Product Design Engineering: Sheet Metal Design

Sheet Metal Design:

Three basic processes compose the majority of sheet metal manufacturing:

Bending
Forming
Blanking (also called shearing)

These processes use tools that apply force to thin sheet material deforming or shearing the material to a specific shape. It's important when designing a sheet metal part to understand the physical limitations of bending, forming, and shearing. Following basic design rules when designing quality sheet metal parts can tremendously impact the processes and costs required to manufacture the part.

Bending Concepts

Bending is the uniform straining of material around a straight axis. The bend must take place in the plastic range of the material for the bend to remain permanent. When the sheet is bent, the inside radius is in compression and the outside radius is in tension. This distorts material differently on each side causing the sheet to become longer overall. Only at the neutral axis does the sheet retain its initial dimensions.

Two common ways to bend sheet metal are V-bending, and punch and die bending. The V-bend is usually accomplished by using a brake press. A brake press has a long bed and a selection of standard V-blocks that can make acute, obtuse, and 90-degree bends. The punch and die technique uses a punch that bends the sheet over a die. A pressure pad holds the material in place during the bend. This is commonly known as forming.

Blanking Concepts

Blanking uses a tool, usually a punch and die, to stamp out a peripheral shape in a single stroke. You can add other features into the tool to produce additional shapes, such as holes and internal profiles, as well as bent flanges in more advanced tools. To blank a profile, break or shear away the material from the parent material. As illustrated in the following figure, the shearing action takes place by stretching the material into a die, which puts the top of the material into compression and the bottom of the material into tension.

This causes a reduction in the cross-sectional area at the edges of the punch and die. Fractures usually occur in the reduced areas, which may eventually break as more force is applied.

Flat Patterns

The flat pattern is a two-dimensional layout of a three-dimensional part defining the shape and size of the flat sheet before it is formed. The flat pattern determines the bend radius length when it is flat and adds this length to the straight sections of the part. A mathematical formula, the Bend Allowance Formula (BAF), calculates the flat length of the bend radius.

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Types of Bending:

Air Bending, Bottom Bending and Coining are the three types of bending most often employed by precision metal fabricators.

Coining:

The term “coining” comes from coin making. In order to put the Lincoln profile on a penny, machines using extremely high tonnage compress a metal disc with enough force to make the metal conform to the image inscribed on the die set.

In the same vein, “coining” with a press brake involves using enough tonnage to conform the sheet metal to the exact angle of the punch and die being used. In coining, the sheet metal is more than just bent, it is actually thinned by the impact of the punch and die, as it is compressed between them along the bending surfaces.

The theory behind coining is that with enough tonnage, your sheet metal will bend to the precise angle of your tooling, so your tooling should be an equal match to the angle you want.

Bottom Bending:

In bottom bending or “bottoming” the punch and die are brought together so that the material makes contact with the punch tip and the sidewalls of the V-opening.

It differs from coining in that the punch and die don't make full contact with the metal, and there isn’t enough tonnage used to actually imprint, or thin the metal.

Because bottom bending uses less tonnage than coining, the material doesn’t entirely conform to the bend angle of the tooling. In fact, with bottoming, the metal experiences what’s referred to as “springback,” which is what happens when it relaxes to a wider angle after being bent. So, with bottom bending, in order to get a certain angle, you need to use tooling that has a slightly more acute angle in order to account for the springback that will naturally occur once the sheet metal is released. For example, you may need your punch and die to be at 88° to achieve a 90° finished form. Different materials and thicknesses result in different amounts of springback.

Air Bending:

With air bending, even less contact is made with the metal than with bottom bending. The tooling only touches the material at three points: the punch tip and the die shoulders. For this reason, the actual angle of the tooling is relatively unimportant. The factor that determines the bend angle is how far the punch descends into the die. The further the punch descends, the more acute the bend angle. Because the depth of stroke (and not the tooling) determines the bend angle, one can get a whole range of bend angles from one set of tooling. Your bend angle is only limited in that you can’t get equal to or smaller than the angle of your punch and die.

Since tonnage doesn’t produce the bend in air bending, you don’t need as much as with coining. And as with bottom bending, there will be a certain amount of springback expected in air bending, so you will likely need to bend to a slightly more acute angle in order to get the final bend you are looking for.

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Bend Allowance Formula

The Bend Allowance is calculated near the center of the material thickness. The neutral axis, which is the location of the calculation, locates where the bending forces, the tension, and compression are zero.

