Designing for Sheet Metal Fabrication

An examination of material, manufacturing process, design considerations, and finishing options for sheet metal prototypes and low-volume production parts

Designing for Sheet Metal Fabrication

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In the manufacturing industry, sheet metal is one of the most versatile materials. The metal can be steel, aluminum, brass, copper, tin, nickel, titanium, or even precious metals. Sheet thickness varies from thin foil to heavy plate to wispy leaf.

The sheets come in a variety of forms: plain, embossed, etched, ribbed, corrugated, or perforated. Applications include transportation, aerospace, appliance manufacturing, consumer electronics, industrial furniture, machinery, and many more.

Why Sheet Metal?

Sheet metal allows the manufacturer to purchase as needed rather than starting with a block of material for machining away. So as long as there is no welding or machining, the remaining metal sheet is still usable; the swarf, however, must be recycled.

A wide variety of sheet metal applications offer advantages over alternative non-metal materials and other types of fabrication methods as well. This process generally has a significantly lower material cost than machining.

A sheet metal manufacturing process can be automated; parts can be created directly from CAD models as with many modern fabrication techniques. The technology involves the use of a variety of materials and a range of processes to shape finished components and products.

Furthermore, sheet metal fabrication is highly scalable. Although the setup for the first piece can be costly, the price per piece decreases rapidly as the volume grows. While many processes have the tendency to drop in cost per piece over time, in a subtractive process such as machining, sheet manufacturing costs usually drop comparatively faster.

How is Sheet Metal Being Used?

The various sheet metal operations include cutting, stamping, punching, shearing, forming, bending, welding, rolling, riveting, drilling, tapping, and machining. Sheet metal components can be inserted with hardware. Depending on the application, components may be brushed, plated, anodized, powder-coated, spray-painted, silk-screened, or otherwise marked. Parts can be riveted, screwed, or welded together to make complete assemblies.

Today, sheet metal fabrication is evolving just like most other technologies. In the past decade, materials, equipment, and tooling have become increasingly specialised. It is imperative to select the right supplier and manufacturing method for your sheet metal components.

The following are some key components of sheet metal fabrication explored in this white paper:

Sheet Metal Fabrication Techniques

By definition, sheet metal starts out flat but can be shaped in a wide variety of ways to meet a variety of needs. In this paper, we discuss materials that are shaped through the bend of a single axis. However, there are numerous ways to mold the material into forms that are not flat or shaped through bending alone.

Metal fabrication techniques such as deep drawing, hydroforming, spinning, and stamping are examples of methods that can be used hot or cold. Processes like these are used to make the body panels for modern vehicles, complex formed objects like metal sinks, and aluminum beverage cans. These techniques can often be iterative, repeating a process several times to progressively alter the shape and size of the metal.

Here are some popular fabrication processes:

Metal Cutting

1. Sheet steel has been cut with shearing for many years but has now been supplanted by faster, more precise methods. 2. Metal can be punched and cut using a punch press and die set. Particularly useful for simple parts that cannot be cut effectively with lasers or water jets. Punch presses can produce parts at 100 to 150 strokes per minute, making them fast and efficient. A punch can also make holes in parts or other cutouts. Using lasers and punches together can produce a complex flat pattern with sized stamping features.

3. A CNC laser cutting process burns away metal with jets of oxygen, nitrogen, helium, or carbon dioxide. Depending on the thickness of the metal, the speed of this process varies, and the cuts can be quite complex, with tolerances of +/- 0.005 in. or better. Additionally, laser tooling doesn’t wear out as much as a mechanical cutter since there is no contact. Lasers are used in sheet metal fabrication in two different ways. The fiber-optic laser can perform precise cutting on thinner and more reflective materials. In order to accommodate thicker gauges, multi-gas or CO2 lasers are more powerful.

4. Photochemical machining involves using CAD stencils to leave a pattern that will be chemically activated to take away unwanted metal.

Metal Bending

sheet metal fabrication_CNC Metal Bending by Omnidex Laser based in Scotland

Metals can generally be bent along straight axes using various presses. Depending on their shape, bends can range from gentle curves, such as those along the vertical axis of a steel can, to sharp corners at 90-degree angles. Generally, these sharp bends are created by press brakes. Continuous bending can be achieved using rolling and forming methods to produce open and closed single-axis curves.

