These basic guidelines for sheet metal fabrication include important design considerations to help improve part manufacturability, enhance cosmetic appearance, and reduce overall production time.
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What is sheet metal?
Sheet metal is one of the shapes and forms that can be purchased. Sheet metal is any metal with a thickness between 0.5 and 6 mm.
Basic Principles
Sheet Metal Fabrication is the process of forming parts from a metal sheet by punching, cutting, stamping, and bending.
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.
Bending
Bending is a process whereby a force is applied to sheet metal which causes it to bend at an angle and form the desired shape. Bends can be short or long depending on what the design requires.
Bending is performed by a press brake machine that can be automatically or manually loaded. Press brakes are available in a variety of different sizes and lengths (20-200 tons) depending on the process requirements.
The press brake contains an upper tool called the punch and lower tool called the die between which the sheet metal is placed.
The sheet is placed between the two and held in place by the backstop. The bend angle is determined by the depth that the punch forces the sheet into the die. This depth is precisely controlled to achieve the required bend.
Standard tooling is usually used for the punch and die. Tooling material includes, in order of increasing strength, hardwood, low carbon steel, tool steel and carbide steel.
Parts to be bent are supplied as flat patterns with bending information. Sometimes bend positions are etched with bend notches, or these notches can be cut out to show the benders where to bend.
Once the laser has cut the flat parts out they can be sent for bending. A press brake forms the flat pattern into a bent part.
The following are some terminology that are used in sheet metal. Designers need to adhere to machinery guidelines when designing for bending. Bends can be characterised by these parameters. Some critical dimensions that need to be considered when setting up sheet metal in CAD software are sheet metal thickness, the k-factor and bend radius. One needs to check that these factors are consistent with the tooling that will be used in manufacturing. This guide gives important guidelines for good design practice.
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.
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:
Parts need to maintain a uniform wall thickness throughout. Generally capabilities of of 0,9mm &#; 20mm in thickness are able to be manufactured from sheet (<3mm) or plate (>3mm) but this tolerance depends mainly on the part.
When considering sheet metal thickness, a single sheet with punches (holes) is a good rule of thumb. Some features such as countersinks are doable but counter bores and other machined features are difficult to produce as they require post machining.
Sheet metal bend brakes are used to bend material into the parts desired geometry. Bends that are in the same plane need to be designed in the same direction to avoid part re orientation, to save both money and time.
Keeping the bend radius consistent will also make parts more cost-effective. Thick parts tend to become inaccurate so they should be avoided if possible. Small bends to large.
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.
To prevent parts from fracturing or having distortions, make sure to keep the inside bend radius at least equal to the material thickness
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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.
It is recommended to use the same radii across all bends, and flange length must be at least 4 times the material thickness.
Minimum bend radii requirements can vary depending on applications and material. For aerospace and space applications, the values may be higher. When the radius is less than recommended, this can cause material flow problems in soft material and fracturing in hard material. Localised necking or fracture may also occur in such cases. It is recommended that minimum inner bend radius should be at least 1 times material thickness.
This is the minimum length of the The bend must be supported all the way until the bend is complete the flange must be long enough to reach the top of the die after it&#;s been fully formed. Brake press operators should know the minimum flange lengths for their tooling before attempting bends that may not work and while it is possible to calculate the minimum flange having an Air Bend Force Chart on hand certainly makes it more convenient.
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.
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.
Metal manufacturing is essential for all areas of the economy. Because of their strength, stiffness, and long-term durability, metal components are used in applications from appliances to construction parts and car body panels. Traditional metal manufacturing techniques include forming, casting, molding, joining, and machining.
Sheet metal forming involves various processes where force is applied to a piece of sheet metal to plastically deform the material into the desired shape, modifying its geometry rather than removing any material. Sheet metals can be bent or stretched into a variety of complex shapes, permitting the creation of complex structures with great strength and a minimum amount of material.
Sheet metal forming is the most cost-effective forming procedure today for manufacturing parts in large quantities. It can be highly automated in factories or, at the other end of the spectrum, manually operated in metal workshops for small series parts. It is a versatile, consistent, and high-quality procedure to create accurate metal parts with limited material waste. From metal cans to protective housing for hardware, parts created by sheet metal forming are found everywhere in our daily lives.
In this article, learn the basics of sheet metals, the various sheet metal forming processes, and how to reduce the cost of sheet metal forming with rapid tooling and 3D printed dies. For a detailed overview and the step-by-step method, watch our webinar or download our white paper.
