The rules of press brake tool selection

Many consider press brake tooling a minor accessory in metal forming when in fact the opposite is true. Although press brakes have evolved into multiaxes, high-precision machines with self-stabilizing features, the tooling is all that ever actually touches the part during bending (see Figure 1).

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The line has blurred between RFA, New Standard, European, and American standard tooling. Many features needed for high-performance bending have migrated to all the various tooling types. Regardless of which tooling and clamping style you choose, be sure it meets at least a few minimum requirements.

High precision. The tools should be manufactured to tolerances within the 0.0004-inch range. This is critical to achieve part accuracy without shimming or other tweaks during setup.

Segmented sections. These allow you to build various lengths out of several precut pieces. Small pieces are safer and easier to handle too.

Self-retaining installation. You should be able to load the tools with the ram up. The toolholding system should hold multiple pieces in place until the clamping pressure is applied (see Figure 2).

Self-seating. As clamping pressure is applied, the punches are mechanically pulled up into position. This eliminates the need to bottom the punch into the die during the setup.

Front loading. You should be able to install tools from the front of the machine. This shortens setup time because you no longer need to spend time sliding tools from the end of the press brake. In most cases, front loading also eliminates the need for forklifts and overhead cranes.

Standard sizes. Common-height tools can reduce the need for machine adjustments when changing jobs. Front support arms, backgauge heights, and safety devices all remain at a common position. And because tools are made to the same heights, you can add off-the-shelf pieces and be sure they will match your existing tools.

Many high-quality press brake tools are made to metric standards. So a nominal sized 0.250-in. V opening is actually 6 mm, or 0.236 in. Moreover, bends in sheet metal have slightly elliptical corner radii, so you only have to get close to get correct. For simplicity, imperial dimensions are rounded in this article.

Note that the discussion that follows focuses on air bending, and for good reason. The trend is to abandon bottoming or coining and embrace air bending whenever possible. Be aware, however, that not all parts can be produced using classic air bending techniques.

Operators throughout the industry use very different tooling to make parts of similar or identical quality. Plenty of operators make acceptable parts with incorrect tooling because they don’t have access to the correct tooling. They make it work; but “making it work” isn’t efficient or repeatable, and it can seriously hinder work flow. Best practices in tooling selection really should have one elegantly simple goal: to achieve the best-quality parts in the least amount of time possible.

What Tools Do You Need and Why?

A maintenance shop will need and use different press brake tools than a custom fabricator will. So before diving into specifics, identify your needs and budgetary constraints.

For instance, you might need additional tools to shorten setup times. You might follow lean manufacturing principles and recognize the benefits of having a separate tool library for each press brake—and hence, be willing to invest in duplicate sets of tools stored at machines. You don’t lose valuable setup time walking to and from the tool crib and elsewhere looking for the correct tools. An added benefit here is that tool style compatibility from machine to machine is no longer necessary, because the tools tend to stay with their intended machine (see Figure 3).

If you need to buy additional, duplicate tools to expand each brake’s dedicated tool crib, choosing them is relatively straightforward. You’ll often find these tools located in convenient places, if not already in the press brakes. Look for the tools with the most wear and tear—those with shiny, bright working surfaces. The body of the tools will likely be clean and bright too. Rusty, dirty tools on the bottom of the rack are not likely candidates.

Die Selection

To get the biggest bang for your buck, choose a minimum number of lower dies that will cover the entire range of metal thicknesses your shop forms. Shops with little tribal knowledge, unforeseen applications, and limited budgets should try selecting lower dies using the 8×2 rule.

First, determine the range of metal thicknesses you want to bend. For example, you might need to bend material 0.030 in. through 0.250 in thick.

Second, assess the smallest V die needed by multiplying the thinnest metal by 8. In this case, 0.030-in. material would need the smallest die, hence: 0.030 × 8 = 0.24, which we’ll round up to 0.25.

Third, assess the largest V die needed by multiplying the thickest metal by 8. In this case, the thickest material of 0.250 in. would need the largest die: 0.250 × 8 = 2.

