sheet metal fabrication design guide

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. 

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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. 

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.

5 Factors For Choosing Sheet Metal Fabrication

­ Written By: Tony Varela

Introduction:

One of the most adaptable building materials in the manufacturing industry, sheet metal has rightfully found its place as one of the most important materials in the industrial age.  Steel, aluminum, brass, copper, tin, nickel, titanium, or other precious metals are traditionally used to make sheet metal.  Thicknesses vary but are mostly broken into two distinctions; thin gauge and heavy plate.  Many different industries rely on the versatility and durability of sheet metal including aerospace, appliance manufacturing, consumer electronics, industrial furniture, machinery, transportation and many more.

Why Choose Sheet Metal?

Sheet metal offers plenty of advantages as compared to both non-metal alternatives and other metal fabrication processes, as well.  When compared to machining, sheet metal is much less expensive in both processing and material costs.  It does not have the extremely high tooling costs of injection molding, which makes sense at high volumes.

As found in machining, rather than starting with an expensive block of material, much of which is wasted in the milling process of removing unneeded material, sheet metal lets you buy what you need and use what you need with relatively low material waste.  The unused sheet can then be used for another project, while the shavings produced in machining, need to be discarded and recycled.

With the advancement of technology used in modern fabrication, automation and new CAD (computer aid design) programs make designing in sheet metal easier and easier.  CAD programs now have the ability to design in the same material you intend to fabricate with and will allow programming of the parts to come straight from the CAD model itself.  No longer is there a need to create a separate set of shop drawings to interpret the design.  Perhaps most significant, in a world of mass production, sheet metal has the ability to scale rapidly.  The greatest cost for sheet metal fabrication is in the first piece.  This is because the cost is all in the setup.  Once the setup is complete, and the costs are spread out across the larger volume of pieces being fabricated, the price drops significantly, greater so than most subtractive processes like machining.

1. How is Sheet Metal Being Used?

Sheet metal can be cut, stamped, formed, punched, sheared, bent, welded, rolled, riveted, drilled, tapped, machined.  Hardware can then be inserted to fix electronic components, metal brackets or other pieces of sheet metal.  To finish sheet metal, it can be brushed, plated, anodized, powder-coated, liquid painted, silkscreen, laser-etched, and pad printed.  And of course, parts can be welded riveted into complex assemblies.

Just like any other technology, the processing of precision sheet metal is constantly evolving.  Materials, processes, tooling, and equipment are becoming highly specialized which is improving the time involved to make common sheet metal parts and speeding up the design process as well.  To fully leverage all the technological advantages, it is important that you select the right supplier and know the differentiation between metal fabricators; architectural sheet metal (HVAC and ductwork), heavy plate fabricators (staircases, fences, heavy structures) precision fabricators (thin gauge sheet metal, enclosures, brackets etc&#;).

Along these lines, this white paper will explore key components of the precision sheet metal fabricator, precision sheet metal fabrication.  This paper will focus on:

• Fabrication techniques
• Common Materials
• Design considerations
• Finishing options

2. Sheet Metal Fabrication Techniques

By definition, sheet metal starts out flat, but before this, it comes from large cast ingot and the rolled into a long ribbon in the desired thicknesses.   These rolled coils are then flattened and sent as large sheets cut to different lengths to accommodate the manufacturing shop&#;s needs. While this paper focuses on bending sheet metal along a single axis, there are processes out there, hot and cold forming techniques that include bending and forming sheet metal along multi-axis points in one process such as deep drawing, hydroforming, spinning and stamping.  These processes are most commonly found in the manufacturing of products like automobile panels, aluminum cans, and complex formed consumer appliances.  Another similar process is progressive stamping which moves a ribbon along a series of stamping which forms and punches different stages.  At the end of these progressive stages, you are left with a finished part.

Cold forming will be the focus of this paper.  Examples of cold-forming processes are as follows

Cutting

• Shearing was one of the longest standing means to cut sheet metal but has since been replaced by faster, more precise methods.
• Punch Press use tools called punch and dies, which punches holes and shapes to make any number of patterns.  Particularly effective for cutting simple patterns that are more economical than cutting on a laser cutter or a water jet. Punch presses can operate at hundreds of strokes per minute making this a suitable center for processing parts quickly.
• Laser Cutting works with a combination of oxygen, nitrogen, helium, or carbon dioxide to burn away material and produce a clean edge.  This form of cutting can hold very tight tolerances.
• Photochemical Machining is a process of controlling the etching using CAD-generated stencils to leave a pattern that is chemically activated to remove unwanted material.