The location of the neutral axis depends on many factors, such as the hardness of the material being bent, the bend radius, and the process used for the bending. Use a factor called the K Factor to calculate the neutral axis in the bend allowance formula. Common values for K Factors work well for parts with standard tolerances. A general rule for the K Factors is to use .33 if the bend radius is less than the material thickness, .44 if the bend radius is one to two times the material thickness, and .5 if the bend radius is more than two times the material thickness. For precise bending, the actual K Factor is determined by trial and error based on the material and the process. The general formula for bend allowance is illustrated in the following figure.

The bend allowance must be calculated for each bend and added to the straight sections of the part to calculate the overall developed length of the flat pattern. The bend lines and bend tangent lines are included in the flat pattern to assist manufacturing personnel in aligning the bending equipment. The following figure illustrates a typical flat pattern with the appropriate information included.

Many variations of these formulas exist and are readily available online. These variations may often seem to be at odds with one another, but they are invariably the same formulas simplified or combined. What is presented here are the unsimplified formulas. All formulas use the following keys:

BA = bend allowance
BD = bend deduction
R = inside bend radius
K = K-Factor, which is t / T
T = material thickness
t = distance from inside face to the neutral line
A = bend angle in degrees (the angle through which the material is bent)

The neutral line (also called the neutral axis) is an imaginary line that can be drawn through the cross-section of the workpiece that represents the lack of any internal forces. Its location in the material is a function of the forces used to form the part and the material yield and tensile strengths. In the bend region, the material between the neutral line and the inside radius will be under compression during the bend. The material between the neutral line and the outside radius will be under tension during the bend.

Both bend deduction and bend allowance represent the difference between the neutral line or unbent flat pattern (the required length of the material prior to bending) and the formed bend. Subtracting them from the combined length of both flanges gives the flat pattern length. The question of which formula to use is determined by the dimensioning method used to define the flanges as shown in the two diagrams below.

Bend allowance

The bend allowance (BA) is the length of the arc of the neutral line between the tangent points of a bend in any material. Adding the length of each flange taken between the center of the radius to the BA gives the Flat Pattern length. This bend allowance formula is used to determine the flat pattern length when a bend is dimensioned from 1) the center of the radius, 2) a tangent point of the radius or 3) the outside tangent point of the radius on an acute angle bend. The BA can be calculated using the following formula:

$BA = A \left( \frac{\pi}{180} \right) \left( R + K \times T \right)$

Bend deduction

The outside set back (OSSB) is the length from the tangent point of the radius to the apex of the outside of the bend. The bend deduction (BD) is twice the outside setback minus the bend allowance. BD is calculated using the following formula:

BD = 2 \left(R + T \right) \tan{ \frac{A}{2}} - BA

The above formula works only for right angles. For bend angles 90 degrees or greater the following formula works, where A is the angle in radians (=degrees*π/180)

BD = R \left(A - 2 \right) + T \left(kA - 2 \right)

K-factor:

K-factor is a ratio of location of the neutral line to the material thickness as defined by t/T where t = location of the neutral line and T = material thickness. The K-Factor formulation does not take the forming stresses into account but is simply a geometric calculation of the location of the neutral line after the forces are applied and is thus the roll-up of all the unknown (error) factors for a given setup. The K-factor depends on many factors including the material, the type of bending operation (coining, bottoming, air-bending, etc.) the tools, etc. and is typically between 0.3 to 0.5.

In sheet metal design, the K-factor is used to calculate how much sheet metal one needs to leave for the bend in order to achieve particular final dimensions, especially for between the straight sides next the bend. Use the known k-factor and the known inner bending radius to calculate the bending radius of the neutral line. Then use the neutral bending radius to calculate the arc length of the neutral line ("circumference of circle" multiplied by the "bend angle as fraction of 360deg"). The arc length of the neutral line is the length of the sheet metal you have to leave for the bend!

The following equation relates the K-factor to the bend allowance:

$K = \frac{ -R + \frac{BA}{\pi A / 180}}{T}$

Generative Sheet Metal Design Rules

Creating sheet metal parts is a metal deformation process. Because the metal is deformed rather than removed (mill or lathe), general rules help you compensate for the manufacturing process. Designing within the process parameters usually reduces tooling cost and, ultimately, the part.