You’ve now determined the smallest and largest die you need—0.25 and 2 in. To fill in what you need in between, you start with the smallest V die and double its size. In this case, that gives you a 0.5-in. die (0.25 × 2 = 0.5). Next, double the 0.5-in. die to get 1.0 in., then double that to get 2.0 in. This gives you a minimum of four different V-die openings to bend 0.030- to 0.250-in. material: 0.25, 0.5, 1.0, and 2.0 in.

Punch Selection

You also use material thickness to determine the minimum number of upper punches. For material 0.187 in. and thinner, you can use an acute offset knife punch with a 0.04-in. radius. The acute angle allows bending past 90 degrees, and the offset allows you to form J shapes. To handle the higher forces when forming material between 0.187 and 0.5 in. thick, consider a straight punch with about a 0.120-in. radius.

Note that for some applications, including those using thicker and high-tensile material, the workpiece tends to crease, crack, or even split in two when using common industry bending standards. It comes down to physics. A narrow punch tip exerts more force on the bend line; combine that with a narrow V-die opening, and the forces rise even more. For challenging applications, and especially when material thicknesses are above 0.5 in., it is best to consult your material supplier on the recommended punch tip radius.

The Rule of 8

In a perfect world, you should be able to select the V-die opening using what we call the rule of 8; that is, the V-die opening should be 8 times the material thickness. To determine this, multiply the material thickness by 8 and choose the closest available die. So if you have 0.060-in.-thick material, you need a die that’s 0.5 in. (0.060 × 8 = 0.48; 0.50 in. is the closest die width); for 0.125-in. material, you need a 1-in. die (0.125 × 8 = 1). This ratio gives the best angular performance, which is why many call it the “sweet spot” for V-die selection. Most published bending charts are centered around this formula.

Simple enough? Well, it would be in that perfect world, and you could live in that perfect world if the sheet metal designers always followed the rule of 8. But alas, in the real world, the exceptions abound.

V-die Opening Determines the Radius

When air bending mild steel, the inside bend radius forms at approximately 16 percent of the V-die opening. So if you air-bend material over a 1-in. V die, your inside bend radius will be about 0.16 in.

Say a print specifies 0.125-in. material. In a perfect world, you’d multiply that thickness by 8 and use a 1-in. V die. Simple enough. But many sheet metal designers like to specify a bend radius equal to the metal thickness. What if the print specifies an inside radius of 0.125 in.?

Again, material air-bends an inside radius that’s about 16 percent of the die opening. This means your 1-in. die can produce a radius of 0.160 in. Now what? Just use a narrower V die. A 0.75-in. die will give you an inside radius that will be close to 0.125 in. (0.75 × 0.16 = 0.12).

Similar thinking applies for prints that specify larger bend radii. Say you need to form 0.125-in.-thick mild steel to a 0.320-in. inside bend radius—more than double the material thickness. In this case, you’d choose a 2-in. die, which would produce an inside bend radius of about 0.320 in. (2 × 0.16).

There are limits to this. For instance, if you find that to achieve the specified inside bend radius you need a V-die opening that’s less than five times the metal thickness, you will compromise angular accuracy, possibly damage the machine and its tooling, and put yourself in a very unsafe situation.

Minimum Flange Length

Keep flange lengths in mind when choosing your V dies. The minimum flange a given V die can form is approximately 77 percent of its opening. So a part being formed over a 1.-in. V die will need at least a 0.77-in. flange.

Many sheet metal designers like to save metal and specify a flange that’s too short, like a 0.5-in. flange in 0.125-in. material thickness (see Figure 4). According to the rule of 8, 0.125-in.-thick material calls for a 1-in. V die—but that 1-in. V die requires the workpiece to have a flange that’s at least 0.77 in. Now what? Again, you can use a narrower V die. For instance, a 0.625-in. die can form parts with flanges as short as 0.5 in. (0.625 × 0.77 = 0.48, rounding up to 0.5).

This also has limits. Just as with tight inside bend radii, if a flange requires a die width that’s less than five times the material thickness, you’ll experience angular accuracy issues, cause possible damage to the machine and its tooling, and put yourself in harm’s way.

