sheet metal fabrication design guide - Geomiq

Basic Principles

Sheet Metal Fabrication is the process of forming parts from a metal sheet by punching, cutting, stamping, and bending. 

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3D CAD files are converted into machine code, which controls a machine to precisely cut and form the sheets into the final part. 

Sheet metal parts are known for their durability, which makes them great for end use applications (e.g. chassis). Parts used for low volume prototypes, and high volume production runs are most cost-effective due to large initial setup and material costs. 

Because parts are formed from a single sheet of metal, designs must maintain a uniform thickness. Be sure to follow the design requirements and tolerances to ensure parts fall closer to design intent and cutting sheets of metal 

Bend line– The straight line on the surface of the sheet, on either side of the bend, that defines he end of the level flange and the start of the bend.

Bend radius – The distance from the bend axis to the inside surface of the material, between the bend lines.

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.Sometimes specified as the inside bend radius. The outside bend radius is equal to the inside bend radius plus the sheet thickness.

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 T, to the material thickness t. The K-factor is dependent upon several factors (material, bending operation, bend angle, etc.) and is greater than 0.25, but cannot exceed 0.50. K factor = T/t

Bend allowance – The length of the neutral axis between the bend lines or the arc length of the bend. The bend allowance added to the flange lengths is equal to the total flat length.

K-Factor

The K-factor is the ratio between the the neutral axis to the thickness of the material.

Importance of the K-factor in sheet metal design

The K-factor is used to calculate flat patterns because it is related to how much material is stretched during bending. Therefore it is important to have the value correct in CAD software. The value of the K-factor should range between 0 – 0,5. To be more exact the K-factor can be calculated taking the average of 3 samples from bent parts and plugging the measurements of bend allowance, bend angle, material thickness and inner radius into the following formula:

Some basic K-factor values are shown here. Use these as a guideline.

Springback

When bending a piece of sheet metal, the residual stresses in the material will cause the sheet to springback 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 springback factor, KS. The amount of springback depends upon several factors, including the material, bending operation, and the initial bend angle and bend radius.

Dimensions:

To prevent parts from fracturing or having distortions, make sure to keep the inside bend radius at least equal to the material thickness 

Bend Angles:

A +/- 1 degree tolerance on all bend angles is generally acceptable in the industry. Flange length must be at least 4 times the material thickness.

Rule of thumb
It is recommended to use the same radii across all bends, and flange length must be at least 4 times the material thickness.

Material Thickness, t

The thickness of the material is not proportional to the tonnage like the v opening. Doubling the thickness does not mean doubling the tonnage. Instead the bending force is related by the square of the thickness. What this means is that if the material thickness is doubled the tonnage required increases 4 fold.

Work Piece Length, L

Like the v opening the tonnage required is directly related to the length of the work piece. Doubling the work length means doubling the required tonnage. It should be noted that when bending short pieces, under 3” in length, the tonnage required may be less than that which is proportional to its length. Knowing this can prevent damaging a die.

Air Force Bending Chart

The Air Force Bending chart is a chart showing the tonnage used for bending different thickness sheet metal. It is useful for sheet metal designers as it specifies the bend radius and tooling to be used for different thicknesses. It is shown here for mild steel. Designers can use this as a guide when designing the minimum flange length possible with the tooling for different V blocks as well as the bend radius. The following charts are based on the Armada Air Force bend guide.

Forming Near Holes

When a bend is made too close to a hole the hole may become deformed. Hole 1 shows a hole that has become teardrop shaped because of this problem.

To save the cost of punching or drilling in a secondary operation the following formulas can be used to calculate the minimum distance required:

For a slot or hole < 25mm in diameter the minimum distance to Hole 2 centre:

D = 2t + r

As a rule of thumb the distance from the outside of the material to the bottom of the cutout should be equal to the minimum flange length as prescribed by the air bend force chart

D = 2,5t + r

When using a punch press, or laser cutting, holes should never be less than that of the material thickness.