Sheet Metal Tolerances

Sheet metal design is usually for parts that do not require tight tolerances. Specifying tight location and form tolerances on a sheet metal part requires additional machining operations that add additional part and tooling costs. Minimize the cost of a sheet metal part by using standard materials and material tolerances whenever possible. Tolerances should be no tighter than necessary to make the part functional.

Forming sheet metal is less accurate than other metal forming operations, such as machining or Electronic Discharge Machining (EDM); however, many parts do not require high accuracy making sheet metal forming an option. Short process time and inexpensive tooling makes forming sheet metal appealing. It's important to understand the limits in the sheet metal forming process and specify tolerances accordingly.

The forming process has three main elements:
- Blanking a flat layout.
- Bending a flange at an angle.
- Creating a radius in the bend.

Blanking a Flat Layout

When blanking a sheet to create a flat layout, the sheet can become wavy. It is important to limit the amount of waviness by specifying a flatness tolerance. Flatness is defined as all points on a surface lying on a single plane. A flatness tolerance is the deviation allowed between two parallel planes. Typical tolerances for the flatness of the blanked profile allow .005 TIR (Total Indicator Reading) for lengths up to 1 inch. For lengths from 1 to 4 inches, TIR is typically .005 per inch, and lengths over 4 inches are allowed .02 + .004 per inch of additional length.

Bending a Flange at an Angle

There are several ways to bend sheet metal. Regardless of the technique used, the typical angle tolerance for bent flanges is +1 degree.

Creating a Radius in the Bend

The final element typical of sheet metal forming is the bend radius. The radius is generally formed with a tool or mandrel as the steel is bent around it. The actual radius is a function of compression of the material against the tool and the tension or stretching of the material on the outside. Typical tolerances used for the limits of the actual radius are based on the radius size.

Blanking Rules
Use the following rules (when possible) to blank the part in one operation. At times, parts contain features that exceed these rules making secondary tooling and operations necessary.

Consider the following when designing a part to blank:

- Open slots and tabs should never be narrower than the material thickness.

- Open slots and tabs should never be longer than five times their width.

- Convex corner radii should be at least one half the material thickness if the material is over 1/16 inch thick.

- Convex corner radii can be sharp if the material is less than 1/16 inch.

Bending Rules
When bending sheet metal, the flange being bent cannot be inside the profile. Flanges being bent must have relief on the bend radii. As the flange is bent, the material inside the bend is compressed and the material outside the bend is in tension, which causes a tear in the flange.

One solution is to move the flange away from the profile to clear the radius. Another solution is to add relief notches to allow the bend radius to compress and stretch. This latter solution is preferred because the location of the flange remains in the same place. It also preserves the design intent and you can include the relief notches in the profile or blanking tool with little additional cost or effort.

Design flanges outside the profile.

- Use relief notches, whenever possible, to reduce the cost of tooling.
- Relief notch length should be the bend radius plus 1T minimum.
- Relief notch width should be 1T or more.

Sufficient height in the flange being bent is essential in order for the bend radius to properly form. If there isn't, adding additional material to the bending operation and then cutting it off in an additional milling operation is required which then increases operation and tooling costs in the part. Therefore, it's important to create flanges with proper height to eliminate additional operations.

- The height of the flange should be at least 2.5T plus inside bend radius for hard materials.
- The height of the flange should be at least 2T plus inside bend radius for softer materials.

The most important element when bending a sheet is the bend radius. A small inside bend radius, combined with thick material, forces the material to compress in a small area during bending. This can cause cracking, or at a minimum, severe bulging at the outside edges of the bend. If this condition is unavoidable and it causes interference with other parts, it may be necessary to modify the flat pattern or the blank to compensate for the bulge. When the bend radius is large in comparison to the material thickness or if the material is 1/16 inch or less, the bulging is usually acceptable and hardly noticeable. The following figure illustrates a flange that bulges due to a small bend radius.

Bending sheet metal with small radii can cause cracking in the bend. Cracking is a function of the inside bend radius, material thickness, and the materials hardness. The following figure illustrates a table of suggested minimum bend radii for several common materials.

Holes
Punching or piercing holes is a quick, economical way to create holes in sheet metal. It is important to remember when punching holes the physics of blanking. The material being removed is actually broken away from the parent material. The finished hole is accurate only for the first 25 to 30 percent of the hole on the punch side, the remainder of the hole is relatively rough and has a burr on the die side. If the part requires more accurate holes, punch an undersized hole and then ream to the finished size.