Punch Selection Rules

For L shapes the rules are … there are no rules. Almost any punch shape will work. So when selecting punches for a group of parts, you always should consider these L-shaped parts last, considering just about any punch shape can handle them.

When forming these L shapes, use a punch that also can form other parts, rather than adding unnecessary tools to the library. Remember, when specifying tooling, less is always best—not just to minimize tooling cost, but also to reduce setup time by reducing the number of tool shapes needed on the shop floor (see Figure 5).

Other shapes do require specific rules for punch selection. For instance, when forming J shapes, the rules are (see Figure 6):

• When the small up-leg is longer than the bottom leg, you need a gooseneck punch.
• When the small up-leg is shorter than the bottom leg, any punch shape will work.
• When the small up-leg is equal to the bottom leg, you need an offset acute punch.

As you can see, the punch selection rules deal mostly with workpiece interference, and this is where bending simulation software can play an important role. If you don’t have access to bend simulation software, you can use your tooling supplier’s drawings with grid backgrounds to check for punch-part interference manually (see Figure 7).

Offset Rules

If you’re using a conventional toolset, you’ll need to use two ram cycles to form offsets or Z shapes. For these shapes, the rules are (see Figure 8):

• The center leg (web) must be larger than half the V-die body width; note that this is the entire die body width, not the V-die opening.
• The side leg must be shorter than the V-die height plus the riser height.
• When the center leg (web) is less than half the width of the V-die body, you’ll need a special tool that forms both bends in one ram stroke. The upside with these form tools is you do not need to flip the plate over. The downside is they require about three times the standard air bending force.

Rules for Bending Across Cutouts and Miters

Any unsupported material inside the V die is subject to deformation; in holes and other cutouts, this deformation manifests itself as blowouts (see Figure 9). When the holes near the bend lines are small, the associated blowout will be small too. Also, most applications will accept some distortion, so there is no definitive rule on the best V-die width to choose when a cutout is on or near a bend line.

When the flanges, cutouts, and miters are clearly too close to the bend line for the metal thickness, you can specify rocker-type dies. The rockers rotate and support the material throughout the entire bending process and, thus, eliminate the blowout.

Figure 9 shows identical parts with cutouts close to the bend lines; the foreground one—with the telltale blowout—was formed using a conventional V die; the background one was formed with a rocker-type die. Also note that the two ovals on the left have the same width (front to back) and are the same distance from the bend line; only their lengths are different. You can clearly see more blowout on the longer oval.

Punch Height for a Given Box Depth

Punch height becomes critical when forming three- and four-sided boxes. In some cases, short punches can form three-sided boxes if one formed side can hang off the side of the press brake during the final (third) bend. If you’re forming four-sided boxes, you need to choose a punch tall enough to span the box height diagonally (see Figure 10):

Sheet metal forming is a manufacturing process that fabricates metal parts from thin metal sheets. It is commonly used in everything from aluminum soda cans to aircraft components.

This article looks at the sheet metal forming process, how it works, where it is used, and its benefits and drawbacks. We also take a look at the sub-processes used to fabricate parts from the metal sheets.

What Is Sheet Metal Forming?

Sheet metal forming, or sheet metal fabrication is the process of using metal sheets to form sheet metal components. This is achieved by forming and cutting sheets into the required shapes and forms using a variety of processes such as: bending, punching, shearing, and hydroforming among others.

Sheet metal forming is extremely common in manufacturing, and sheet metal parts can be found in a wide range of applications, such as: the automotive industry, aerospace industry, consumer goods, and household appliances.

What Is the Purpose of Sheet Metal Forming?

The purpose of sheet metal forming is to create parts made from thin sheet metal for use in a wide range of applications. Sheet metal forming is an extremely versatile process that takes advantage of the strength and malleability of sheet metal to create durable parts that are lower in cost than comparable manufacturing processes such as forging.

How Does Sheet Metal Forming Work?

Sheet metal forming makes use of a number of processes to transform a flat sheet of metal into metal parts that are durable and lightweight. Bending, shearing, punching, and hydroforming are all examples of processes that are used to form or cut metal sheets into the desired shape.

