A number of types of presses of various sizes are available for the wide range of products demanded by manufacturers. Presses can be classified in the following manner:
1. According to the Source of Power:
Presses may be classified as manual driven, mechanical type or hydraulic and pneumatic type. The manual driven (hand operated) fly-presses are used for producing small components requiring simple piercing, blanking and similar operations.
These are low capacity presses ranging from half ton to about 10 tons. In arbor type of press a second member is used to reciprocate the punch. The screw is turned by means of a handle which is secured to an arm carrying the screw.
Generally a heavy weight is attached to the arm to produce the flywheel effect. In the fly-screw type press the effort is applied at the rim of the flywheel. The angle of the helix in this press is more than the angle of friction so that the potential energy of the shaft and flywheel is changed into kinetic energy as the ram is coming down.
The presses of this type are generally mounted on benches and are, therefore, also called bench model presses. These are mainly used for fitting, assembling, punching and embossing very thin sheets below 1 mm thickness.
Mechanical driven presses are the most commonly used type and range in capacity from 5 to 100 tons. The various parts of this press are motor, pinion, gear, flywheel, crank shaft and ram. It is further classified according to the method of movement of ram. These presses are suitable for all types of press works except for deep drawing operating where slow motion is desired.
Hydraulically driven presses are used for heavy duty work (forging work etc.) In hydraulic presses the force is exerted through high pressure fluid driving a cylinder-piston system.
Low capacity presses of this type are used for operations like stamping, flattening, coining and deep drawing. In these presses, the power is stored in the accumulator. Pneumatically driven presses are confined largely to those operations which can be performed with a fast and short (hammer like) action.
2. According to the Number of Rams and Slides:
The presses may be:
(a) Single action type,
(b) Double action type, and
(c) Triple action type.
A single action press has one slide (or ram) and is the most popular type of press used for stamping operations. A double-action press incorporates two slides, one inside the other. The punch is attached to the inner slide.
The outer slide is used to clamp the flange of a part which has been drawn by the punch using a pressure plate. Such a press is very suitable for combination dies e.g. is deep drawing operation, it is necessary to hold the blank with an outer pressure pad while the punch is drawing the desired shape down into the die.
The outer slide can be used for this blank holding function and will remain stationary in its lowest position until the inner (working) slide has moved past lower dead centre and it begins to ascent. A triple action press has three slides.
The two upper slides move downwards while third slide which is located in the base of the press moves upwards to perform a supplementary operation. This arrangement permits three different but related operations to be performed together. A typical use of this press may be for combination embossing, blanking and drawing where the lower ram actuates the embossing die.
3. According to the Mechanism of Power Press (Method of Transmitting Power for Actuation of Ram):
In mechanical presses, the power is derived from a motor driving flywheel or gear and is delivered to the ram in following ways:
(a) Crank shaft and connecting rod mechanism
(d) Toggle joint
(e) Knuckle joint
(f) Rack and pinion or screw arrangement
Crank and eccentric mechanisms are used for both single and multiple action presses and also for light as well as heavy duty work. Their mechanism is very simple. The ram is carried in vertical guides on the front and is coupled, through the connecting rod to the crank-shaft, which is carried in bearings near the top of the front columns.
In order to provide for vertical adjustment of the ram the length of the connecting rod can be varied. Keyed to the driving end of the crankshaft is a heavy flywheel which serves as a reservoir of energy. A brake is generally fitted at the opposite end of a crank shaft to smoothen the motion and eliminate jerkiness.
From Fig. 29.1, it will be observed that:
i. The maximum force is exerted at the end of the stroke.
ii. There is considerable dwell (i.e. time at the end of stroke is more) and thus this drive is best suited for getting permanent impressions (operations such as coining, embossing).
In toggle press, the working mechanism and the characteristics are as shown in Fig. 29.2. From the movement of toggle and punch, it is very obvious that this drive is best suited for drawing operation where the drawing operation is carried out by the punch and the holding operation by toggle movement. Other mechanism do not require much explanation.
4. According to the Position of Ram:
The presses may be classified as of vertical, horizontal or inclined type to facilitate the handling of work material.
5. According to Its Use:
A press may be used for punching, blanking, drawing, forming, coining, embossing purposes and the press may be named accordingly.
6. According to Design of Frame:
Press frames are made either from high-grade cast iron or from steel. Frames are designed in various styles according to the class of work to be done. Open fronted C-shaped presses are available either as rigid or inclinable.
Open front provides adequate working clearance for the tools and the press may be approached from three sides. It generally consists of a single column at the back of work-table. The other possibility is two columns in rear which are rigidly fixed to each other both at the top and at the bed.
The inclinable open back frame is inclined backwards and this permits the pressing to be removed by gravity between working strokes. In double sided presses the bed is equally accessible from front and back. The press frame is generally square shaped made of two vertical column plates fixed at the top and at the bed. In small presses, the frame is generally cast integral while in big presses, the various members of frame are held together with the help of tie rods.
Broadly the classification according to shape of frame is as:
(а) Straight side frame,
(b) Arch type frame,
(c) Gap type frame.
7. According to Capacity:
The rated tonnage capacity of the press should always be specified as it indicates the maximum pressure which the press is designed to safely take up. The capacity of a press is generally given as so many tonnes maximum pressure, though this method of expressing effort is rather misleading. It will be observed that the torque given to the crankshaft remains the same but the amount of resistance that ram can withstand depends upon the inclination of crank.
As the ram reaches bottom most position, maximum tonnage is available. Force required to draw or shear should be less than the tonnage capacity of the press, otherwise the weakened member will fail. The load that press may carry depends upon the size and strength of its farm, crankshaft and ram connection, and it is on the strength of these that the makers base their specification of the capacity.