Metal Stamping Overview
The metal-stamping process is described in general, focusing on types of stamping presses and tooling, as well as other equipment that comprises a complete stamping-press line. The use of forming lubricants and the process of part design is also discussed. A stamping operation requires talented people and the right equipment to perform successfully. How does the stamping process work, and how are equipment and personnel employed to make sure quality parts are stamped consistently? A stamped sheetmetal part requires able creators backed by talented personnel who allow their machinery—from stamping presses to tooling–to reach its technological potential.
Designing A Part
The birth of a stamped metal part is the designer’s drawing board, perhaps the result of a request from a specifier. The designer must plan a particular part while considering a multitude of factors. How is the part expected to interact with other parts and best fit into a component or assembly? Must the part be light in weight? What forces must it withstand? How long must it last? What size should it be? What is the environment, and how will material selection influence how the part performs in that environment? What technology and machinery is available in order to construct this part in an efficient and cost-effective manner? Will the part be coated or must it be cleaned? If so, how does that affect the choice of material and types of lubrication required for manufacture?
Quality People and Equipment Required
Questions asked and answered, the proposed part enters the realm of manufacturing. The effective metal-stamping operation, especially an operation serving multiple clients with multiple requirements, boasts an array of flexible equipment engineered and maintained to efficiently produce a variety of parts. The employees overseeing and operating shop-floor machinery must be well-trained to take advantage of the technology.
Stamping Presses Serve Specific Needs
Obviously, stamping presses are the heart of any sheetmetal-stamping operation. Presses come in varying tonnages, configurations and means of operation. The majority of stamping presses can be classified as mechanical or hydraulic. Mechanically driven presses typically boast higher operating speeds—surpassing 2000 strokes/min. to produce parts rapidly. Relatively simple 2D parts are ideally created in mechanical presses, parts such as razor blades or electrical contacts. Hydraulically powered presses traditionally offered force control throughout the entire forming stroke, unlike traditional mechanical presses that ramp up force as the press ram descends on the work material. Though typically slower than mechanical presses, hydraulic presses, with this total force control, have been the machinery of choice to produce deeper 3D parts with cup or sink recesses. Producing parts with depth in a stamping press is referred to as drawing.
In recent years, these formerly cut-and-dried distinctions between mechanical and hydraulic presses have blurred as new press and press-control technologies enable each to assume characteristics of the other. A newer development, servo-powered presses, which are technically mechanical presses often referred to as servo presses, utilize servo drives to bring benefits of hydraulic and mechanical presses into a single machine.
Increasingly Complex Tooling
Of course, a stamping press would just be a machine that makes noise were it not for the tooling inside. One-hit dies represent the simplest form of tooling, where one press hit pounds out a complete part, or at least a shape that travels to secondary machinery for completion or to another one-hit die in another press. Progressive dies, containing multiple stations, add features to a part with each press hit as the base material travels along the die in a strip, knopwn as a carrier strip. In this manner a part is progressively formed. Transfer dies can be considered a combination of one-hit and progressive dies. Here, a material blank—without a carrier strip–travels from die to die, eventually forming a complete part. Many large drawn parts are produced via transfer dies. Transfer dies require mechanisms to physically lift a part from one die station and deposit it into the next. This is accomplished through the use of a transfer press—essentially a specialized mechanical press—or via a part-transfer system attached to an existing press.
Over the years, owing to new technology and efforts to reduce costly and time-consuming secondary operations that take place away from the press, more and more work occurs in the tooling. The result: more complicated and costly stamping dies. Given this fact, die design, maintenance, protection and utilization are so important to the metal-stamping process that a manufacturer will have on staff personnel dedicated to tooling issues.
The high cost of hard tooling such as stamping dies, and the care required to allow this tooling to produce part after part to rigid specifications, demand attention to detail in this area. To protect tooling, a die designer or metal stamper will incorporate various controls and sensors into the process. Often, sensors will be embedded into tooling to ensure presence, and correct orientation and shape of the part material. Die components such as punches may be built with and/or coated with special material. These tool coatings allow for creation of higher-quality stampings while increasing tool life. Many stamping operations, especially those tasked with performing multiple jobs on a single press line, incorporate quick-die-change (QDC) equipment. Such equipment—rolling bolsters, die carts, clamps, etc.—allow rapid changeout of tooling from one job run to the next in order to keep presses running.
Forming Lubricants Is Key to Part Quality
and Equipment Life
Proper lubrication within the tooling is essential to the protection of dies and presses, and also to the production of quality parts. Depending on the part material, type of part to be produced and type of tooling employed, specific forming fluids are used. Various lubricant formulations exhibit properties that best serve specific stamping requirements. In some cases, material is coated with lubricant prior to entering the press. In other cases, forming fluids are applied to the part material and tooling during the stamping process.
Lubricant selection reaches beyond part, tooling and material considerations. More and more, safety and regulatory concerns affect selection. Lubricants may be required to be reclaimed or disposed of safely, and must not pose a hazard to employees or the environment. Stamping-lubricant and lube-system suppliers have, therefore, developed fluid formulations and methods of delivery and reclamation to address these concerns.
