses provide working force through the application of fluid pressure on a piston by means of pumps, valves, intensifiers, and accumulators. These presses have better performance and reliability than mechanical presses.

• Pneumatic Presses: These presses utilize air cylinders to exert the required force. These are generally smaller in size and capacity than hydraulic or mechanical presses, and therefore find use for light duty operations only.

Classification on the basis of number of slides

• Single Action Presses: A single action press has one reciprocation slide that carries the tool for the metal forming operation. The press has a fixed bed. It is the most widely used press for operations like blanking, coining, embossing, and drawing.

• Double Action Presses: A double action press has two slides moving in the same direction against a fixed bed. It is more suitable for drawing operations, especially deep drawing, than single action press. For this reason, its two slides are generally referred to as outer blank holder slide and the inner draw slide. The blank holder slide is a hollow rectangle, while the inner slide is a solid rectangle that reciprocates within the blank holder. The blank holder slide has a shorter stroke and dwells at the bottom end of its stroke, before the punch mounted on the inner slide touches the work piece. In this way, practically the complete capacity of the press is available for drawing operation.

Another advantage of double action press is that the four corners of the blank holder are individually adjustable. This permits the application of non uniform forces on the work if needed.

A double action press is widely used for deep drawing operations and irregular shaped stampings.

• Triple Action Presses: A triple action press has three moving slides. Two slides (the blank holder and the inner slide) move in the same direction as in a double – action press and the third or lower slide moves upward through the fixed bed in a direction opposite to that of the other two slides. This action allows reverse – drawing, forming or bending operations against the inner slide while both upper actions are dwelling.

Cycle time for a triple – action press is longer than for a double – action press because of the time required for the third action.

Classification on the basis of frame and construction

• Arch – Frame Presses: These presses have their frame in the shape of an arch. These are not common.

• Gap Frame Presses: These presses have a C-shaped frame. These are most versatile and common in use, as they provide un-obstructed access to the dies from three sides and their backs are usually open for the ejection of stampings and / or scrap.

• Straight Side Presses: These presses are stronger since the heavy loads can be taken in a vertical direction by the massive side frame and there is little tendency for the punch and die alignment to be affected by the strain. The capacity of these presses is usually greater than 10 MN.

• Horn Presses: These presses generally have a heavy shaft projecting from the machine frame instead of the usual bed. This press is used mainly on cylindrical parts involving punching, riveting, embossing, and flanging edges.

Press Selection:

Proper selection of a press is necessary for successful and economical operation. Press is a costly machine, and the return on investment depends upon how well it performs the job. There is no press that can provide maximum productively and economy for all application so, when a press is required to be used for several widely varying jobs, compromise is generally made between economy and productivity.

Important factors affecting the selection of a press are size, force, energy and speed requirements.

Size: Bed and slide areas of the press should be of enough size so as to accommodate the dies to be used and to make available adequate space for die changing and maintenance. Stroke requirements are related to the height of the parts to be produced. Press with short stroke should be preferred because it would permit faster operation, thus increasing productivity. Size and type of press to be selected also depends upon the method and nature of part feeding, the type of operation, and the material being formed.

Force and Energy: Press selected should have the capacity to provide the force and energy necessary for carrying out the operation. The major source of energy in mechanical presses is the flywheel, and the energy available is a function of mass of flywheel and square of its speed.

Press Speed: Fast speeds are generally desirable, but they are limited by the operations performed. High speed may not, however, be most productive or efficient. Size, shape and material of workpiece, die life, maintenance costs, and other factors should be considered while attempting to achieve the highest production rate at the lowest cost per piece.

Mechanical versus Hydraulic Presses:

Mechanical presses are very widely used for blanking, forming and drawing operations required to be done on sheet metal. For certain operations which require very high force, for example, hydraulic presses are more advantageous.

Table 1 gives a comparison of characteristics and preferred application of the two types of press.

Press Feeding Devices:

Safety is an important consideration in press operation and every precaution must be taken to protect the operator. Material must be tried to be fed to the press that eliminates any chance of the operator having his or her hands near the dies. The use of feeding device allows faster and uniform press feeding in addition to the safety features.