Laser cutting is a type of production that uses a laser to cut different metals. The laser has a high energy beam which easily burns through the material. Laser cutting can be used on materials such as metal, aluminium, plastic, wood, rubber, etc. Lasers use computer numerically controlled programming (CNC) to determine the shape and position ls of the cutouts. Material thicknesses of up to 20mm can be lasercut. There are advantages and disadvantages in using lasercutting. CO2 lasers are more traditional, and can cut thicker materials but do not deliver such an accurate cut as fibre lasers. Fibre lasers can generally cut thinner materials and have much higher cutting speeds than CO2 .

Advantages and Disadvantages

Advantages of lasercutting over cutting mechanically include better workholding, reduced workpiece contamination, better precision and reduced chance of warping as the heat affected zone is small. Some disadvantages are that lasercutting does not always cut well with some materials (for example not all aluminium) and it is not always consistent. Despite the disadvantages lasercutting is highly efficient and cost effective.

Material Restrictions

Materials that are not suitable for lasercutting include mirrored or reflective materials, Masonite boards, composites containing PVC.

Acceptable Materials

Generally the following materials are suitable for lasercutting: metal, stainless steel, some thicknesses of aluminium, wood and some plastics.

Localized hardening

Localised hardening takes place on the edges where the where the laser has cut. This hardening produces a durable and smooth edge without the need for finishing after using the laser cutter

Distortion

A heat-affected zone (HAZ) is produced during laser cutting . In carbon steel, the higher the hardenability, the greater the HAZ. Distortion from laser processing is a result of the sudden rise in temperature of the material near the cutting zone. Distortion is also created by the rapid solidification of the cutting zone. In addition, distortion also can be attributed to the rapid solidification of material remaining on the sides of the cut.

Kerf

During laser cutting a portion of the material is burnt away when the laser cuts through, leaving a small gap. This ‘gap’ is known as the laser kerf and ranges from 0.08 – 0.45mm depending on the material type, thickness and other conditional factors. A minimum distance of 1-2mm between parts needs to be left to avoid accidental crossover cutting. 

It is also advised to keep parts 2-5mm away from the edge of the material due to some sheets being warped or slightly off in their sizing. One should always cut parts in the boundary of the sheet size and not use the sheet edges as a border.

Curls

Curl Feature Guidelines

Curling sheet metal is the process of adding a hollow, circular roll to the edge of the sheet. The curled edge
provides strength to the edge and makes it safe for handling. Curls are most often used to remove a sharp
untreated edge and make it safe for handling. It is recommended that: The outside radius of a curl should not be smaller than 2 times the material thickness. 

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A size of the hole should be at least the radius of the curl plus material thickness from the curl feature. A bend should be at least the radius of the curl plus 6 times the material thickness from the curl feature

Hemming is nothing but to fold the metal back on itself. In Sheet Metal hems are used to create folds in sheet metal in order to stiffen edges and create an edge safe to touch. Hems are most often used to remove a sharp untreated edge and make it safe for handling. Hems are commonly used to hide imperfections and provide a generally safer edge to handle. A combination of two hems can create strong, tight joints with little or minimal fastening. Hems can even be used to strategically double the thickness of metal in areas of a part which may require extra support. It is recommended that:

For tear drop hems, the inside diameter should be equal to the material thickness.

Holes & Slots: Dimensions

Keep hole and slot diameters at least as large as material thickness. Higher strength materials require larger diameters. 

Clearances

Holes and slots may become deformed when placed near a bend. The minimum distance they should be placed from a bend depends on the material thickness, the bend radius, and their diameter. Be sure to place holes away from bends at a distance of at least 2.5 times the material’s thickness plus the bend radius. Slots should be placed 4 times the material’s thickness plus the bend radius away from the bend. Be sure to place holes and slots at least 2 times the material’s thickness away from an edge to avoid a “bulging” effect. Holes should be placed at least 6 times the material’s thickness apart.

