Steel production and processing are continuous operations where the last step is coiling. Steelmakers and processors use tension when coiling to avoid producing “soft” or collapsing coils. Coiling induces tensile and compressive stresses into the strip, and these stresses can contribute to blank or part distortion in subsequent processes. Unless sufficient winding tension adjustments are made, the degree of these stresses change throughout the coil – whereas the outer laps of the coil may be on the order of 6 feet ( mm) in diameter, the inner laps typically are wound on a 20 inch to 24 inch (500 mm to 600 mm) diameter mandrel. In addition, the magnitude of these stresses increases with higher strength products, leading to coil shape imperfections like coil set and crossbow.
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Coil set is a bow condition parallel with the rolling direction, and curves downward in the same direction as the upper outside lap of an overwound coil (Figure 1a). Here, the top surface of the coil or strip is stretched more than the bottom surface, and typically becomes more severe as the coil is processed and the lap diameter decreases. Crossbow is a bow condition perpendicular to the rolling direction, and curves downward in the same direction as the upper outside lap of an overwound coil, with the center portion of the sheet raised a measurable amount above the sheet edges (Figure 1b).
The first operation when unwinding a coil is some type of shape correction to ensure flatness before further processing. There are two main types of equipment used to create a flat coil – a straightener and a precision leveler. While these two types of equipment are similar, a precision leveler has additional capabilities. Both bend the coil back and forth over a series of work rolls to alternately stretch and compress the upper and lower surfaces (Figure 2). Critical equipment parameters include roll diameter, roll spacing, backup rolls, roll material type, gear design, backup rolls, overall system rigidity, and power requirements. The amount of force required to relieve the residual stresses is a function of the sheet thickness and yield strength. Equipment sufficient for shape correction on conventional grades may not be sufficient to completely flatten the advanced steel grades available now and in the future.
Straighteners and levelers have a series of rolls that progressively flex the strip to remove the residual stresses. Each successive roll pair has an adjustable gap to deform the sheet to a targeted amount with the goal of resulting in a flat coil once the steel passes through all the rolls. The entry end has the smallest gap, putting in the most deformation. The last pair of rolls has the largest gap, usually set for metal thickness. The gap profile varies based on thickness, yield strength, and equipment (Figure 3). Many equipment manufacturers have generated tables to guide the operator as to the best settings for various yield strength/thickness combinations.
Removing coil set requires permanent yielding in the outer 20 percent of the top and bottom surfaces of the metal. The central 80 percent of the thickness remains unchanged.T-14 Straighteners are appropriate for this type of shape correction (Figure 4). Only end bearings support the simplest straighteners, with no backup rolls used. Closing the entry roll gap risks deflection of the unsupported center, potentially leading to creating edge waves in the coil.
Eliminating crossbow and other shape imperfections like buckles or waves requires permanent yielding in the outer 80 percent of the top and bottom surfaces, with only the central core — 20 percent — remaining in the elastic range.T-14 Precision levelers, which applies tension to the strip as it bends around more smaller diameter rolls, can achieve this deformation (Figure 5). While this deformation can get the coil shape closer to flat, it also reduces the inherent formability of the grade. Processors should use only the least amount of deformation necessary to correct the shape to retain sufficient formability for stamping or other operations.
Yield point elongation (YPE), Lüders lines, and stretcher strains are names describing the same phenomenon seen in some annealed or aged metals. A related defect called fluting occurs in V-bending. Leveling at-risk coils with repeated cycles of bending and unbending, like shown in Figure 3, may be an effective way to minimize stretcher strains or fluting. However, process control is critical, since excessive leveling work hardens the coil and results in increased strength and reduced ductility. On the other hand, insufficient leveling does not address the defects related to the yield-point phenomenon.
Recent studies K-24, K-48, K-49 describe the importance of sufficient leveling, using real-world examples as well as simulation to model the phenomena and show potential corrective actions, as shown in the following animations.K-50
Figure 6 shows an animation of V-bending without any roller leveling. The fluting defect occurs, since the formed panel shape does not conform to the punch. Figure 7 is an animation of leveling with roller penetration deep enough to produce deformation equivalent to an 85% plastic fraction. Figure 8 presents a closer view of the V-bending, highlighting improved formed panel shape conformance to the punch. The references cited above detail the simulation methodology.
