10 Things to Consider When Buying sheet metal forming process

There are many different choices to make when using sheet metal to fabricate a product. Decisions have to be made at the design stage, choices such as material selection, material thickness or what kind of finish is required. As part of the design process, it’s important to consider which production technologies might be used when manufacturing the part, and considering the impacts of that process on the part design. The choice of production processes will impact part price, tolerances, part geometry, lead time, tooling requirements, even part finish and strength. Often, the initial part design will need to be revisited after  thinking through the implications.  

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Sheet metal fabrication processes can be classed as two main groups:

• Cutting and forming processes
• Joining and assembly processes

Not all processes will suit all sheet metal parts and assemblies. For most parts and assemblies, however, there will be a number of potential choices. Different fabrication processes will produce slightly varying results in terms of the finished design approach. As well as this, different processes will also come with different costs, production times and other production limitations or opportunities that need to be considered.

The first group of sheet metal fabrication processes we’ll look at is cutting and forming processes.

In cutting and forming processes, sections of sheet metal are cut, bent, molded or otherwise manipulated to meet design specifications. The simplest example of a cut or formed piece of sheet metal is an angle iron. To create an angle iron, a long, straight piece of sheet metal is bent (formed) in one direction.

Cutting and forming processes are often the first stage of processing that sheet metal products undergo, coming before joining and assembly processes. 

Most cut or formed sheet metal parts are more complicated than angle irons, meaning several different cutting or forming processes are applied to a single piece of sheet metal over several stages. A single cutting or forming process may be repeated in multiple stages, or more than one cutting or forming process may be used. A sheet metal piece may have several laser cuts performed on it, and then it may be bent at more than one point, for example.  

Even parts as simple as an angle iron may be produced by combining two to three different processes. Angle irons can be produced through laser cutting and CNC bending, stamping, shearing and forming, etc.

In this section, we’ll look at the following cutting and forming processes:

Cutting

• Laser Cutting (CO2 and Fiber Lasers)
• Mechanical Cutting
• Water Jet Cutting
• Plasma Cutting
• Shearing

Forming

• CNC Sheet Bending
• Roll Forming
• Precision Manual Stamping
• Precision Progressive Stamping
• CNC Punching

Laser cutting uses a high-powered, industrial laser to cut sheet metal into the required shape and dimensions.

There are two broad categories of laser cutting: fusion cutting and ablative laser cutting. In fusion cutting, the laser melts the material, and high-pressure gas is used to sheer the melted material away. In ablative laser cutting, a pulsive laser is used to remove material layer by layer.

Modern laser cutters are either CO2 lasers or fiber lasers. CO2 lasers use infrared light to cut materials. They are less powerful and slower than fiber lasers, but they leave a smoother surface finish when cutting thicker materials. Fiber lasers are faster, generally more powerful, and can cut thicker overall material.

Advantages:

• Faster and more efficient than mechanical cutting methods.
• Smoother surface finish than mechanical and most other computer-controlled methods.
• Highly accurate, precise results that accommodate tight tolerances.
• No custom die required, and diverse shapes can be obtained.
• Works on a wide variety of metals as well as other materials.
• Accommodates a wide range of material thicknesses.
• Well suited for metal annealing, etching as well as engraving for serial numbers, barcodes, part marking, etc.
• With modern CNC controls, complicated individual cuts are possible as well as complicated production runs.

Disadvantages:

• Potential for heat induced structural changes to occur to the material, impacting the final product.
• Reflective materials can be difficult to cut
• Laser cutting is not suitable for most plastics or woods
• Requires in-process adjustment when cutting sheet metal with varying thicknesses.
• Work hardening of the edges may require additional processing before treatments like powder coating or painting can be applied.
• Other cutting methods may be better for high-volume production.

Plasma cutting uses an accelerated jet of hot plasma to cut through electrically conductive material, such as metal.

During plasma cutting, compressed gas is forced at high speed, in a jet, towards the material being cut. An electric arc is created in the gas between an electrode in the cutter and the electrically conductive material being cut. The electricity ionizes the gas and as electricity passes through it, creating a jet of plasma that becomes heated enough to melt the material being cut. Temperatures in the stream of plasma can reach over 20,000 °C.