Bend notches

Notching is a shearing operation that removes a section from the outer edge of the metal strip or part. In case, distance between the notches to bend is very small then distortion of sheet metal may take place. To avoid such condition notch should be placed at appropriate distance from bend with respect to sheet thickness. Notching is a low-cost process, particularly for its low tooling costs with a small range of standard punches.

Clearances 

Notches must be at least 3.175mm away from each other. For bends, notches must be at least 3 times the material’s thickness plus the bend radius. Tabs must have a minimum distance from each other of 1mm or the material’s thickness, whichever is greater.

Recommendations for Notch Feature:
Notch width should not be narrower than 1.5 * t.

Length of notches can be up to 5 * t. Recommended corner radius for notches should be 0.5 * t.

Notches must be at least the material’s thickness or 0.04”, whichever is greater, and can be no longer than 5 times its width. Tabs must be at least 2 times the material’s thickness or 0.126”, whichever is greater, and can be no longer than 5 times its width.

Welding is a complex craft that requires knowledge of various techniques and practices. In order to do their job successfully, welders must have an in-depth understanding of the various techniques and practices used in the industry, which includes types of weld joints.

According to the American Welding Society (AWS), a joint can be defined as, “The manner in which materials fit together.” The applications of welding are endless, and different jobs require different types of welds and joints.

There are five major welding joint types: butt joint, tee joint, corner joint, lap joint and edge joint. Butt joints are the most common and can be made in various ways depending on factors like groove shape and width. Tee joints are formed when two pieces intersect at a 90° angle and can be created using different welding styles. Corner joints meet in a corner and can be formed as V-groove or square butt joints.

Keep reading to learn about each of the different weld types and how they can apply to a career in the field.

Why Understanding the Applications of Different Weld Joint Types Is Important

The term “weld joint design” refers to the way pieces of metal are put together or aligned with each other. Each joint’s design affects the quality and cost of the completed weld. Selecting the most appropriate joint design for a welding job requires special attention and skill.

That’s why it’s important to have a good understanding of different weld joint types. Understanding the applications can help welders produce durable, high-quality welds.

There are five basic welding joint types commonly used in the industry, according to the AWS:

• Butt joint welding
• Tee joint welding
• Corner joint welding
• Lap joint welding
• Edge joint welding

The 5 Basic Joint Weld Types

1. Butt joint welding

A butt joint, or butt weld, is a joint where two pieces of metal are placed together in the same plane, and the side of each metal is joined by welding. A butt weld is the most common type of joint that is used in the fabrication of structures and piping systems. It’s fairly simple to prepare, and there are many different variations that can be applied to achieve the desired result.

Butt welds are made in a variety of ways, and each one serves a different purpose. Varying factors include the shape of the groove, layering and width of the gap. Listed below are some typical examples of butt weld joints:

• Square
• Single bevel
• Double bevel
• Single J
• Double J
• Single V
• Double V
• Single U
• Double U grooves

The area of the metal’s surface that is melted during the welding process is called the faying surface. The faying surface can be shaped before welding to increase the weld’s strength, which is called edge preparation. The edge preparation may be the same on both members of the butt joint, or each side can be shaped differently.

Reasons for preparing the faying surfaces for welding include the following:

• Codes and standards
• Metals
• Deeper weld penetration
• Smooth appearance
• Increased strength

In some cases, the exact size, shape and angle can be specified for a groove. If exact dimensions are not given, the groove can be made to the necessary size. However, it’s important to remember that the wider the groove, the more welding it will require to complete.

As the metal becomes thicker, you must change the joint design to ensure a sound weld. On thin sections, it is often possible to make full penetration welds using a square butt joint. When welding on a thick plate or pipe, it is often impossible for the welder to get 100% penetration without some type of groove being used.

When it comes to butt joints, commonly occurring defects may include burn-through, porosity, cracking or incomplete penetration. However, these can be avoided by modifying the welding variables.