The progressively higher yield strengths for AHSS are challenging the capabilities of straighteners and precision levelers that were not designed for flattening these high strength materials. Equipment manufacturers have been studying and developing solutions to address this issue. There are a series of factors related to the design of straighteners and precision levelers affected by advanced steel grades:
Roll Diameter – Leveling rolls for AHSS generally are smaller in diameter than those used for mild steel, providing a smaller radius around which to bend the material. This is because exceeding the higher yield strength of Advanced High Strength Steels requires a more aggressive bend.
Roll Spacing – Work roll center-spacing will be closer for AHSS than for comparable mild steels. Closer spacing leads to the requirement of more force to reverse-bend the material, resulting in greater power requirements for processing.
Roll Support – Larger journal diameters with larger radii and bearing capacity will withstand the greater forces and higher power required to straighten AHSS.
Roll Depth Penetration – The upper rolls must have enough travel to be able to penetrate the lower fixed rolls sufficiently so the deformation exceeds the yield strength of the AHSS grade. This penetration may need to be as much as 50 to 60 percent greater than for mild steels.
Roll Deflection – Given the greater force requirements for straightening AHSS, work roll deflection becomes a concern especially with smaller-diameter rolls more likely to flex and deflect. Processing wider sheet also increases the deflection risk. Excessive work roll deflection results in undesirable side effects such as edge waves, increased journal stresses and premature gear failure. Backup rollers prevent excessive work roll deflection.
Roll Material – Higher strength materials and special heat treatment should be employed to ensure rolls can withstand greater stresses for longer periods without experiencing fatigue failure.
Gear Materials – Gears that drive the rolls should be produced from heat treated high strength materials to produce smooth running, chatter free roll drive for long life under high loads.
Gear Positioning – Closer roll center spacing requires higher power transmission and results in a smaller gear-pitch ratio, which reduces gear power ratings.
Gear Sizes – To compensate for the gear positioning issue, flattening AHSS grades requires wider gear faces as well as stronger outboard support of journals and idler shafts to produce higher gear power ratings.
Frame Rigidity – The higher strength of advanced steels results in stresses throughout all the components of the processing unit. Frame rigidity is vital to prevent work roll deflection.
Equipment manufacturers have also developed design solutions that address processing of AHSS. As an example, several manufacturers have designed equipment with removable cartridges allowing for swapping between sets containing differently sized rolls, gears, and support structures. As they switch jobs from AHSS to conventional steels, they swap in the appropriate cartridge. This also allows for off-line roll cleaning and maintenance.
Remember that the likelihood of coil set and residual stresses in the coil increases with strength. Operators must take proper precautions when cutting the strapping banks used in coil shipment to avoid “clock-springing.”
Newer processing equipment may contain additional hold-down arms or other features to protect both plant personnel and equipment from damage.E-11
Stamping AHSS materials can affect the size, strength, power and overall configuration of every major piece of the press line, including material-handling equipment, coil straighteners, feed systems and presses.
Higher-strength materials, due to their greater yield strengths, have a greater tendency to retain coil set. This requires greater horsepower to straighten the material to an acceptable level of flatness. Straightening higher-strength coils requires larger-diameter rolls and wider roll spacing in order to work the stronger material more effectively. But increasing roll diameter and center distances on straighteners to accommodate higher-strength steels limits the range of materials that can effectively be straightened. A straightener capable of processing 600-mm-wide coils to 10 mm thick in mild steel may still straighten 1.5-mm-thick material successfully. But a straightener sized to run the same width and thickness of DP steel might only be capable of straightening 2.5 mm or 3.0-mm thick mild steel. This limitation is primarily due to the larger rolls and broadly spaced centers necessary to run AHSS materials. The larger rolls, journals and broader center distances safeguard the straightener from potential damage caused by the higher stresses.
Through the cold-forming process of metal stamping, sheet metal is transformed into different shapes. The metal-stamping process uses a combination of dies and stamping presses to convert pieces of flat metal into usable shapes for a variety of industries.
From the automotive industry to aerospace projects to medical machinery, many industries throughout the globe heavily rely on metal stamping for their equipment. There is an increasing need worldwide for mass-produced quantities of complex parts, and metal stamping is a cost-efficient way to do so.