- A punched hole diameter must be at least the material thickness or a .062 minimum.
- For hard materials (PSI 90,000 and above), the minimum diameter must be two times the thickness.
- A + .002 hole diameter is possible for the top 25% to 30% of the hole on the punch side.
- Measure holes on the punch side for accuracy.
- Tolerance holes as minimum diameters.
- Specify burrs as a maximum allowable value.
- Call out hole locations from center of hole to center of hole.
- Determine hole location from the center of the blank, not the edge of the blank.

Feature Location
The location of a punched or blanked feature must be at least the material thickness from the edge of the blank due to the bulging that occurs as the metal is deformed. If a hole must be close to an edge, it's recommended that you add a tab to the material or change the hole to an open slot.

The general rule for minimum distances between features or edges is the larger the feature the greater the distance. There should also be a clearance distance established for features relative to bends. The closer a feature comes to a bend, the more distortion occurs in the feature as the part is bent into shape.

- The larger the feature periphery, the farther you should stay away from other edges.
- Keep features with curved profiles 1T to 1.5T away from other edges.
- Keep features with rectangular profiles 1.5T to 3T away from other edges.
- Keep features up to 1 inch (long or diameter) 1.5T plus bend radius away from bends.
- For larger features, add an additional 1T for each inch in length.
- Features positioned relative to bends should have a location tolerance of + .01.

Generative Sheet Metal Design - General Information
The most important part of a design is to effectively communicate your design intent to manufacturing and quality control personnel. The best sheet metal part design is only effective if the part can be manufactured efficiently. It is important to adhere to specific sheet metal terminology and notation. The manner in which tolerances are called out on the drawing and the areas of the part that are dimensioned can cause a perfectly good part to be scrapped, or worse, a bad part to be accepted.

Bend Terminology
Several terms are unique to sheet metal: Inner Mold Line (IML), Outer Mold Line (OML), Center Line of Bend, and Neutral Axis. These terms are defined as follows:

- The IML is the intersection of the two inner surfaces of the bend.
- The OML is the intersection of the two outer surfaces of the bend.
- The Center Line of Bend refers to the axis of the bend radius where the machine or die is set that creates the bend.
- The neutral axis is where the tensile forces (at the outside of the bend) and the compressive forces (on the inside of the bend) are zero.

Sheet Metal Dimensioning
The basic rules of design apply when dimensioning a sheet metal part for manufacturing or inspection. There are several rules that are unique to sheet metal. This figure illustrates the following rules:

- Dimensions should be given between surfaces having a functional relation to each other.
- Dimensions should be given between surfaces controlling the relationship of mating parts.
- Radial dimensions should always be to the inside bend radius.
- Dimensions should be consistent to the inside or outside surfaces.
- Do not dimension from the inside of the part to the outside of the part.
- Use the OML intersections to dimension the length of angled flanges.

Standard Notes
When dimensioning sheet metal parts, standard notes in the title block of the drawing can account for many features and conditions. You can also record them as common shop procedures. You can group many sheet metal designs as families of parts because they use similar features or are made of similar materials. By including a standard note in the title block that states "Unless otherwise specified...," you can focus on exceptions to the normal. Typical techniques used to handle standard conditions are listed below.

Bend Radii
When designing parts made from the same material, it is typical to design to a common inside bend radius. A typical title block note may state:
Unless otherwise specified.
All inside bend radii to be .062.

Corner Radii
It is more economical to produce a sheet metal part with an allowance on the outside corners rather than forcing them to remain sharp. A typical title block note states:
Unless otherwise specified.
1/16 corner radii allowed.

Feature Tolerance
When feature locations and sizes are designed with very tight tolerances, additional operations are usually required to finish the part. These additions increase time and cost to an otherwise simple process. Most sheet metal feature size and location tolerances can be specified by a note stating the following:
Unless otherwise specified.
Hole location + .010 on centers.
Hole size + .005 diameter.

Shape Deviation

Because the sheet metal process deforms material, it is not practical to assume features maintain their true shape. Holes tend to stretch in an oblong shape if they are near a bent edge. This condition can be captured in a note stating the following:

Unless otherwise specified.
Shape deviations acceptable within feature size limits.

