Die Science: Stamping stainless steel (part II)

Expect More Springback

Springback, also known as elastic recovery, is the result of the metal wanting to return to its original shape. Because stainless steel has inherently higher strengths and work- or strain-hardens more than plain low-carbon steel, you must overbend or overform the metal beyond the intended bend angle or shape. The amount of overbending is a judgment based on the grade of stainless steel , its hardens and thickness, and the geometry you want to achieve.

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For straight-line bending, a rotary-style bender is recommended (see Figure 1 ). This type of bender allows you to make simple, accurate adjustments with respect to the bend angle. In addition, it requires much less force than other benders to create the bend. This is especially desirable when forming hardened or high-carbon martensitic grades of stainless.

When stretch forming or drawing stainless steel, try to stretch as much surface area of your part as evenly as possible. This will reduce the amount of springback.

Avoid Overworking the Metal

Because stainless steel often work hardens much more than low-carbon steel does, it is very susceptible to compression or stress cracking. Both of these are the results of severe cold working. Certain grades of stainless may require annealing between forming stations to soften the steel and allow it to be re-formed as needed.

shows a typical stress crack. In addition, certain types of stainless steel may be susceptible to stress corrosion cracking (see). This is a combination of work hardening and corrosion often caused by chlorides in the lubricant.

Expect Greater Cutting and Forming Forces

Stainless steel typically has much greater tensile and yield strengths than low-carbon steel does. For this reason, it is critical that you choose a press that has adequate force available to perform the work.

A good rule of thumb is to double the force needed for plain low-carbon steel. Also, if you are drawing, double the blank holder force.

Slow Down

As a rule, the more slowly you form stainless steel, the better. For example, trying to deep-draw a 3-inch-diameter cup to a depth of 2.5 in. using a press with a ram speed of 70 feet per minute (FPM) will prove to be futile.

Avoid using fast blanking presses when forming stainless steel. Hydraulic presses are the best for deep drawing stainless steel. Depending on the stainless that you are forming, try not to exceed a maximum forming speed of 40 FPM.

Select the Right Lubricant

Because stainless steel often has very smooth surface topography, it is sometimes necessary to add wetting agents to the lubricant. For deep drawing cosmetically fussy parts such as kitchen sinks, poly film or thin-film barrier lubricants often are used. These thin sheets of plastic both lubricate and protect the surface (see Figure 3 ).

In deep-drawing applications, it is often necessary to use lubricants containing chlorine or sulfur. These heat-activated additives help to reduce the friction when the forming temperature get extreme. For deep drawing, lubricants such as mineral oil and motor oil will most likely prove to be useless because of their low working temperatures.

Select the Right Tool Steel

Stainless steel by its chemistry is very abrasive because it contains chromium. Hard chrome often is used to coat large stamping dies such as those for body panels. This coating of chrome gives the die greater resistance to both abrasive and adhesive wear.

When selecting a tool steel for forming stainless, use steel that has both good toughness and wear resistance. Whenever possible, avoid using conventional D-2 tool steel&#; not because of its lack of toughness or wear resistance, but because of its high chromium content. The high chrome in the stainless will interface with the chrome in the D-2, often resulting in cold welding or surface-to-surface migration. This will cause the tool steel to &#;pick up&#; some of the stainless sheet, you can solve the problem by coating the tool with physical vapor deposition (PVD) or chemical vapor disposition (CVD) coatings.

A great material to use for forming stainless steel is aluminum bronze, because it is completely dissimilar to stainless (see Figure 4 ). It often is used to make wear plates for cams and slides, as well as heels and thrust blocks. When choosing the hardness of aluminum bronze, keep in mind that harder grades might crack under severe shock load.

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When using conventional tool steels and whenever feasible, use powdered metal (PM) grades such as the CPM® or vanadium grades, because they have much greater toughness and wear resistance than conventional tool steel grades. For severe metal forming and cutting operations, it might be necessary to coat the tool steel too. Coatings reduce the friction between the stainless steel work material and the forming punches. They also often add a thin coat of high-wear carbide to the forming punches. Solid carbide is great for forming stainless steel, but because of its poor shock resistance, it often is a poor candidate in operations undergoing a great deal of impact shock.

Use Bigger Cutting Clearances

In most cases, cutting clearances are from 12 percent to 15 percent preside for soft grades and up to 20 percent for full hard grades. Generally speaking, the harder the metal, the bigger the cutting clearance.

Stainless steel is not necessarily harder to form and cut than plain carbon steel, but it is different. Pay careful attention to the type, grade, and hardness of the stainless steel than you are using. Understanding these important differences is the key.

Until next time&#; Best of luck!

The process of turning sheets of metal into a useful part or component is called sheet metal stamping. The metal is fed into a press, where the stamping tool, also known as a die, creates the desired shape. The die is pressed into or through the metal with tremendous force. The force used in the process is measured in tons.
Except for some specialized processes, sheet metal stamping doesn&#;t use heat. Instead, it is done with a cold-forming technique. Even though no heat is used, the part can come out hot because of the friction that&#;s created between the metal and the die from the force of the press.