Metal stamping is a complicated process that can be completed through a variety of
metal-forming techniques. Some examples include bending, punching, blanking, spinning, hydroforming, or piercing the metal to alter it to the ideal shape.
In this article, we’ve outlined the metal stamping process and some different types of metal stamping techniques to help you get a better understanding of how this process works.
The metal stamping process also referred to as metal pressing, consists of taking flat sheet metal and feeding it into a stamping press. This press then uses pressure and die to transform the metal into the predetermined shape.
Some examples of stamping techniques used include punching, coining, embossing, bending, blanking, and flanging.
To ensure the metal piece turns out the way it is supposed to, stamping professionals need to use the design tool that comes with CAD/CAM engineering technology. These designs must be as accurate as possible for each part to come out looking and operating how it should.
Because the metal stamping process can turn out a high volume of the same part in a short amount of time, it is used in a variety of industries.
Metal stamping can help produce:
Not only can metal stamping machines stamp, but they can also cut, punch, cast, and shape metal sheets into their desired shape and size. Stamping machines are designed to be as accurate as possible, and they can also be programmed to ensure accuracy.
There are a variety of metal stamping techniques that are utilized for particular applications. We’ve outlined some examples below.
This metal stamping technique breaks down the process into a series of stations that each serve a different function. The stock strip carries the part from one station to the next until it is finished and removed from the strip.
The original metal strip travels from its coil and makes its way into the die press. From there, the metal moves horizontally across each station. As soon as the desired bend, punch, or cut is made, the part moves onto the next station until it’s completed.
The part is still connected to the metal strip while it is being altered, so each movement must be as accurate as possible. This precision and accuracy assure that the end part is accurate and usable.
Progressive die stamping is a beneficial way to produce mass quantities of parts at a low cost, making it an attractive option for many. The die lasts for a long time and it’s easy to repeat the process as many times as needed. This type of metal stamping also produces minimal wasted scrap.
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However, without a high level of precision and accuracy during progressive die stamping, the parts may not be accurately made.
Another type of metal stamping is transfer die stamping. It is similar to progressive die stamping, but a mechanical transport system moves the part from station to station instead of an automated strip. This type of stamping is useful for making parts that need to be free from the metal strip during the stamping process.
This may be a single die on its own or it may require multiple dies or machines in a row to create a makeshift production line.
The lower tooling costs associated with transfer die stamping make it an excellent option for production. They are also versatile and can produce features such as cut-outs, ribs, knurls, threading, and pierced holes. This means that no secondary operations are required, further cutting back on cost.
Transfer die stamping is used in markets such as agriculture, automotive, locomotive, electronics, medical, recreational vehicles, and more. It’s beneficial in creating larger, bulkier parts that may require the use of multiple presses.
Another type of metal stamping is fine blanking, also referred to as fine-edge blanking. This specialty type of metal stamping is ideal for achieving flatness and a full-sheared edge in a way that other metal stamping techniques cannot.
This technique uses cold extrusion and it is a single-step process, resulting in a lower fabrication cost.
Fine blanking is typically completed using a hydraulic or mechanical press, or a combination of the two. There are three recognizable movements used in the fine blanking process:
The product is manufactured and needs to have complete contact with the surface on the part’s edge to be effective.
One example of a part that is often made using fine blanking is a gear. Gears require flatness in certain areas and every tooth needs to be fully engaged to be effective, making fine blanking the perfect candidate for this type of production.
The fine blanking press runs at much higher pressures than more conventional stamping techniques, so the machinery must be designed to withstand these higher pressures.
Four-slide stamping, also referred to as multi-slide stamping, uses four tools simultaneously to form the piece into the proper shape. As the metal sheet is fed through the four-slide, it is quickly bent by each shaft.
It is ideal for producing products that require intricate cuts or complicated bends. While other metal stamping techniques use a vertical slide during production, four-slide stamping uses four different sliding tools to shape the metal piece.
The tool slides used in four-slide stamping are highly productive and straightforward, meaning this is an ideal method for creating complex or multi-part bends, even bends that are larger than 90 degrees. Multiple bends, punches, folds, or cuts can be completed in just one step.