Hemming &#; The edges of the sheet metal are folded over itself or folded over another piece of sheet metal in this forming operation to achieve a tight fit or a stronger, rounded edge.  Hemming is a technique to join parts together, improve the appearance, or increase the strength and reinforce the edge of the part.  Two standard hemming processes include roll hemming and conventional die hemming.   Roll hemming is carried out incrementally with a hemming roller.  An industrial robot guides the hemming roller and forms the flange.  Conventional die hemming is suitable for mass production.  With die hemming, the flange is folded over the entire length with a hemming tool.

Bending &#; Most sheet metal bending operations involve a punch and die type setup when forming along one axis.  Punch and dies come in all sorts of geometries to achieve varied different shapes.  From long gently curves to tight angles at, below, or above 90-degree angles bending metal can achieve many different shapes.   Press brakes are generally needed when a sharp angle is desired.   Rolling and forming methods are used when a long continuous radius is desired in one direction, or along one axis.

3. Common Types of Sheet Metals

There are many different metals and alloys that come in sheet form and are ultimately used in the fabrication of manufactured parts.   The choice of which material depends largely on the final application of the fabricated parts, things to consider include formability, weldability, corrosion resistance, strength, weight, and cost.   Most common materials found in precision sheet metal fabrication include:

Stainless Steel &#; There are a number of grades to choose from, for the purpose of this white paper we will focus on the top three found in precision sheet metal fabrication:

• Austenitic stainless is a non-magnetic &#; any of the 300 series steel &#; that contains high levels of chromium and nickel and low levels of carbon. Known for their formability and resistance to corrosion, these are the most widely used grade of stainless steel.
• Ferritic &#; Stainless steels that are magnetic, non-heat-treatable steels that contain 11-30% chromium but with little or no nickel. Typically employed for non-structural uses where either good corrosion resistance is needed such as with seawater applications or decorative applications where aesthetics are the main concern.  These metals are most commonly found in the 400 series stainless steel.
• Martensitic &#; A group of chromium steels ordinarily containing no nickel developed to provide steel grades that are both corrosion resistant and hardenable via heat-treating to a wide range of hardness and strength levels.

Cold Rolled Steel &#; A process in which hot rolled steel is further processed to smooth the finish and hold tighter tolerances when forming.  CRS comes in and alloys.

Pre-Plated Steel &#; Sheet metal material that is either hot-dipped galvanized steel or galvanealed steel, which is galvanized then annealed.  Galvanization is the process of applying a protective zinc coating to steel in order to prevent rust and corrosion.  Annealing is a heat treatment process that alters the microstructure of a material to change its mechanical or electrical properties, typically reducing the hardness and increasing the ductility for easier fabrication.

Aluminum &#; An outstanding strength to weight ratio and natural corrosion resistance, aluminum sheet metal is a popular choice in manufacturing sectors meeting many application requirements.  Grade offers excellent corrosion resistance, excellent workability, as well as high thermal and electrical conductivity.  Often found in transmission or power grid lines.  Grade is a popular alloy for general purposes because of its moderate strength and good

workability.  Used in heat exchanges and cooking utensils.  Grade and are commonly found in metal fabrication.  Grade is the most widely used alloy best known for being among the stronger alloys while still formable, weldable, and corrosion-resistant.  Grade is a solid structural alloy most commonly used in extrusions or high strength parts such as truck and marine frames.

Copper/Brass &#; With a lower zinc content brasses can be easily cold worked, welded and brazed.  A high copper content allows the metal to form a protective oxide later (patina) on its surface that protects it from further corrosion.  This patina creates an often highly desirable aesthetic look found in architectural or other consumer-facing products.

4. Design Considerations for Sheet Metal Fabrication

Engineers designing sheet metal enclosures and assemblies often end up redesigning them so they can be manufactured.  Research suggests that manufacturers spend 30-50% of their time and 24% of the errors are due to manufacturability.  The reason behind these preventable engineering errors is usually the wide gap between how sheet metal parts are designed in CAD programs and how they are actually fabricated on a shop floor.  In an ideal scenario, the designing engineer would be familiar with the typical tools that will be used to fabricate the sheet metal parts while also taking advantage of designing within the CAD programs available sheet metal settings.

