In this article we will discuss about:- 1. Introduction to Power Press 2. Power Press Operations 3. Components 4. Flywheel Considerations 5. Transfer Presses 6. Selection.
Introduction to Power Press:
Power presses are used for producing large quantities of articles quickly, accurately and economically from the cold working of mild steel and other ductile materials. The components produced range over an extremely wide field and are used throughout industry.
Sometimes the pressing may be complicated and more than one pressing operations may be required. Now-a-days practice is to produce most of the sheet parts of any shape by using specially designed press tools and other combination of operations.
For economical production of quantities of pressings, consideration has to be given to the rate of production, the cost of the press tools to be employed, and the expenditure involved in setting them.
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It is also necessary to plan the operations to reduce scrap material to a minimum and to use waste material for other smaller pressings. For any operation to be performed on press, the selection of the proper press and the design of the tool or die to be mounted on it are very important.
Power Press Operations:
Press tools may be designed for carrying out the following or more operations:
(i) Piercing (removal of a local piece of the material to form a hole of some shape).
(ii) Cutting and shearing.
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(iii) Blanking (production of the contour in flat blank), clipping, shearing, etc.
(iv) Bending (material is bent in one plane).
(v) Curling.
(vi) Drawing (production of a deep cup component).
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(vii) Deep drawing, shallow drawing, redrawing.
(viii) Extrusion.
In addition, the operations of cupping (production of a cup from disc), coining and trimming (truing up the edges of a pressing), coining, interlocking, riveting, forging, impact extrusion may also be carried out.
Components of Power Press:
Any press is comprised of a frame, a bed or bolster plate and a reciprocating member called ram or slide which exerts force upon work material through special tools mounted on the ram and bed. The linear movement to the ram is transferred by the energy stored in the rotating flywheel in case of mechanical press or by a hydraulic system in the hydraulic press. Generally the access to the working space between the bed and the ram is provided by a frame.
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Major components of a press are:
1. Rectangular Bed:
It is a part of the frame and generally opens in the centre to allow the scrap or blank to fall down. The bed also supports a bolster plate.
2. Bolster Plate:
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It is needed for mounting the press tools and accessories and, therefore, should be of sufficient thickness to withstand the forces coming on it. Bolster plates are available in standard overall sizes and openings, and are sometimes provided with slots running from front to back. It is generally never changed once it is fixed on the bed.
3. Ram (Slide):
It moves through its fixed stroke, a distance depending upon the size and design of the press. The position of the ram can be changed and its stroke is generally fixed. However in certain presses provision the change to stroke length exists.
The distance between the top of the bed (bolster) and the bottom of the slide with its stroke down is called the shut height of a press. This is an important dimension for the tooling equipment designer as the die along with punch holder and die holder have to be designed according to this dimension.
4. Knock Out:
It is a mechanism operating on the upstroke of a press which ejects work pieces or blanks from press tool.
5. Cushion:
It is a press accessory located beneath or within the bolster for producing an upward motion or force and is actuated by air, oil, rubber or springs or a combination thereof.
Flywheel Considerations:
It is used to supply energy for that period of operation which requires more energy and during other period, it stores the energy. If machine lacks sufficient flywheel energy, it will come to stop and will not be able to complete the operation. Actually by employing flywheel, we can work with motor of less capacity and at the same time supply maximum tonnage at the desired need of operation.
For a greater working distance (in case of drawing operation) and for faster operation (in case of automatic blanking or piercing operation) more energy and power must be provided.
In case of blanking operation, the work is completed in a very brief portion of stroke. So in this entire energy is to be tapped from flywheel and flywheel supplies instantaneously whole energy required for operation and for remaining period of cycle, it will restore. Drawing operation takes considerable part of the cycle. Here as time is sufficient, therefore, energy can be taken from motor and deficient energy supplied by the flywheel.
Allowable Speed Reduction in Flywheel:
Its value for intermittent operation is taken as
= 20% and for continuous operation = 10%
E = energy (kg cm.)
D = dia of flywheel (cm.)
W = weight of flywheel in kg.
N = speed in R.P.M., R = radius of gyration
(Energy which can be supplied by flywheel)
Also E = P x L x K (From operation)
P = average force and L = stroke length
K is constant which accounts for various friction losses in machine members.
If energy of flywheel is less than P x L x K then best thing would be to increase the speed N.
Transfer Presses:
These are high production machines and particularly suitable for light work. They are, however, also available for medium and heavy duty work. The transfer press forms finished product from a single piece of stock which is transferred from one station to another by means of appropriate transfer devices such as push bars, fingers and gripping mechanisms.
It is possible to use cut blanks which may be fed direct to the machine using stock feeding mechanism or a coil of strip may be used from which the blanks must first be produced. Safety devices are fitted to protect the tools against any possible damage should any part of the actuating mechanism prove faulty. Transfer machines can often be used for deep drawing operation without the necessity of introducing intermediate annealing operations.
To minimise the occurrence of accidents, as far as possible, mechanical feeding arrangements, should be used and the dangerous areas should be railed round so that operators are unable to place their hands inside.
Selection of Power Press:
Considering the economic aspect, the press tool designer has to make proper selection of the type of the press to be used and also the kind of press tools to provided. For instance, it may be more economical to use a tool which will complete a number of operations at one stroke of the press than to employ a number of cheaper and simpler tools to perform the same work in a series of operations.
Alternative methods have to be considered carefully with reference to the quantity of pressings required. The design of the tool should be simplest possible and the method of operation the most efficient one.
As regards the selection of the proper size and type of press for a given kind of work, the following points need to be considered:
(i) The strength of the press or its tonnage capacity (maximum pressure in tonnes)
(ii) The energy available for the flywheel capacity.
(iii) Speed of operation, i.e. crank pin speed.
(iv) The capacity of the motor.
(v) The size and type of die required.
(vi) The amount of stroke necessary.
(vii) The distance between the top of the bed (bolster) and the bottom of ram with its stroke down.
(viii) The method of feeding, the direction of the size of the sheet blank or work piece.