Burrs and Sharp Edges
Because the edges of the material can become sharp and dangerous to handle during the sheet metal process, it is important to specify an edge condition to protect manufacturing personnel and the end-user of the product. You can define edge conditions in a general note in a variety of ways. Some methods are listed as follows:

- Break all sharp edges and corners .015.
- Remove all burrs.
- Burrs not to exceed .005.
- Condition for handling.
- Break all sharp corners except cutting edges.
- Specify burr up or down.

Common Shapes
The sheet metal process lends itself well to simple shapes that are typically formed using standard tools and processes. Standard shapes are used to add stiffness to the sheet metal parts and provide support or clearance for mating parts. The types of common shapes that are addressed in this course are: bends, flanges, hems, joggles, and beads.

Bends
Bends are the most common formed features in sheet metal design. They are defined as the uniform straining of material around an axis. Simple bends are typically formed using a brake press and can be formed at any angle as long as the bent material does not interfere with any surrounding material. Complex or multiple bends are formed using a punch and die, where you are forming the material rather than bending it.

Inset Flanges
Inset flanges are used mainly to support mating parts or provide mounting locations. These features are pierced into the sheet and then bent. The inset flange requires more sophisticated tooling than the simple bend but it is still cost effective compared to machining and/or welding.

Hems
Hem features modify the edges of sheet metal parts. They are used primarily for stiffening the part but are also useful for creating smooth edges where safety or contact with other parts is an issue. There are three basic shapes of a hemmed edge: J-hem, teardrop hem, and curled hem. The J-hem and the teardrop hem can be bent closed but the curled hem cannot.

Joggles
Joggle features are used for stiffening and to allow clearance for mating parts. The aerospace industry uses these features extensively because of the large number of sheet metal parts assembled to create a smooth contour. These features are usually formed using special joggle dies that stretch the material into shape.

Beads:
Beads are protrusions or depressions stamped on a sheet metal face. A bead can add strength to a sheet metal part or add control to the forming operation. There are three types of beads: U-Shaped, V-Shaped, and Circular.

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Bending is a metal forming process in which a force is applied to a piece of sheet metal, causing it to bend at an angle and form the desired shape. A bending operation causes deformation along one axis, but a sequence of several different operations can be performed to create a complex part. Bent parts can be quite small, such as a bracket, or up to 20 feet in length, such as a large enclosure or chassis. A bend can be characterised by several different parameters, shown in the image below.

Bend line - The straight line on the surface of the sheet, on either side of the bend, that defines the end of the level flange and the start of the bend.
Outside mold line - The straight line where the outside surfaces of the two flanges would meet, were they to continue. This line defines the edge of a mold that would bound the bent sheet metal.
Flange length - The length of either of the two flanges, extending from the edge of the sheet to the bend line.
Mold line distance - The distance from either end of the sheet to the outside mold line.
Setback - The distance from either bend line to the outside mold line. Also equal to the difference between the mold line distance and the flange length.
Bend axis - The straight line that defines the center around which the sheet metal is bent.
Bend length - The length of the bend, measured along the bend axis.
Bend radius - The distance from the bend axis to the inside surface of the material, between the bend lines. Sometimes specified as the inside bend radius. The outside bend radius is equal to the inside bend radius plus the sheet thickness.
Bend angle - The angle of the bend, measured between the bent flange and its original position, or as the included angle between perpendicular lines drawn from the bend lines.
Bevel angle - The complimentary angle to the bend angle.

The act of bending results in both tension and compression in the sheet metal. The outside portion of the sheet will undergo tension and stretch to a greater length, while the inside portion experiences compression and shortens. The neutral axis is the boundary line inside the sheet metal, along which no tension or compression forces are present. As a result, the length of this axis remains constant. The changes in length to the outside and inside surfaces can be related to the original flat length by two parameters, the bend allowance and bend deduction, which are defined below.

Neutral axis - The location in the sheet that is neither stretched nor compressed, and therefore remains at a constant length.
K-factor - The location of the neutral axis in the material, calculated as the ratio of the distance of the neutral axis (measured from the inside bend surface) to the material thickness. The K-factor is dependent upon several factors (material, bending operation, bend angle, etc.) and is typically greater than 0.25, but cannot exceed 0.50.
Bend allowance - The length of the neutral axis between the bend lines, or in other words, the arc length of the bend. The bend allowance added to the flange lengths is equal to the total flat length.
Bend deduction - Also called the bend compensation, the amount a piece of material has been stretched by bending. The value equals the difference between the mold line lengths and the total flat length.