• Blank and Stamping Feeds.

Feeding of blanks or previously formed stampings to presses can be done in several ways. Selection of a specific method depends upon factors like production rate needed, cost, and safety considerations.

Manual feeding: Feeding of blanks or stampings by hand is generally limited to low production rate requirements which do not warrant the cost of automatic or semi- automatic feeding devices. Manual feeding, however, is accomplished with the use of a guard or, if a guard is not possible, hand feeding tools and a point – of – operation safety device. Some commonly used hand feeding tools are special pliers, tongs, tweezes, vacuum lifters and magnetic pick – ups.

Chute feeds: For feeding small blanks or stampings, simple chutes are often used. The blank slides by gravity along rails in the bottom of the chute. Slide chutes are designed for a specific die and blank and are generally attached permanently to the die so as to reduce setup time. Slide angle of 200 - 300 is sufficient in most cases. Chute feeds need barrier guard enclosure for operation protection, with just enough opening in the enclosure for the blanks to slide through to the die.

Push feeds: These feeds are used when blanks need orientation in specific relation to the die. Work piece is manually placed in a nest in a slide, one at a time, and the slide pushed until the piece falls into the die nest. An interlock is provided so that the press cannot be operation until the slide has correctly located the part in the die. To increase production rate, push feeds can be automated by actuating the feed slide through mechanical attachment to the press slide.

Lift and transfer devices. In some automatic installations vacuum or suction cups are used for lifting of blanks one at a time from stacks and then moved to the die by transfer units. Separation of the top blank from a stack is achieved by devices which are operated magnetically, pneumatically or mechanically.

• Dial Feeds: Dial feeds consist of rotary indexing tables (or turntables) having fixtures for holding work pieces as they are taken to the press tooling. Parts are placed in the fixtures at the loading station (which are located away from the place of press operation) manually or by other means like chutes, hoppers, vibratory feeders, robots etc. Such feeds are being increasingly used because of higher safety and productivity associated with them.

• Coil Stock Feed

Two main classifications of automatic press feeds for coil stock are slide (or gripper) and roll feeds. Both of these may be press or independently driven.

Mechanical slide feeds: Press – driven slide feeds have a gripper arrangement which clamps and feeds the stock during its forward movement and releases it on the return stroke. Material is prevented from backing up during the return stroke of the gripper by a drag unit like a frictional brake. Grippers reciprocate on rods or slides between adjustable positive stops to ensure accuracy. Slide feeds are available in a variety of sizes and designs. These are generally best for narrow coil stock and short feed lengths.

Hitch – type feed: This feed differs from press – driven mechanical slide feed in that actuation is by a simple flat cam attached to the ram or punch holder instead of by the press. On the downward stroke of the ram, one or more springs are compressed by the cam action, then on the upstroke, the springs provide the force to feed stock into the die.

These feeds are best suited for coil stock of small to medium thickness and for relatively short feed progression. These are one of the oldest and least expensive feeding devices still used very widely. Due to their low cost, they are generally left permanently attached to the dies, thus reducing setup time.

Pneumatic slide feeds: These feeds are similar to mechanical slide feeds in that they have grippers or clamps that reciprocate on guide rails or slides between adjustable positive stops to push and / or pull stock into a die. However, these differ in that they are powered by an air cylinder, with actuation and timing of valves by cam – operated limit switches.

These feeds are best for short progression, and find wide applications in job shops because of their low cost and versatility.

Roll feeds: In these feeds, coil stock is advanced by pressure exerted between intermittently driven, opposed rolls which allow the stock to dwell during the working part of the press cycle. Intermittent rotation (or indexing) of the feed rolls, with the rolls rotating in only one direction, is accomplished in many ways. In one common design, the rolls are indexed through a one – way clutch by a rack – and – pinion mechanism that is actuated by an adjustable eccentric on the press – crankshaft.

These feeds are available in several types and sizes to suit almost any width and thickness of stock. Though their initial cost is slightly higher, their greater durability and lower maintenance cost account for their extensive use.