The tools for these processes will be set up according to the needs of the design, after which the sheet will undergo processing. Finally, a number of finishing steps will be applied. This might include: deburring, surface treatments, or welding.

What Thickness of Metal Is Suitable for Sheet Metal Forming?

Sheet metal forming is suitable for metal thicknesses of 0.6 mm to 6.35 mm. This is a general guideline, and the actual suitable thickness will change depending on the type of metal used, the manufacturer's capabilities, and the complexity of the metal part to be fabricated.

What Equipment Is Used in Sheet Metal Forming?

Sheet metal forming makes use of a wide range of equipment depending on the design of the part and the process being applied. The following list highlights some of the equipment used in sheet metal forming:

1. Punches and dies are commonly used for shearing and cutting operations whenever specific shaped holes are required.
2. Bending machines are used for bending operations.
3. Rollers are used for shaping sheets into conical or cylindrical shapes.
4. Shearing tools are used for shearing in a straight line.

What Are the Sheet Metal Forming Processes?

Sheet metal forming involves a number of manufacturing processes. The following list contains some of the primary sheet metal forming operations:

1. Curling

Curling is a process that adds a circular, hollow roll to the edge of sheet metal. This is done to deburr the edges, but also to add strength and make the sheets safer to handle. Metal sheets are usually fed into specialized machines that gradually roll or bend the edges to form a smooth, rounded profile.

Curling is commonly used in HVAC, appliance manufacturing, and architectural applications to create edges on panels, trim, or housings. Most metals can undergo curling, depending on their ductility and thickness, including: steel, aluminum, and brass.

Curling adds strength to the edges of parts and improves rigidity and safety. Complex shapes or tight rolls can be challenging to achieve through curling. Specialized tooling and machinery may also be required to perform curling, depending on the application.

2. Laser Cutting

Laser cutting is a process of using a high-powered laser to cut shapes into sheet metal. A sheet metal blank is fastened onto the laser cutter machine bed. A computer numerical control (CNC) system controls the movement of the laser beam, performing precise, clean cuts in a pre-programmed pattern.

Laser cutting is used in applications that require precise shapes, patterns, or holes to be cut into sheet metal. It is commonly used in the automotive, aerospace, and electronics industries. Laser cutting can be applied to many metals, such as: steel, stainless steel, aluminum, or copper. Galvanized steel can be cut with laser cutting, but this is not recommended, as the high heat can lead to the protective coating being damaged, as well as toxic zinc-oxide fumes being released.

Laser cutting offers high accuracy, versatility, and repeatability. Minimal post-processing is needed, and there is minimal material waste. However, the initial equipment cost can be high.

To learn more, see our guide on Sheet Metal Laser Cutting.

3. Bending

Bending involves using bending tools in order to create bends or curves along a straight axis of the metal sheet. Press brakes are commonly used, into which the metal is set or clamped and then bent to the desired angle. Bending is commonly used for making automotive body parts, enclosures, and electrical components. The process is suitable for most types of metals commonly used in sheet metal fabrication, such as: stainless steel, brass, aluminum, and galvanized steel.

Sheet metal bending is quick, accurate, low-cost, and requires fairly simple tooling. Some disadvantages include limitations to the thickness of metal that can be bent, as well as the need for consistent thickness.

To learn more, see our guide on Sheet Metal Bending.

4. Ironing

Ironing is a process used to improve the surface finish of sheet metal parts and achieve uniform thickness. A metal part is pressed through a die or series of dies which incrementally reduces clearance. By passing through the die, the walls of the part are thinned and elongated, without altering the shape significantly.

Ironing is used for producing aluminum cans, but also for other applications in which a consistent thickness and a good surface finish are required. Metals with high ductility are suitable candidates for ironing, including steel and aluminum.

Ironing improves dimensional accuracy, surface finish, and thickness uniformity in metal parts, but requires specialized tooling and machinery. It is also unsuited for parts that require significant shape changes.