When bending a piece of sheet metal, the residual stresses in the material will cause the sheet to spring-back slightly after the bending operation. Due to this elastic recovery, it is necessary to over-bend the sheet a precise amount to achieve the desired bend radius and bend angle. The final bend radius will be greater than initially formed and the final bend angle will be smaller. The ratio of the final bend angle to the initial bend angle is defined as the spring-back factor, KS. The amount of spring-back depends upon several factors, including the material, bending operation, and the initial bend angle and bend radius.

Bending is typically performed on a machine called a press brake, which can be manually or automatically operated. For this reason, the bending process is sometimes referred to as press brake forming. Press brakes are available in a range of sizes (commonly 20-200 tons) in order to best suit the given application. A press brake contains an upper tool called the punch and a lower tool called the die, between which the sheet metal is located. The sheet is carefully positioned over the die and held in place by the back gauge while the punch lowers and forces the sheet to bend. In an automatic machine, the punch is forced into the sheet under the power of a hydraulic ram. The bend angle achieved is determined by the depth to which the punch forces the sheet into the die. This depth is precisely controlled to achieve the desired bend. Standard tooling is often used for the punch and die, allowing a low initial cost and suitability for low volume production. Custom tooling can be used for specialized bending operations but will add to the cost. The tooling material is chosen based upon the production quantity, sheet metal material, and degree of bending. Naturally, a stronger tool is required to endure larger quantities, harder sheet metal, and severe bending operations. In order of increasing strength, some common tooling materials include hardwood, low carbon steel, tool steel, and carbide steel.

Bend Allowance and K Factor calculation:

[K-Factor = t/T] , (t = Half the thickness, T = Thickness). CATIA calculate K factor according to DIN standard [K-Factor = (0.65 + log (R/T)/2)/2]

[BA (Bend Allowance) = 2 * PI (A/360)(R + KT)] , (PI = 3.14, A = Angle between two walls, R = Bend radius, K = K Factor, T = Thickness of sheet)

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Example: Link (http://sheetmetal.me/formulas-and-functions/bend-allowance/)

Understanding the Bend Allowance and consequently the Bend Deduction of a part is a crucial first step to understanding how sheet metal parts are fabricated. When the sheet metal is put through the process of bending the metal around the bend is deformed and stretched. As this happens you gain a small amount of total length in your part. Likewise when you are trying to develop a flat pattern you will have to make a deduction from your desired part size to get the correct flat size.

The Bend Allowance is defined as the material you will add to the actual leg lengths of the part in order to develop a flat pattern. The leg lengths are the part of the flange which is outside of the bend radius.

In our example below a part with flange lengths of 2” and 3” with an inside radius of .250” at 90° will have leg lengths of 1.625” and 2.625” respectively. When we calculate the Bend Allowance we find that it equals .457”. In order to develop the flat pattern we add .457” to 1.625” and 2.625” to arrive at 4.707”. As you can see the Bend Allowance and Bend Deduction are closely related below.

The Bend Allowance Formula takes into account the geometries of bending and the properties of your metal to determine the Bend Allowance. You will need to know your Material Thickness (MT), the Bend Angle (B<), the Inside Radius (IR), and the K-Factor (K). The Material Thickness will be measured in decimal form, not by the gauge number. For more information on gauges and their decimal equivalents and tolerances view our Gauge Chart page. The Bend Angle will be something that you determine based on what the complimentary angle of your part is going to be.

It is important to convert from the included angle to the complimentary angle before performing any calculations. The Inside Radius will be the finished radius of the included angle. For information on how the Inside Radius is determined see our post on the Air Bend Force Chart. Finally the K-Factor is a property of the material which you are bending. This property determines how the material is stretched when bending. See our post on the K-Factor for better understanding as well as charts and formulas.

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Click Sheet Metal Parameters.

The Sheet Metal Parameters dialog box is displayed.

The third tab concerns the bend allowance.

Bend Allowance:

The bend allowance corresponds to the unfolded bend width.

bend < 90deg

bend > 90deg

L is the total unfolded length

A and B the dimensioning lengths as defined on the above figure. They are similar to the DIN definition.

K Factor:

Physically, the neutral fiber represents the limit between the material compressed area inside the bend and the extended area outside the bend. Ideally, it is represented by an arc located inside the thickness and centered on the bend axis.