Metal Hemming

By rolling a metal shape, you can create a smoother, stronger edge. In open hems, an air space is left between the folded metal, while in closed hems, the folded metal is pressed into tight quarters. When a piece of metal is curled, it produces a rounded edge, also known as a barrel hem. Alternatively, it can serve a specific function, for example to hold a pin within a door hinge, eliminating the sharp edges.

What Are the Different Types of Sheet Metal?

Forming metal sheets and fabricating sheet metal parts can be done from a number of metals and metal alloys. The choice of material is determined by the application requirements, and factors like formability, welding, corrosion resistance, weight, and cost are taken into account. Materials commonly used for sheet metal include:

In sheet metal fabrication, stainless steel is categorized into two types: standard and spring-like.

Designing for Sheet Metal Fabrication Services_Stainless Steel Part_Omnidex

• Any 300 series steel can be non-magnetic, and this is the type of stainless most commonly used. As far as manufacturing is concerned, no heat or other stress relief is required. The stainless steel grade 316  has the highest corrosion resistance and maintains its strength even at elevated temperatures. While grade 304 is somewhat less corrosion-resistant, it is highly formable and weldable.

• Magnetized stainless steel 400 series is the most commonly used type in sheet metal fabrication. Although grade 410 has lower corrosion resistance, it is heat treatable. A brush-finished appliance surface made from grade 430 stainless steel is an inexpensive alternative to other stainless steel types. Due to their elastic rather than plastic deformation characteristics, these materials must be over bent in order to achieve their final form.

• When forming spring-like steels, heat is needed to relieve stresses, since the steel hardens quickly. Grades include 301, 17-4, 1095, and 1075. Stainless steel springs typically require specialized equipment and processes as well as a significant amount of over bending to achieve the final shape.

During cold rolling of steel, the finish is smoothed and the tolerance is tightened when forming. A wide range of alloys are available for CRS. 1018 and 1008 are the most common.

Designing for Sheet Metal Fabrication Services_Cold rolled steel Part_CRS_Omnidex
Cold rolled steel Part

Galvanized steel or galvanealled steel is used for these sheets, which is galvanized and then annealed.

Designing for Sheet Metal Fabrication Services_Pre-plated steel Part_Omnidex
Pre-plated steel Part

Aluminum is a moderately priced material that has a range of properties across a range of grades to meet specific application requirements. In Grade 1100, strength can be somewhat low, but it is chemical and weather resistant, weldable, and ductile, which allows deep drawing.

Designing for Sheet Metal Fabrication Services_Aluminum Part_Omnidex
Aluminum Part

The 3003 grade is strong and formable, weldable, corrosion-resistant, and affordable. The 5052 grade is significantly stronger while still being formingable, weldable, and corrosion-resistant. The alloy grade 6061 is corrosion-resistant and strong, but cannot be shaped. It is weldable, though it sacrifices some strength when welded.

For engineers and designers looking for a red metal, they typically select electrolytically tough pitch copper (ETP), such as C110 or C101. As an alternative, cartridge brass is sometimes used in less frequent cases.

Designing for Sheet Metal Fabrication Services_Brass copper Part_Omnidex
Brass copper Part

What Are the Different Types of Sheet Metal?

Fabricating sheet metal involves no addition or subtraction. It starts with flat material and, by definition, maintains the material thickness throughout the part. Welding multiple metal sheets together may allow some parts to be selectively thickened, but this is a costly and unusual practice.
Unlike other production processes, sheet metal fabrication has its own design criteria. Early identification of a part’s features and functions can facilitate a production-ready design. However, if a design has challenging features, a manufacturing vendor should be able to point these out and offer suggestions for making the design more user-friendly. A supplier may even offer design analysis software that highlights areas for design improvement quickly. Here are some design considerations:
    1. A sheet metal fabrication process is most cost-effective when it utilizes “universal” tools instead of part-specific ones. If the process of welding or riveting multiple parts together becomes too complex, consider using universal tools.
    2. Due to the fact that bends stretch the metal, features must be placed away from bends in order to prevent distortion. The convention to use is 4T – four times the thickness of the material.
    3. As a press brake bends sheet metal by pressing it into a die with a linear punch, it is not possible to create closed geometries.
    4. Compared to machining or 3D printing, the tolerances of sheet metal are much broader. Tolerances can be influenced by the thickness of the material, the machines used, and the number of steps in the manufacturing process. Tolerance information is typically provided by manufacturers.
    5. The bend radius should be uniform, such as 0.030 in. It’s recommended to use the (industry standard) across a single part to reduce the number of setups and speed production.
    6. If possible, keep a distance from bend to edge four times the thickness of the material. Doing so, eliminates the need to take off excess material.
    7. A thin material can crack or warp when it is welded. It would be better to use another assembly method.
    8. Be sure to follow the minimum requirements of the manufacturer for installation locations and material thickness when using PEM hardware.