What Is Sheet Metal Stamping?

Sheet Metal Stamping Process

There are basically only three components to sheet metal stamping&#;the sheet metal, die, and press machine&#;but any single part can require multiple steps to arrive at its final form. The following guide explains a few common processes that might occur during metal stamping.

• Forming: Forming describes the process of transforming the flat metal into another shape by applying force. It is accomplished in one of several ways, depending on the design specifications for the part. With a series of operations, the metal can be altered from a relatively simple shape into a complex one.
• Blanking: Blanking is the most basic technique and initiates when the sheet or blank is fed into the press where the die cuts out the desired shape. The resulting piece is called a blank. The blank may be the desired part, also known as a fully finished blank, or it may continue to the subsequent step of forming.
• Drawing: Drawing is a more complicated operation and is how vessels or deep depressions are formed. Tension is used to carefully draw the material into a cavity to change its shape. Though the material might stretch while it&#;s drawn, technicians try to avoid stretching as much as possible to keep the material intact. Sinks, cooking equipment, and oil pans for vehicles are usually made with drawing.
• Piercing: Piercing is almost the opposite of blanking, but instead of saving the blanks, technicians use the material around the outside of the punched area. As an example, think of cutting biscuits from a rolled-out circle of dough. During blanking, the biscuits are saved; during piercing, on the other hand, the biscuits are discarded, and the hole-riddled remains are the desired outcome.

Role of Dies in Metal Stamping

Whether you&#;re using the forming, drawing, piercing, or blanking process in sheet metal for your project, production will involve metal dies. Mounted on presses, dies are the fundamental tools that metal stamping operations rely on to precisely cut, form, and shape an array of metal parts. Manufacturers use custom dies to achieve components that match the exact specifications of your design, making anything from straightforward products like clips to intricate electronic parts.

These durable tools facilitate high-volume, precision production and support economy of scale. To help ensure that metal stamping processes are generating high-quality products, however, it&#;s important to use the right die type for your project.

Types of Dies

To address the unique requirements of highly varied projects, there are multiple types of die available for manufacturers to use. Three common varieties in metal stamping operations are compound, combination, and progressive dies.

Compound

Compound dies provide compound functionality&#;every single press stroke carries out several operations at once. As a result, they can quickly and efficiently generate even complex metal parts. When choosing a die for your needs, keep in mind that compound dies are more adept at cutting than forming.

Combination

Combination dies are alike compound dies in their single-stroke operational capabilities. However, this die variety is well-suited to both forming and cutting. This enables manufacturers to use them for products that require multiple metal forming techniques, such as bending, forming, piercing, and sheet metal blanking processes.

Progressive

These cold stamping dies move a strip of raw material through its various stations, with the die performing one stamping action per stroke at each one. The metal workpieces stay attached to the strip until they pass through the final die station, at which time they&#;re complete and cut loose. In this setup, multiple workpieces are going through the press simultaneously, with each at a different stage of the stamping process on a mold set.

Understanding Metal Stamping Presses

Just as there are multiple die types, you can use different press varieties to complete your stamping project. The three main options are:

• Mechanical presses: These highly efficient presses utilize a motor-connected flywheel device to facilitate energy transfer and storage. They&#;re compatible with progressive and transfer die stamping processes and can produce between 20 and 6,000 tons of pressing force, depending on their size. Manufacturers can quickly ramp up production as mechanical presses offer the greatest operational speeds of the three presses and have the capacity for large runs of virtually identical, repeatable goods. Mechanical presses rapidly generate shallow components with simple designs from coil and sheet metal, such as hardware and appliance or automotive components.
• Hydraulic presses: Compared to mechanical presses, hydraulic presses are slower but more versatile. They enable you to modify the die opening and stroke length and deliver a wider range of pressure levels. This accommodates deeper, more intricately shaped components requiring higher material flow. As long as your project&#;s schedule allows for this production method, hydraulic presses are the ideal choice for producing goods like bowls, cylinders, and tanks.
• Mechanical servo presses: These presses combine the advantageous features of the previous two types, offering nearly the same speeds as mechanical presses with the versatility of hydraulic presses. Programmable mechanical servo presses, however, also heighten your ability to control everything from strokes and operational speeds to slide motion and positioning. These capabilities give manufacturers more flexibility in part production as well as their choice of press speed and die type.

The ideal metal pressing process service provider will have the equipment, custom capabilities, and extensive expertise you need to sufficiently achieve your project goals.

Precision Metal Stamping Services From Aranda Tooling, Inc.

When your industry requires high-caliber precision parts, you need a stamping company that meets the most stringent quality standards. Aranda Tooling, an ISO :-registered company, has been manufacturing precision tools for companies worldwide since . Today, we produce over 1 million highly detailed parts every week for even the most technologically advanced applications.

To find out more about our stamping services, please visit our services page, or request a quote online. If you have any questions, contact us at Aranda Tooling, Inc. today.

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