The K factor defines the neutral fiber position:

W = a * (R + k * T)

where:

W is the bend allowance
R the inner bend radius
T the sheet metal thickness
a the inner bend angle in radians

If b is the opening bend angle in degrees:

a = p * (180 - b) / 180

When you define the sheet metal parameters, a literal feature defines the default K Factor and a formula is applied to implement the DIN standard. This standard is defined for thin steel parts. Therefore the K Factor value ranges between 0 and 0.5.

The DIN definition for the K factor slightly differs.

W = a * (R + k' * T/2)

Therefore k' = 2 * k and ranges from 0 to 1.

This formula can be deactivated or modified by right-clicking in the K factor field and choosing an option from the contextual menu. It can be re-activated by clicking the Apply DIN button. Moreover, the limit values can also be modified.

When a bend is created, its own K Factor literal is created.

Two cases may then occur:

If the Sheet Metal K Factor has an activated formula using the default bend radius as input parameter, the same formula is activated on the bend K Factor replacing the default bend radius by the local bend radius as input.
In all other cases, a formula "equal to the Sheet Metal K Factor" is activated on the local bend K Factor. This formula can also be deactivated or modified.

Bend Deduction:

When the bend is unfolded, the sheet metal deformation is thus represented by the bend deduction V, defined by the formula:

L = A + B + V

(refer to the previous definitions).

Therefore the bend deduction is related to the K factor using the following formula:

V = a * (R + k * T) - 2 * (R + T) * tan ( min(p/2,a) / 2)

This formula is used by default. However, it is possible to define bend tables on the sheet metal parameters. These tables define samples: thickness, bend radius, open angle, and bend deduction. In this case, the bend deduction is located in the appropriate bend table, matching thickness, bend radius, and open angle. If no accurate open angle is found, an interpolation will be performed.

When updating the bend, the bend deduction is first computed using the previously defined rules. Then the bend allowance is deduced using the following formula:

W = V + 2 * (R + T) * tan ( min(p/2,a) / 2)

When the bend deduction is read in the bend table, the K factor is not used.

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Converting a surface model into a sheet metal:

1). Open the model of a surface.

2). Select Hopper from Rolled Walls.

3). For selection, select the surface.

4). Click OK. A sheet form of the surface is created.

5). From Views toolbar, select Fold/Unfold.

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Considering the Bend Deduction and Bend Allowances is a critical first step in designing sheet metal parts as it affects nearly every following step in the fabrication process. More so, it will allow you to achieve the correct size and dimensions needed in the flat pattern. The flat pattern is what the part looks like before any bends have happened. The lengths in the flat pattern will be different from in the bent state. This is because metal material when formed in a bending process is both stretched and compressed depending on the thickness and the type of material.

The Bend Deduction BD is defined as the difference between the sum of the flange lengths (from edge to the apex) and the initial flat length. In other words, the material you will have to remove from the total length of the flanges in order to arrive at the proper length in the flat pattern.

In the example below, the part has flange lengths of 2” and 3” with an inside radius of 0.250” at 90° will have a length of 5”. When the Bend Deduction is calculated we find that it equals 0.293” in length. In order to develop the flat pattern, we will subtract 0.293” from 5” to arrive at 4.707”. The image below shows the close relation between Bend Deduction and Bend Allowance.

What are sheet metal gauges?

Gauges are used to measure the material thickness of a sheet of metal. These units are neither standard of metric and are completely independent of those typical measurement systems. Keeping a gauge conversion chart nearby is an easy way to determine the actual thickness of a sheet of metal in inches or millimeters. For example, a 14 gauge stainless-steel is .07812 inches thick. The gauge number 14 holds no relevance to the actual measurements.

It is important to know that the gauge thicknesses also vary depending on the type of sheet metal being referenced. Take for instance 12-gauge thickness across the material types listed below; stainless steel is 0.105″ thick, aluminum is 0.080″, copper is 0.108″, and brass is 0.081″.

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Tube Pipe Bending:

The bends have been of small bend radius when there was no operational reason for this, as well as having difference bend radii when they could have been the same. Tube bending has some rules that should ideally be followed to avoid the manufacturer having to produce expensive dedicated tooling.

If there are multiple bends, these should, where possible, be of the same bend radius.
If a very tight bend is required, it may be better to consider an angular welded joint or joints.