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How to Finish Sheet Metal Parts?

Sheet metal can be finished in several ways and for different reasons. There are finishes that protect vulnerable materials from rust or corrosion and others that are applied for simple aesthetic reasons. In some cases, finishing serves both purposes. Some treatments are simply alterations to the metal surface itself; others consist of another material or process that is applied to the metal. Finishing treatments include:

It consists of shooting jets of abrasive material at the metal to roughen and clean the surface. Sandblasting is typically used on stainless and carbon steel and is often used as a preliminary step before painting to remove impurities and improve adhesion.

It is similar to sandblasting in function, but uses abrasive brushes to clean and score the metal surface. It can serve as a final finish on materials like aluminum and stainless steel and is commonly used as an appliance finish.

Polishing yields a glossy surface and is used on metals like stainless steel, aluminum, and copper. It can serve as the final finish or as preparation for other finishing processes such as plating. It is generally unsuitable for metals that are to be painted because it does not enhance adhesion.

Powder coating electrostatically applies a dry powder—typically a thermoplastic or thermoset polymer—to the metal surface and then cures it with heat. The process results in surface that is more durable than conventional paint but may not have paint’s aesthetic qualities.

Plating can be done electrolytically or electroless for various purposes. It can inhibit corrosion, improve solderability, harden a surface, prevent wear, reduce friction, or aid paint adhesion. Plating processes for sheet metal include:

    1. Passivation, a cleaning process that prevents corrosion in stainless steel by removing free irons from the surface of the material
    2. Chromate coating, a conductive coating used on aluminum to protect against corrosion.
    3. Anodizing, an electrochemical process used on aluminum and other non-ferrous metals that provides insulation and prevents corrosion
    4. Zinc, a self-sacrificing anti-corrosion coating for steels it is applied by galvanizing or galvanealing and is often combined with a Chromate coating over the zinc
    5. Nickel, often a cosmetic coating and can serve as a substrate for plating processes that cannot adhere to a given metal
    6. Tin, a solderable, conductive coating

Final Thoughts on Choosing Sheet Metal Fabrication

Making a decision to use sheet metal in an application is the first step in the process. Function dictates design, which in turn dictates the process. It is important to balance factors such as strength, weight and cost when choosing a material and gauge. The process is complex, but it can be made simpler by using finite element analysis (FEA) and CAD models. However, prototyping is where the real test lies.
Although today’s engineering tools are powerful, you can only determine whether a design meets expectations when you can see and handle the part in question. Is the material strong enough? How light is it? Is the design balanced, look good, and feel right? Does it affect other components? Before ordering hundreds or thousands of parts, even relatively simple components should be tested in an actual environment. Sheet metal parts sometimes require several iterations of prototypes before they are just right, and speed is a major factor in speed to market.

It is still possible to turn small quantities of prototypes into low-volume, end-use production parts once a design is developed, prototyped, and tested. Several different methods can be used to produce the same part, resulting in differing costs. It would be easy to choose if all metal shops provided the same range of services. Due to this, multiple quotations are necessary, as each will have distinct expertise and capabilities.

A Few Common Applications for Sheet Metal

In the modern world, sheet metal is one of the most versatile, functional materials and processes. From washers to entire roofs, designers and engineers develop components of all sizes. In addition to commodity products, it also appears in highly specialized products. Product lines leveraging sheet metal:

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