The more that is known about the fabrication process during the design phase the more successful the manufacturability of the part will be.  However, if there are issues with the way certain features were designed, then a good manufacturing supplier should be able to point those out and suggest good alternatives to address them.  In some cases, the suggestions may

same time and unneeded costs.  Here are some considerations while designing sheet metal for fabrication:

• Sheet metal fabrication is most cost-effective when standard tool sizes are used as opposed to costly custom tools that need to be made specifically for the job. If a single part becomes too complex, consider welding or riveting parts together that can be made using standard, or universal tools.
• Because bends will stretch material, features such as holes, cut-outs, inserted hardware should be located well enough away from bends to prevent distortion of the hole. To help with this rule, remember &#;4T&#; which means located features four times the material thickness away from any bends.
• Press brakes create bends by pressing sheet metal into a die with a linear punch, so the design does not allow the creation of closed geometry.
• Sheet metal tolerances are far more generous than machining or 3D tolerances. Factors affecting tolerances include material thickness, machines used, and the number of steps in the fabrication process.  Suppliers generally will provide detailed tolerance specifications as it related to their shop and machines.
• A uniform bend radius such as 0.030 in. (industry standard) should be used on every bend of a part to reduce multiple setups and accelerate production.
• Welding thin materials can lead to cracking or warping. Consider other joining methods when working with thin materials.
• Consider material thickness and manufacturers&#; minimum requirements when installing PEM hardware.

5. Finishing Sheet Metal

There are several different methods and reasons to finish sheet metal parts.  Depending on the material chosen, some finishing techniques protect the material from corrosion or rust while other finishing materials are done for aesthetic reasons.  In some cases, finishing can achieve both purposes.  There are finishing processes that include simple alterations to the surfaces of the materials.  Other finishing processes consist of applying a separate material or process to the metal.  Standard finishing techniques include:

• Brushing is used to deburr and remove surface defects from sheet metal parts.  The surface pattern created is a uniform parallel grain resulting from the brush moving against the metal surface in one direction.
• Plating is a process of coating a layer of metal to an object of different types of metal.  This process is done, commonly, for aesthetic purposes or more practical reasons such as corrosion resistance and protection from wear and tear.
• Polishing sheet metal as a means of finishing is where a layer of oxidation and also a thin layer of the material itself is ground off.   In doing so it smooths out the metal, removing imperfections, and making it gleam once more.  Generally, start with a coarse grit of sandpaper, like a 40 to 80 grit and then work towards a finer grit to finish off the polish effect.
• Powder Coating creates a hard finish that is tougher than conventional paint.  Powder coating comes in any color and therefore makes a great custom finish method when both durability and aesthetics are desirable finishes.
• Abrasive Sand Blasting, more commonly known as sandblasting or media blasting, is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface or remove surface contaminants.

Concluding Thoughts

Selecting a material, in this case, sheet metal is the first step in any design process.  The process begins with the function of the part you are intending to design.  The function of the part will help determine the needed design.  Choosing a material and gauge are critical steps that involve balancing factors like strength, weight, and cost.  This is not a simple process but can be streamlined by using CAD models with the above design considerations found in this white paper.  The next real test, however, is prototyping.

While today&#;s engineering tools are powerful, it is only when you can see and handle a part that it becomes known whether the design will meet expectations.  Is it strong enough?  Light enough? Does it look, feel, and balance the way it should?  Does it sacrifice other components?  Even relatively simple components benefit from real-world try out before committing to hundreds or thousands of parts.   In some cases, it may take several prototype iterations to get the sheet metal part right.  With a good manufacturing supplier, this process

can be kept at a minimal impact on the overall project but getting it right earlier in the prototype process.

It is tempting for larger enterprises to outsource design to engineering service providers so they can focus on core activities.  However, selecting the right partner helps avoid further widening the gap between the ideal design and fabrication process and the all too common real-world scenario of poor designs making to the fabrication floor without resolution of design flaws.  Working with partners willing to collaborate, interested in knowing more about the manufacturing process, and involved in developing sheet metal products.  When selecting fabrication suppliers, look for companies with a proven track record in producing parts and who bring a vast wealth of fabrication knowledge to ensure fewer hiccups in the design to the fabrication process and product is brought to market faster.

Source:

The Aluminum Association. &#;Aluminum Alloys 101,&#; (n.d.) Retrieved from https://www.aluminum.org/resources/industry-standards/aluminum-alloys-101

Australian Stainless Steel Development Association. &#;Types of Stainless Steel&#; () Retrieved from https://www.assda.asn.au/stainless-steel/types-of-stainless-steel/austenitic

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