These behemoth straightside mechanical presses can stamp out part after part for automotive and other markets.
Photo courtesy of The Minster Machine Co., Minster, OH.
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 are 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.
- In purchasing a new or used metal-stamping press, did you weigh the benefits of a hydraulic press over a mechanical press, or vice-versa? Did you consider a servo press as an option?
- How much effort does your company make to protect tooling and ensure metal-stamping quality via the use of sensors and press controls?
- Do you consider which type of forming fluid to use with each particular metal-stamping job, or do you tend to use the same stamping lubricant no matter the types of parts produced?