5. Hydroforming

Hydroforming is a process of shaping metals into complex forms by using high-pressure fluid. A metal blank is placed within a die cavity, whereafter high-pressure fluid is pumped onto the blank, forcing it into the shape of the die.

Hydroforming is commonly used in the automotive, medical, and aerospace industries, for which complex shapes are often required. Metals with high ductility are suitable for hydroforming, like: aluminum, stainless steel, and brass.

Hydroforming can create complex shapes with uniform wall thickness, with reduced waste and relatively low cost compared to other forming methods. Disadvantages of hydroforming include the specialized equipment that is needed, which requires a high initial investment, as well as the specialized expertise needed to perform hydroforming.

6. Deep Drawing

Deep drawing is a process used to create hollow cylindrical shapes. It works by placing sheet metal over a die and pressing the metal blank into the die cavity using a punch, resulting in a hollow cylindrical shape with no reduction in the thickness of the sheet metal.

Deep drawing is used for creating containers, beverage cans, and automotive parts such as door panels. The process can be suitable with any metals with high ductility and malleability. Aluminum, stainless steel, copper, and brass are commonly used in deep drawing.

Deep drawing allows for the creation of complex hollow shapes with consistent wall thickness and precise dimensions. It requires minimal labor and is cost-effective at high production volumes.

7. Shearing

Shearing is a cutting process used to cut sheet metal along a straight line. The sheet metal blank is positioned on a shearing machine, which has two blades that slide past each other to cut the metal. The blank is clamped in place, and the upper blade of the shearing machine is lowered onto the blank to make the cut.

Shearing is used to cut metal sheets into smaller pieces or to trim edges. It is widely used in the manufacturing, construction, and automotive industries. Steel, aluminum, and stainless steel, along with most other metals can be sheared. The thickness of the metal and the capability of the shearing machine are the limiting factors as to what can be sheared.

Shearing is a rapid process, produces minimal waste, and is cost-effective at high volumes, but can cause edge deformation and burrs, especially in thicker materials. Shearing only cuts along a straight line, and must be combined with other processes to produce complex shapes.

8. Punching

Punching is the process of removing material from sheet metal to create holes, slots, or specific shapes. Sheet metal is placed between a punch and a die. The die supports the sheet metal, while the punch is forced through the sheet metal into the die to create the hole or shape.

Punching is also used for creating enclosures, brackets, and panels. Most metals can be used for punching, with the thickness of the sheet and the capabilities of the punching machine being the limiting factor.

Punching is an automated, rapid process. It has good repeatability and is a highly efficient process, especially for high-volume production. On the other hand, tooling costs can be high, especially for custom shapes. Some post-processing may also be required, especially for intricate designs.

What Are the Materials Suitable for Sheet Metal Forming?

Sheet metal forming can be applied to a wide variety of metals, depending on the application and specific properties required from the sheet metal part. Some common metals used in sheet metal forming are:

1. Stainless Steel: Widely used in the medical and food industries. It has high strength, corrosion resistance, and good formability.
2. Aluminum: A number of properties make it ideal for sheet metal forming, including good malleability, corrosion resistance, thermal and electrical conductivity, and an excellent strength-to-weight ratio. It is used in applications such as: household and industrial appliances, components for the aerospace industry, and in power lines.
3. Hot-Rolled Steel: Relatively easy and cheap to produce. It is commonly used whenever price is more important than precision, such as structural components in the construction industry, automotive chassis components, and railroad tracks.
4. Cold-rolled steel: Goes through more manufacturing steps than hot-rolled steel, which increases its tensile strength. It is usually used in home appliances, structural components, and the aerospace industry.
5. Galvanized Steel: Durable metal with good corrosion resistance. It is commonly used in roof structure applications, air conditioning, and refrigeration.
6. Copper: Very malleable, which makes it a good option for sheet metal forming. It has good electrical and thermal conductivity and is usually used for electrical applications.

Can Galvanized Steel Be Used in Sheet Metal Forming?

Yes, galvanized steel can be used in sheet metal forming. It is commonly selected for its high durability and corrosion resistance, as well as its relatively low cost of production. Some common applications of galvanized steel parts include: roofing applications, air conditioning, refrigeration, and industrial machinery.