2. Tee joint welding

Tee welding joints are formed when two pieces intersect at a 90° angle. This results in the edges coming together in the center of a plate or component in a T shape. Tee joints are considered a type of fillet weld, and they can also be formed when a tube or pipe is welded onto a base plate.

Image Credit: Maine Welding Company

With this type of weld, it’s important to always ensure there is effective penetration into the roof of the weld. There are a handful of welding styles that can be used to create a tee joint:

• Plug weld
• Slot weld
• Bevel-groove weld
• Fillet weld
• J-groove weld
• Melt-through weld
• Flare-bevel-groove weld

Tee joints are not usually prepared with grooves, unless the base metal is thick and welding on both sides cannot withstand the load the joint must support. A common defect that occurs with tee joints is lamellar tearing—which happens due to restriction experienced by the joint. To prevent this, welders will often place a stopper to prevent joint deformities.

3. Corner joint welding

Corner joints have similarities to tee welding joints. However, the difference is the location of where the metal is positioned. In the tee joint, it’s placed in the middle, whereas corner joints meet in the ‘corner’ in either an open or closed manner—forming an L shape.

These types of joints are among some of the most common in the sheet metal industry, such as in the construction of frames, boxes and other applications. There are two ways of fitting up an outside corner joint—either it forms a V-groove (A) or forms a square butt joint (B), as seen in the diagram below.

The styles used for creating corner joints include V-groove, J-groove, U-groove, spot, edge, fillet, corner-flange, bevel-groove, flare-V-groove and square-groove or butt.

4. Lap joint welding

Lap welding joints are essentially a modified version of the butt joint. They are formed when two pieces of metal are placed in an overlapping pattern on top of each other. They are most commonly used to joint weld two pieces with differing thicknesses together. Welds can be made on one or both sides.

Image credit Science Direct

Lap joints are rarely used on thicker materials and are commonly used for sheet metal. Potential drawbacks to this type of welding joint include lamellar tearing or corrosion due to overlapping materials. However, as with anything, this can be prevented by using correct technique and modifying variables as necessary.

5. Edge joint welding

In an edge joint, the metal surfaces are placed together so that the edges are even. One or both plates may be formed by bending them at an angle.

The purpose of a weld joint is to join parts together so that the stresses are distributed. The forces causing stresses in welded joints are tensile, compression, bending, torsion and shear, as seen in the image below. 

The ability of a welded joint to withstand these forces depends upon both the joint design and the weld integrity. Some joints can withstand certain types of forces better than others.

The welding process to be used has a major effect on the selection of the joint design. Each welding process has characteristics that affect its performance. The rate of travel, penetration, deposition rate and heat input also affect the welds used on some joint designs. The following styles are applicable for edge joints:

• U-groove
• V-groove
• J-groove
• Corner-flange
• Bevel-groove
• Square-groove
• Edge-flange

Due to overlapping parts, this type of joint is more prone to corrosion. Welders must keep in mind other defects like slag inclusion, lack of fusion and porosity, which can also occur.

How This Applies to a Career in Welding

Understanding the physics of joint design is essential for welders, as this allows them to recognize and anticipate the various forces that will be applied to a weldment in the field. Engineers use static and dynamic loading computer programs to anticipate the weldment’s strength requirements.

Today’s welders are expected to understand the types of forces being applied to the weldment and to determine the best joint design to prevent these forces from causing a structural failure. An improper configuration of a weld joint can cause weld and material defects, such as cracking or lamination—and skilled welders must know how to adjust variables to avoid these defects.

Learning to work with different welding joints and weld types takes practice and in some cases requires the completion of a formal training program, such as the Welding Technology training program offered at Universal Technical Institute (UTI). If becoming a welder sounds like the right career for you, this program can provide you with the training you need to get ready in just 9-10 months.1

From welding safety to principles of welding to complex welding applications, the courses will walk you through key concepts you’ll need to know as a welder—including welding joints.

Receive Training at a UTI Welding Campus

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All campuses feature industry-aligned technology and equipment to help prepare you for an entry-level role in the field after you graduate.

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