Selection Of Presses And Its Setting

Module 6 Selection of presses and its setting

Contents • • • •

Selection of Presses and settings for various dies Equipment for various sheet metal operations Overloading of presses Automation and Safety in press shop

Selection of presses and settings • Blanking—Servo. •  Servo mechanical presses can perform short-stroke, high-speed stamping as well as mechanical presses, in addition to long-stroke, slow-speed stamping. • Flexibility for blanking and shallow forming. • A servo mechanical press's speed can be controlled and reverse tonnage effects minimized after material fracture, in blanking operations, a greater percentage of a press's overall tonnage rating can be applied than on a mechanical press. • For example, on a 250-ton mechanical press, only half the tonnage rating, 125 tons, can be applied. A 250-ton-rated servo-mechanical press can blank up to 220 tons, depending on the application.

• Fine blanking—Servo.  – Fine blanking takes advantage of the programmable slide motion of a servo press.

• Punching, Shearing, Blanking—Pneumatic. – Pneumatic presses perform punching, shearing, blanking, forming, piercing, embossing, and complex roll forming.

• Drawing, Deep Drawing—Hydraulic.  – It's well-known that in deep-drawing operations, deformation at a low velocity improves metal flow because of the effect of punch velocity on friction conditions. – To draw and deep-draw components such as cookware, gas tanks, and pressure cylinders, strong forces are needed high in the stroke through the BDC. A hydraulic press's control of the force and speed throughout the entire stroke makes it conducive for drawing and deep drawing.

• Drawing or Blanking—Servo. – Because the servo can give stroke lengths from 1 to 12 inches long and apply full tonnage at 1 SPM to maximum speed, blanking can be done one day, deep draws the next.

• Blanking, Piercing, Coining, Shallow Forming—Mechanical.  – Flywheel drive mechanical presses are suitable for piercing, blanking, bending, and very shallow drawing with progressive dies. "When you have only a blanking or piercing operation, you only have to consider the material thickness in the flat plane. – The best application practice is to keep the press stroke as short as possible. This reduces slide velocity, snap-through, reverse load, and vibration and helps achieve maximum production rates. – Because blanking, piercing, coining, and shallow forming are performed very close to the bottom of the stroke, a mechanical press gives better results. – A mechanical press makes a very sharp, shallow, and well-defined coin impression better because it stamps faster. Also, for blanking, burr formation is reduced at a mechanical press's higher speeds. "Conversely, burr formation during blanking, punching, and other cutting operations can occur with a hydraulic press."

PRESSES FOR SHEET METAL WORKING

• 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 arbor 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 – 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 unobstructed access to the dies from three sides and their backs are usually open for the ejection of stampings and / 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: Selection of a press is necessary for successful and economical operation. 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 work piece, 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.

Applications of this metal include medical tables, bodies of automobile and truck, roofs of buildings, and airplane fuselages. Based on the application for which it is used, the thickness of the sheet metal can vary. They can come in coiled strips or flat pieces. Various tools are used to bend or cut this metal in different shapes.

Tools Used for Cutting, Bending/Shaping the Sheet Metal • There are four main types of tools required for processing sheet metals. • Sheet Metal Bending Tools

– Sheet metal bending is a process in which the metal is bended by keeping its volume constant.

Bench Top Budget Brakes –This is a wide bench top with a fixed lower beam and a variable upper beam. While bending aluminum, a radius is created with the help of the upper beam of the bench top sheet metal brake. Bracket Makers –This is another type of equipment used for bending sheet metal. Brackets of various shapes and specifications can be formed by the bracket maker equipment. The 90° tools and radius tools of this equipment are very helpful in bracket formation.

• Tools for Sheet Metal Forming – – Sheet metal forming is the process in which the geometry of a sheet metal is altered by application of force. This force is sufficient to plastically deform the material by stressing the metal beyond its yield strength. In this process, material is not removed from the metal.

Shrinker Stretchers – Shrinker stretcher tool is used to form sheet metal, without the need of heating, cutting, or welding. Smooth radius curves can be formed with the help of this tool. The tool is mainly used to make inside curves. To do this, the metal shrinking is done on one side.

• Sheet Metal Shearing Tools – • Shearing is the process in which sheet metal is cut in a straight line. Generally, this cutting process is used to make cuts that are parallel to the existing edge. The process also allows you to make angled cuts.

Throat less Hand Shears –The throat less hand shears feature a heavy steel frame. As the name suggests, the tool has a throat less design. These tools allow you to cut effortlessly, owing to its top blade that is arched. Throat less Rotary Shears –This tool is a bit more flexible than the hand shears. It allows you to cut any shape. Along with the forged steel frame, the throat less rotary shears, feature an adjustable cutter, and a ratcheting head.

Overloading of presses • The primary purpose of the system is to prevent damage to the press in the event of an overload. • Overload can be caused by many conditions, including: – – – – – – –

Setting the shut height too low. Loading a die that requires too much tonnage. Getting slugs or foreign material lodged in the die. Misfeeding material or allowing it to buckle. Positioning the die improperly. Using incorrect material Using material that changes in hardness or thickness.

• A press overload can occur on one side of a multipoint press and not the other. In fact, this is the most common overload condition. The die's total tonnage requirement is not the only measure. For example, a 400-ton die will overload a 400-ton, two-point press if more than 200 tons of the load are under one connection.

Automation • Advantages in Press Line Automation: – Increase line efficiency and reliability by at least 96% for the complete line – Increase the rationalization of production in press-shops with existing presses through mechanic and electric retrofit activities – Experience in retrofit activities, as well as in new lines – Achievement of line maximum rates (production ramp-up) through simple guided procedures and reduced timing – Stamping of up to 16 parts/min, with dedicated robots and software that optimizes the cycle-time – High performance

Safety in press shop • The keys and adjusting wrenches must be removed from the machine before operating it. • Stop the machine before measuring, cleaning or making any adjustments. • Do not handle metal turnings by hand as they can cause injury. Use brush or rake to remove turnings. • Keep hands away from the cutting head and all moving parts. • Cutting tools and blades must be clean and sharp, so that they can be used without force. • Ensure that the guards are in position and in good working condition before operating. • Know the location of emergency stop switch. • Do not wear loose clothing or jewelry that can be caught in the rotating parts. • Confine long hair • Keep work area clean. Floors must be level and have a non-slip surface. • There must be enough space around the machine to do the job safely.

• Avoid awkward operations and hand positions. A sudden slip could cause the hand to move into the cutting tool or blade. • The person working with the machine must not be distracted. • Machines must not be left unattended. Switch off the machine before leaving. • Rotating parts of machines must not be stopped with hands after switching off. • Compressed air must not be used to clean machines, as this can force small particles to fly off and can cause injury. • Safety glasses must always be used while working with machinery for protection from flying particles. • Safety glasses must be worn by all personnel entering an area where machines are operated. • Ear protection must be worn for protection from high noise. • Safety shoes must be worn if there is handling of heavy materials. • Hand gloves must NOT be used while working with machinery, due to the chances of getting caught in the nip point.