What is metal stamping
Metal stamping is a cold-forming process that makes use of dies and stamping presses to transform sheet metal into different shapes. Pieces of flat sheet metal, typically referred to as blanks, are fed into a sheet metal stamping press that uses a tool and die surface to form the metal into a new shape. Production facilities and metal fabricators offering stamping services place the material to be stamped between die sections, where the use of pressure shapes and shears the material into the desired final shape for the product or component.
This article describes the metal stamping process and steps, presents the types of stamping presses typically employed, looks at the advantages of stamping compared to other fabrication processes, and explains the different types of stamping operations and their applications.
Basic Concepts of Metal Stamping
Metal stamping, also referred to as pressing, is a low-cost high-speed manufacturing process that can produce a high volume of identical metal components. Stamping operations are suitable for both short or long production runs, can be conducted with other metal forming operations, and may comprise one or more of a series of more specific processes or techniques, such as:
Punching and blanking refer to the use of a die to cut the material into specific forms. In punching operations, a scrap piece of material is removed as the punch enters the die, effectively leaving a hole in the workpiece. Blanking, on the other hand, removes a workpiece from the primary material, making that removed component the desired workpiece or blank.
Embossing is a process for creating either a raised or recessed design in sheet metal, by pressing the raw blank against a die that contains the desired shape, or by passing the material blank through a roller die.
Coining is a bending technique wherein the workpiece is stamped while placed between a die and the punch or press. This action causes the punch tip to penetrate the metal and results in accurate, repeatable bends. The deep penetration also relieves internal stresses in the metal workpiece, resulting in no spring back effects.
Bending refers to the general technique of forming metal into desired shapes such as L, U, or V-shaped profiles. The bending process for metal results in a plastic deformation which stresses above the yield point but below the tensile strength. Bending typically occurs around a single axis.
Flanging is a process of introducing a flare or flange onto a metal workpiece through the use of dies, presses, or specialized flanging machinery.
Metal stamping machines may do more than just stamping; they can punch, cut and shape metal sheets. Machines can be programmed or computer numerically controlled (CNC) to offer high precision and repeatability for each stamped piece. Electrical discharge machining (EDM) and computer-aided design (CAD) programs ensure accuracy. Various tooling machines for the dies used in the stampings are available. Progressive, forming, compound, and carbide tooling perform specific stamping needs. Progressive dies can be used to create multiple pieces on a single piece simultaneously.
Types of Stamping Operations
Progressive die stamping
Progressive die stamping uses a sequence of stamping stations. A metal coil is fed into a reciprocating stamping press with progressive stamping dies. The die moves with the press, and when the press moves down the die closes to stamp the metal and form the part. When the press moves up, the metal moves horizontally along to the next station. These movements must be precisely aligned as the part is still connected to the metal strip. The final station separates the newly-fabricated part from the rest of the metal. Progressive die stamping is ideal for long runs, because the dies last long without getting damaged, and the process is highly repeatable. Each step in the process performs a different cut, bend, or punching operation on the metal, thus gradually achieving the desired end-product shape and design. It is also a faster process with a limited amount of wasted scrap.
Transfer Die Stamping
Transfer die stamping is similar to progressive die stamping, but the part is separated from the metal trip early on in the process and is transferred from one stamping station to the next by another mechanical transport system, such as a conveyor belt. This process is usually used on larger parts that may need to be transferred to different presses.
Types of Stamping Presses
The three common types of stamping presses include mechanical, hydraulic, and mechanical servo technologies. Usually, presses are linked to an automatic feeder that sends sheet metal through the press either in coil or blank form.
Mechanical presses use a motor connected to a mechanical flywheel to transfer and store energy. Their punches can range in size from 5mm to 500mm, depending on the particular press. Mechanical pressing speed also varies, usually falling between the range of twenty and 1,500 strokes per minute, but they tend to be faster than hydraulic presses. These presses can be found in an array of sizes that stretch from twenty to 6,000 tons. They are well-suited for creating shallower and simpler parts from coils of sheet metal. They’re usually used for progressive and transfer stamping with large production runs.
Hydraulic presses use pressurized hydraulic fluid to apply force to the material. Hydraulic pistons displace fluid with a force level proportional to the diameter of the piston head, allowing for an advanced degree of control over the amount of pressure, and a more consistent pressure than a mechanical press. Additionally, they feature adjustable stroke and speed capabilities, and can typically deliver full power during any point in the stroke. These presses usually vary in size from twenty to 10,000 tons and offer stroke sizes from about 10mm to 800mm.