Sheet metal refers to thin, flat metal pieces that are formed by industrial processes. These can be extremely thin sheets, considered foil or leaf, to up to 6 mm (0.25 in) sheets. Pieces thicker than 6 mm are considered plate steel or "structural steel.&#; Sheet metal thickness is normally specified in millimeters around the world, while the US uses a non-linear measure known as the gauge. The larger the gauge number, the thinner the metal sheet.
Sheet metal is widely used in the manufacturing of cars, aircraft, trains, hardware enclosures, office tools, furniture, house appliances, computers, machine components, beverage cans, and in construction (ducts, gutters, etc.).
Plate metal is generally used in applications where durability is more important than weight, for example in larger structural parts of ships, pressure vessels, and turbines.
Many different metals can be processed into sheet metal, including aluminum, steel, brass, tin, copper, nickel, titanium, and for decorative purposes, also gold, silver, and platinum.
Sheet metal work stock is normally rolled and comes in coils that can be cut and bent into a variety of shapes.
Sheet metal forming includes treatments such as bending, spinning, drawing, or stretching implemented by dies or punching tools. Forming is mostly performed on a press and parts are formed between two dies.
The sheet metal forming process is straightforward:
A sheet of metal is cut out from a stock metal to create individual blanks.
The blank is placed in the forming machine in between two tools.
Subjected to the high forces of the machine, the upper die (also known as the punch) pushes the sheet metal around the matching lower tool and bends it in the desired shape.
As a downside, sheet metal forming is an equipment-intensive operation. The procedure requires machinery and specialized tools that are part-dependent. As shown above, the tool&#;also referred to as the form or die&#;is the part of the forming machine acting to bend the sheet.
Typically, manufacturers produce their forming tools out of metal by CNC machining in house or outsourcing to service providers. This upfront tooling is expensive and generates significant lead times.
Driven by innovation, industries using metal components need more intricate parts with increased agility in fabrication means. Reconsidering tooling techniques can be a powerful lever for this.
Although large size parts such as car body panels are associated with heavy tooling, most metal workshops also produce all kinds of small units requiring lower bending forces. Replacing those metal tools with plastic parts printed in house for prototyping and low volume production can shorten development times and drive down production costs.
In-house 3D printing enables engineers to prototype metal parts and iterate tool designs in a matter of hours, achieving complex geometries while reducing reliance on outsourced providers. Professional desktop printers are affordable, easy to implement, and can be quickly scaled with the demand.
Manufacturers are already using stereolithography (SLA) polymer resins to substitute metal jigs, fixtures, and replacement parts in factories. In processes such as injection molding or thermoforming, using test molds in plastic is an effective practice to validate designs and solve DFM challenges before committing to expensive metal molds. Savings in material costs from metal to plastic are significant.
Watch the video to see how Shane Wighton from the Formlabs engineering team formed a sheet metal part using 3D printed tools for concept validation.
SLA 3D printing technology presents some interesting properties for sheet metal forming. Characterized by high precision and a smooth surface finish, SLA printers can fabricate tools with excellent registration features for better repeatability. Thanks to a broad material library with various mechanical properties, choosing a resin tailored to the specific use case can optimize the result of the forming. SLA resins are isotropic and fairly stable under load compared to other 3D printing materials. Plastic tooling can also eliminate a polishing step, as plastic dies do not mark the sheet as metal.
The mechanism is similar to the general sheet metal forming workflow. The difference lies in the design and print of the two-part tool made of upper and lower dies. The blank sheet is then placed between both plastic dies, and pressed with a hydraulic press or other forming equipment.
White PaperThis research work tests and demonstrates the viability of SLA 3D printed dies to form sheet metal parts.
Download the White PaperIn this white paper, learn how to combine rapid tooling with traditional manufacturing processes like injection molding, thermoforming, or casting.
Download the White PaperRethinking toolmaking is a powerful way for reducing costs in metal manufacturing. Beyond the agility provided by prototyping expensive tools, 3D printed plastic dies can be efficient and affordable substitutes to expensive metal tools. For sheet metal forming, 3D printed tools offer multiple opportunities for applications from bent brackets to embossed parts, louvers, grille, and off the shelf set of dies for a press brake.
In our free white paper, we demonstrate how we successfully fabricated a metallic blade guard with the help of 3D printed plastic dies. We could potentially produce dozens of these parts with a single set of dies, bringing short-run production in house. Download the white paper now for the detailed case study and the step-by-step method and watch the webinar for specific design considerations and application examples.
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