Another advantage of four-slide stamping is the tooling itself is relatively inexpensive and there is minimal waste produced during the process.
Stamping presses play an integral role in the process, and there are three common types: mechanical, hydraulic, and mechanical servo. The distinction between the three types comes from the power source.
In most cases, the stamping press is linked to an automatic feeder that moves the sheet metal through the press. We’ll get into detail on each type of stamping press below.
Mechanical stamping presses have a motor that’s connected to the mechanical flywheel. The flywheel then stores that energy to help the machine operate. These presses can produce a variety of punch sizes, ranging anywhere from 5mm to 500mm depending on the specific press being used.
The speed of mechanical presses ranges as well. It can operate as slow as 20 strokes per minute or operate as fast as 1,500 strokes per minute. In the case that additional energy is needed when the speed is lower, the operator can add an auxiliary flywheel to the drive. Mechanical presses are typically faster than hydraulic presses, however.
The press stroke length is kept as short as possible because it has such an impact on speed. On average, each stroke is 2 inches.
The operator of the mechanical press will use electronic control, a clutch, and a brake to help engage and disengage the press drive as necessary.
Mechanical stamping presses are most often used to create simple and shallow parts made from coils of sheet metal, most notably for large production runs for transfer and progressive stamping.
As the name implies, hydraulic stamping presses use the hydraulic fluid as the power source. The technology used in hydraulic presses has significantly improved over time, with each improvement offering an upgrade in electronics and valves.
The hydraulic pistons release fluid that has a proportional amount of force to the piston head’s diameter. This allows for a high level of control over how much pressure is applied at any given moment. Despite so much flexibility in its stroke and speed capabilities, hydraulic presses can still deliver full power at any point in the stroke.
The pressure used in hydraulic presses may be preset by the user. The velocity can also be tweaked to adjust how fast the slide travels while the die closes. When the right pressure level is achieved, the valve helps activate pressure reversal to avoid overloading.
Hydraulic presses range anywhere from 20 tons to 10,000 tons. Their stroke sizes are anywhere from 10mm to 800 mm. This type of press is ideal for creating small, intricate parts thanks to its changeable stroke length and consistent pressure.
Mechanical servo stamping presses don’t use flywheels as their power source. Instead, they rely on high-capacity motors to ultimately create intricate designs at a faster pace than standard hydraulic presses.
Servo presses are highly programmable, making it easy to have control over the stroke, slide position, motion, and speed of the press. These machines operate on a link-assisted drive system or a direct drive system.
Their high speed and high levels of customization make mechanical servo presses the most expensive option out of the three.
Now let’s discuss the dies used in metal stamping. There are two types of dyes used: single-station dies or multi-station dies.
A single-station die is made from compound dies and combination dies; a compound die is capable of performing multiple operations in a single press. For example, if a steel washer needs to be made in a single press, it would need multiple cuts, thus requiring compound dies.
Combination dies use cutting and non-cutting operations in one stroke. For example, a combination die may produce both a cut and a flange when shaping a metal blank.
The other type of die used in metal stamping is multi-station dies. Progressive dies and transfer dies are included in this category. Examples of operations resulting from this die-set include cutting, punching, or notching.
Another noteworthy die used in stamping presses is steel rule dies, commonly referred to as knife dies. This type of die was first used on soft non-metal materials such as paper, cardboard, or leather. Over time, knife dies have also been utilized on copper, brass, and aluminum.
Let’s now compare the pros and the cons of metal stamping.
Advantages of metal stamping include:
On the other hand, there are also some downsides to metal stamping. Disadvantages of stamping include:
Now that we’ve covered the basics of metal stamping, let’s look at how this process is used and applied. It is used in a variety of ways, especially for the creation of three-dimensional designs. Chances are high that there are appliances or light fixtures in your home that were created through metal stamping.
It is utilized in the automotive industry, the lighting industry, the automotive industry, the medical field, the electronic industry, telecommunications services, and so many more. From simple components to extremely intricate machines and parts, metal stamping has seemingly endless applications.
Metal stamping is a widely used process that has applications relevant to your daily lifestyle. If you’re interested in learning more about metal stamping or any of our other metal capabilities, Schaumburg Specialties is happy to assist you.
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