Hydraulic presses are usually used for smaller production runs to create more complicated and deeper stampings than mechanical presses. They allow for more flexibility because of the adjustable stroke length and controlled pressure.
Mechanical servo presses use high capacity motors instead of flywheels. They are used to create more complicated stampings at a faster speed than hydraulic presses. The stroke, slide position and motion, and the speed are controlled and programmable. They are powered by either a link-assisted drive system or a direct drive system. These presses are the most expensive of the three types discussed.
Types of Stamping Dies
Dies that are used in metal stamping operations can be characterized as either single-station or multiple-station dies.
Single-station dies include both compound dies and combination dies. Compound dies perform more than one cutting operation in a single press, such as the case of the multiple cuts needed to create a simple washer from steel.
Combination dies are ones which incorporate both cutting and non-cutting operations into a single press stroke. An example might be a die that produces a cut as well as a flange for a given metal blank.
The steel strip material used for the cutting surface is designed to match the desired shape, and a slot is cut into the die shoe to hold the steel rule material. The characteristics of the material to be cut, such as its thickness and hardness, help establish the steel rule thickness to be used in the cutting blade.
The choice of metal stamping materials used depends on the desired attributes of the finished piece. Stamping is not limited as a fabrication process to just metals - there are numerous materials that can be processed through stamping techniques, such as paper, leather, or rubber, but metals are by far the most commonly used.
In general, metals tend to maintain their malleability and ductility after stamping. Those used in precision stamping usually range from soft to medium hardness and carry a low coefficient of flow. Some of the customary metals and metal types fabricated through stamping include:
● Precious metals, such as silver, gold, and platinum
● Ferrous metals, especially iron-based alloys like stainless steel
● Non-ferrous metals, such as bronze, brass, and zinc
● Non-standard alloys, such as beryllium nickel and beryllium copper
Ferrous metals are commonly used in stamping operations, as their low carbon content means they are among the least expensive options available resulting in low unit production costs.
Several important factors and design considerations need to be addressed when performing metal stamping operations.
Post-stamping production operations can include having the stamped product go through deburring, tapping, reaming, and counterboring processes. These allow for the addition of other parts to be added to a stamped piece or for the correction of imperfections in finish or removal of sharp edges that may impact safety.
Deburring involves the removal of shards of cut material that remain on the workpiece after the stamping operation has been completed. Sharp edges may require grinding to remove burrs or may need to be flanged over to produce a smoothed edge and to direct the burred edge to the inside fold where it will not cause injuries or be noticed cosmetically.
Overly narrow projections should generally be avoided in stamped products, as these may be more easily distorted and impact the perception of quality in the finished product.
Avoidance of sharp internal and external corners in stamped product designs can help reduce the potential for the development of larger burrs in these areas and sharp edges that require secondary treatment to remove. Also, a great potential for stress concentrations exists in sharp corners, which may cause cracking or subsequent failure of the part through extended use.
Overall dimensions for the finished product are going to be limited by the available dimensions of the sheet metal sheets or blanks, and these limits need to be factored for the material consumed in folds on edges or flanges and any additional material removal or use. Very large products may need to be created in multiple steps and mechanically joined together as a second step in the production process.
For punching operations, consider both the direction of punching as well as the size of the punched feature. Generally, it is best to do punching in one direction, so that any sharp edges produced by the punch will all be on the same side of the workpiece. These edges can then be hidden for appearance purposes and kept away from general access by workers or product end-users where they might represent a hazard. Punched features should reflect the thickness of the raw material. A general rule is that punched features should be at least twice the material thickness in size.
For bends, the minimum bend radius in sheet metal is roughly the same as the material thickness. Smaller bends are more difficult to achieve and may result in points of stress concentration in the finished part that may subsequently cause issues with product quality.
When drilling or punching holes, performing these operations in the same step will help to assure their positioning, tolerance, and repeatability. As general guidelines, hole diameters should be no smaller than the material thickness, and the minimum spacing of holes should be at least twice the material thickness apart from each other.
Bending operations should be performed with awareness of the risk or distorting the material, as the material on the interior and exterior surfaces of the bend point are compressed and stretched respectively. The minimum bend radius should be approximately equal to the thickness of the workpiece, again to avoid stress concentration build up. Flange lengths should be more like three times the workpiece thickness as a good practice.
Some of the benefits of stamping include lower die costs, lower secondary costs, and a high level of automation compared to other processes. Metal stamping dies tend to be relatively less expensive to produce and maintain than those used in other common processes. Stamping machines are relatively easy to automate and can employ high-end computer-control programs that provide greater precision, faster production, and quicker turnaround times.