Functional Stamping-Press Line Has
Many Components
Stamping a part involves more than a press, tooling and forming fluids. A fully outfitted press line includes feed machinery that delivers part material to the press. This includes equipment to transport stage and deliver coiled material into the press, or other equipment to feed a press individual material blanks. In multiple-press lines, robots or other part-handling machinery transport part material from press to press, then capture finished parts for placement in bins or racks. Conveyors or other material-handling equipment also move parts or collect and transport scrap.
2. Metal Stamping Presses and Stamping
Operations
Metal-Stamping presses can be classified according to drive mechanism—mechanical, hydraulic, servo–and press-frame construction—gap-frame, c-frame, straightside. These classifications, detailed here, impart certain important characteristics to the press. Also described are stamping processes—blanking, piercing, notching, bending, drawing, coining.
Presses function as the signature pieces of shop-floor equipment in a stamping operation. The types of metal-stamping presses depend on the nature of the stamping work. Stamping presses function by providing energy to force a ram downward, providing force for the stamping dies and tooling. Attached to the press ram is an upper die. The ram descends toward the lower die. Located between these die halves is the part material. As the die halves meet, a part is cut, shaped or otherwise worked within the tooling. The ram then ascends, a part is removed or the part material is indexed, and the stamping cycle repeats.
Sounds simple? But, generating the force necessary to stamp metal parts, especially given newer and higher-strength steels demands attention to press design and method of press motion. Designs and drive mechanisms impart characteristics to stamping-press operations that affect the ability to form parts. With stampers demanding flexibility in these expensive pieces of capital equipment, press manufactures have responded, and press technology has evolved to serve stampers’ diverse needs.
Stamping presses generally conform to two basic designs. Gap-frame presses, also referred to as c-frame presses because of their shape, are connected from bottom to top at one location, behind the work area. These presses feature lower capacities and typically perform as stand-alone machines, often manually fed. Straightside presses are supported on each side of their rectangular footprint, and given their robust construction, they are less susceptible to deflection arising from off-center loading (a condition where stresses drive the ram out of its normal, parallel condition) than gap-frame presses. Due to their beefed-up framework, straightside presses offer high capacities and often operate in press lines, either teamed with other straightside presses or outfitted with ancillary equipment.
Hydraulic Presses
Hydraulic presses depend upon the pressurization of hydraulic fluid in cylinders to provide force to the ram. These presses, by controlling the pressure, allow force control, and full force if need be, throughout the motion of the ram as the part is formed. This amount of vertical motion is referred to as the stroke length. The notion of force control throughout the stroke is important. In certain applications, such as working with difficult-to-form material, or when performing drawing operations, force control throughout the stroke, and the ability to provide full power throughout the stroke, is needed for proper forming. Force control and full-force capability, along with relatively simple maintenance and lower energy costs, are major reasons why stampers may consider choosing a hydraulic press over its mechanical kin.
At one time, hydraulic presses were seen as maintenance-intensive, given the occurrences of leaking seals and hoses attributed to the fluid-handling nature of the machines. In recent years, better hydraulic-system designs and improved seals and connectors have all but eliminated such problems. Applications for hydraulic presses include deep drawing, high-tonnage blanking at lower speeds, and short job runs, since press speed is less critical given smaller part volumes. Hydraulic presses are available in capacities from 20 to 10,000 tons, with work strokes from 0.4 to 32 in.
Mechanical Presses
Drive methods of mechanical presses differ from those of their hydraulic counterparts. In most mechanical stamping presses, flywheels, driven by motors, store energy that is then transferred to ram motion. Because flywheels expend energy with each downstroke of the ram, they slow down. That energy must be restored, and is, by the motor in time for the next stamping-press cycle to begin.
Mechanical presses operate at much faster speeds—well above 1,000 strokes/min.–than hydraulic presses, but work strokes are shorter, due to the fact that full force develops in a mechanical press near the bottom position of the press stroke. With their high-speed capability, mechanical presses get the call for many high-volume metal-stamping jobs where parts are flat or at least somewhat shallow. Mechanical presses fare well in stamping flat or low-depth parts. That is because mechanical presses can only provide full forming force in a smaller stroke range than hydraulic presses. Typical applications for mechanical presses include high-speed blanking, precision flat-part production and shallow drawing. Mechanical press capacities range from 20 to about 6,500 tons with strokes from 0.2 to 20 in.
Servo Presses
Recent years have brought new technology in the form of servo-driven presses, or servo presses, to metal stampers. Servo presses, though technically classified as mechanical presses, employ servo drives to provide power, negating the need for flywheels. Advantages of servo presses include the ability to control the stamping press’ stroke length and velocity. Another plus: Servo presses allow for dwell time at the bottom of a press stroke, where forming work occurs. This is ideal when material must be given time to flow or stretch into a part shape. Features such as these bring benefits of mechanical and hydraulic presses into a single machine, providing flexibility to the stamper.
Types of Metal-Stamping Operations
Given the proper tooling, metal-stamping presses can perform a multitude of part-shaping operations.
- Blanking: Cutting flat sheetmetal into a defined size and shape. Typically performed in one hit of the press, the result may be a finished part or a blank destined for further forming or processing into its final shape.
- Piercing: Similar to blanking, the pierced piece instead is scrapped, with the surrounding material as the part.
- Notching: Similar to piercing, but here material is removed from the edges of the workpiece.
- Bending: Sometimes referred to as forming, tooling bends workpiece material into various angles.
- Drawing: The press essentially stretches sheetmetal to a depth.
- Coining: The die forms an imprint on the workpiece.
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