Of course, a general rule is the tougher the tube material the harder it is to bend and more liable to buckle, crinkle or tear. So the most popular (and hence cheapest) Tube sizes are as follows:

The standard preferred bend is 2D and this also is the cheapest to produce due to standard tooling supporting this size of bend.

However, we would normally dimension the inside Radius not tube centerline. So for a Ø20 Tube a 2D bend would be R40 to the Tube Centerline, We would dimension this at R30 to the inside of the bend. It is also good practice if a differing bend to 2D is required, to go in increments of D; 3D, 4D etc. Finally, if the function allows, give a very open bend tolerance, For example in the Ø20 Tube of 2D bend, give a tolerance such as R20 to R50 (this equating to a 1.5D to 3D bend).

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Generative Sheet Metal Design:
Parameter standards help you enforce standard techniques and practices used when creating sheet metal features by simplifying how one defines the feature parameters. Therefore streamlining the design process. You can also verify sheet metal features, or the entire part, against these standards.

The sheet metal process lends itself well to simple shapes that are typically formed using standard tools and processes. The easiest way to add stiffness to a sheet metal part is to create a wall, extrusion or flange thus making the part much more rigid.

Sheet metal features are detail features that you add or subtract from a sheet metal part. Cutout features remove material from a sheet metal part. Corner relief features allow parts to fold and unfold without overlapping. Corner features add or remove material on a body by replacing selected edges/faces with cylindrical or conical surfaces. Chamfer features create or remove material by creating a flat face between faces. Pattern features duplicate features and geometry (using various methods) while still maintaining associativity. This feature type is useful when the part you are designing has a lot of symmetry. The different pattern types available are Rectangular, Circular, and User.

Generative Sheet Metal Stampings:
Sheet metal stamping allows you to add detailed features to a sheet metal part. They are created by embossing sheet metal.

The different stampings available on the Stamping toolbar are Flanged Hole, Bead, Circular Stamp, Curve Stamp, Surface Stamp, Bridge, Flanged Cut Out, Louver, User Stamp, Dowel, and Stiffening Rib. One benefit in using these features is that you can position them parametrically.

Within the mapping bracket project, we incorporated the use of another workbench to create additional sheet metal features. The Generative Shape Design tools (Extract, Join and Project) are useful when creating bead profiles along the walls of a folded part as well as creating unfolded/folded profiles. Creating the unfolded profile took many steps to develop. In some cases, actually flattening the part and developing the cut profile on the flat part may be an easier and better practice to use.

Save As DXF:
Saving the part file as a DXF allows you to import it as a flat pattern drawing. This can save a significant amount of time when transferring drawings between multiple CAD packages provided your system can support file translating.

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1). Click on Sheet Metal parameters.

2). Select Bend Allowance tab. There is a option as K Factor.

3). If you select Parameters tab and change the values of Thickness and Default Bend Radius. The value of K factor will also change.

Example:
1). Click on xy plane and then click on Sketch.

2). Create a rectangle on the plane and define the length of the rectangle 102mm then exit the sketch.

3). Create a wall based on the sketch

4). Click on Wall on Edge and create a new wall on an edge. Insert value of 50 for Height and select third option from length type. Then click on OK.

5). When you measure the length of the first wall you will notice that length of the wall is decreased 2mm (From 102mm to 100mm). As 2mm is the bend reduce defined in Sheet metal parameters.

6). The value of bend allowance (Internal bend radius is 2mm).

7). Therefore, external bend radius is 3.5 = Bend allowance (Internal bend radius) + Thickness

8). The length of the wall is 50mm which is defined previously.

9). Unfold the sheet metal by clicking on Fold/Unfold... command.

10). Now measure the bending section (Bend Allowance). The length is 3.982mm. The value is calculated by CATIA. The length and the whole length is related to parameters of bend allowance and K factor. This value is calculated in next step as follows.

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1). Now return to the previous example.
Open Sheet metal parameters window. K Factor = 0.35 (DIN Standard)

2). To see the formula which CATIA uses to calculate K factor right click on K Factor and select Edit.

3). Now calculate Bend Allowance. As you see the calculated value is exactly the same as value measured in previous example.

4). If you want to edit K factor. Click on Sheet metal parameters. In Bend Allowance tab right click on K Factor section, select Formula and then Deactivate.

5). Now you can edit K Factor value. Click OK.

6). Now, measure the bend allowance. The value is 4.32mm.

7). If you want to use DIN standard again, in Bend Allowance tab click on Apply DIN button.

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