Is Tool Steel Suitable for Sheet Metal Forming?

No, tool steel is simply not suitable for sheet metal forming. Tool steel is extremely hard and brittle, and it would break if subjected to the common metal-forming processes. Sheet metal forming relies on the ductility and malleability of metal sheets to form them into metal parts. Tool steels are generally hard with low ductility and malleability. This makes them less than ideal for sheet metal forming.

How Useful Is Sheet Metal Forming?

Sheet metal fabrication is extremely useful for a wide range of applications, as evidenced by how commonly the manufacturing process is used. Sheet metal forming is a relatively low-cost, extremely versatile process that can produce an almost limitless variety of parts.

Products made by sheet metal forming can be seen all around, in everything from beverage cans, household appliances, and metal furniture, to HVAC systems, structural components, and components for the aerospace industry.

Which Industries Typically Used Sheet Metal Forming?

Sheet metal forming is used in an incredibly wide range of industries, thanks to its versatility, relatively low cost, and the durability of the produced parts. Some industries that commonly use sheet metal forming are:

1. Aerospace: Uses sheet metal forming due to the general precision of the forming processes, along with the high strength-to-weight ratio of the produced parts. It is commonly used for fuselages and structural components in aircraft and spacecraft.
2. Automotive: Uses sheet metal forming for body panels, chassis components, and other structural parts of the vehicles. The durable, lightweight components produced by sheet metal forming make the method an ideal choice.
3. Construction: Makes use of sheet metal forming to produce durable, weather-resistant components, such as: roofing sheets, structural components, and ductwork.
4. Medical Equipment: Housings and enclosures for medical equipment, as well as medical devices, are commonly made by sheet metal forming, due to its ability to meet strict standards and requirements.
5. Household Appliances: Sheet metal forming is commonly used for making enclosures, frames, and other structural components used in household appliances due to its versatility, precision, and durability.

Do Construction Industries Use Sheet Metal Forming?

Yes. The construction industry uses sheet metal forming for a wide range of applications. The relatively low cost and versatility of sheet metal forming, combined with the durability of sheet metal components make it an obvious choice for use in the construction industry.

Structural components such as: beams, columns, and brackets are made using sheet metal forming processes and are used for their high strength and load-bearing capabilities. Roofing sheets fabricated via sheet metal forming are used for their longevity and weather resistance. Ductwork for HVAC systems is commonly made using sheet metal forming.

What Are the Advantages of Sheet Metal Forming?

Sheet metal forming has a number of advantages and benefits. Here are some advantages of sheet metal forming:

1. Relatively inexpensive when compared to other manufacturing processes such as casting or forging.
2. Very flexible in terms of design and application. Sheet metal can be formed in various ways with a wide variety of metals in order to produce a wide range of parts.
3. Sheet metal parts are extremely durable, with good tensile strength and environmental resistance.
4. Suitable for producing small batches, which makes it ideal for rapid prototyping.
5. The use of thin sheets of metal makes sheet metal parts lightweight, while still remaining durable. This is ideal for applications in which a high strength-to-weight ratio is required.

What Are the Disadvantages of Sheet Metal Forming?

Sheet metal forming does have some drawbacks. Here are some disadvantages of sheet metal forming:

1. To start manufacturing with sheet metal forming, a high initial investment is required to acquire the necessary equipment and tooling.
2. Very complex designs can be unsuitable for sheet metal forming and result in higher costs and production times.
3. Sheet metal forming is a fairly labor-intensive process with many manual steps. This can make it slower than other processes such as stamping.

Is Sheet Metal Forming Expensive?

No. Sheet metal forming is relatively inexpensive and is known as a low-cost manufacturing process. The cost of sheet metal forming will change depending on the design of the part, the material used, and the number of processing steps involved. Fabrication costs can be kept low by sticking to fairly simple design elements and using standard sizes and options.

Summary

This article presented sheet metal forming, explained it, and discussed how it works and its processes. To learn more about sheet metal forming, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities, including sheet cutting and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

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