Because the material being cut needs to be electrically conductive, not all materials can be cut with a plasma cutter. Common materials cut with plasma cutters are steel, aluminum, brass, copper, nickel and titanium.

Advantages:

• Very neat cut edges are possible.
• Fast cutting speeds are possible compared to many alternative cutting methods.
• Narrow kerf widths.
• Good levels of precision.
• Cheaper than laser cutting and water jet cutting.

Disadvantages:

• Not suitable for non-electrically conductive materials.
• The high temperatures can cause problems in the material being cut – less suitable for thin and medium-thin sheet metal parts.  
• Plasma cutters produce radiation, meaning that workers have to wear protective equipment.
• Less precision and slower cutting speeds than laser cutting.
• Post-processing may be required at cut edges.

Water jet sheet metal cutting uses a highly pressurized mixture of water and abrasive material to cut metal. The mixture is forced, by a hydraulic pump, through a system of pipes that pressurizes the water up to as much as 60,000 psi. The water is released as a thin jet, that is similar in essence to a laser, and this jet cuts the surface it lands on through abrasive action.

Water jet cutting is highly accurate and versatile, and modern machines can be programmed to produce repeated, identical cuts across an entire production run.

The big advantage water jet cutting has over other comparable cutting methods (laser and plasma cutting) is that the process does not produce as much heat. Where laser cutting or other heat producing methods will cause a problem for a sheet metal part, water jet cutting provides an alternative.

Water jet cutting can be done on materials other than metal and is often used on stone, glass, plastics, wood or rubber. For softer materials, pure water can be used as a cutting medium, without the addition of abrasive material.  

Advantages:

• Highly accurate, precise and intricate cuts are possible.
• Heat-induced structural effects on sheet metal are avoided.
• Small kerf widths are possible.
• Suitable for a range of metal types as well as other materials.
• Produces smooth cut edges, reducing the need for post-processing.
• On large production runs, water jet cutting can be cost effective.
• Thick pieces of metal can be cut – it’s possible to cut up to 300mm with water jet cutting, which is more than laser cutting.
• Water jet cutters can be combined with software to automatically manage complicated production runs.

Disadvantages:

• Cutting times are slower than with other methods.
• Thick cuts are not as clean as other cutting methods – tapering often occurs down the cut edge.
• For the best finish results, post-processing will be required.
• Can be more expensive than other cutting methods.
• Less suitable for small-scale production runs due to cost and set up times.
• Manufacturers with water jet cutting equipment may be harder to find.
• Less suitable to engraving and annealing.

In mechanical cutting, machines with parts that have power-driven, moving cutting edges are used to shape metal to match design specifications. The cutting edge of the machine simply removes material from a piece of sheet metal through mechanical action. The methods employed are less sophisticated than up to date methods but can be useful in some manufacturing scenarios.

Some examples of mechanical cutting machines are lathes, milling machines and drill presses.

In a lathe, a piece of metal being cut is spun and moved against a stationary blade that performs a cut. Milling machines work in the opposite way to lathes. A cutting blade is spun and moved against a piece of sheet metal, and it performs a cut as it comes into contact with it. With a drill press, a drill piece is used as a cutting edge and is pressed into a piece of sheet metal to perform a cut.

Mechanical cutting methods all vary according to how the piece of equipment being used performs a cut and work in a relatively straightforward way. Some other examples of mechanical cutting machines are grinders, saws and planers. 

Advantages:

• Mechanical cutting methods can be fast to set up, making them useful for small-scale production runs.
• A high level of accuracy is possible with modern cutting machines, and they can be useful where a high level of precision is required.
• Can be used on a wide variety of metal types as well as other materials.
• Many factories will have these machines available for use.
• Custom tooling usually isn’t required.
• It is easy to produce prototypes with mechanical cutting methods.
• Some mechanical cutting methods may have reduced heat induced structural effects on sheet metal parts.

Disadvantages:

• It may not be possible to perform certain cuts with mechanical cutting machines – lathes cannot cut in the center of a workpiece, for example.
• Several different mechanical cutting processes may need to be combined to complete the production of a part.
• Post cutting processing is often required with mechanical cutting.
• Specialist labor may be required.
• Slower processing times may be unsuitable for large-scale production runs.
• There may be variation between parts, depending on the exact production methods used.

CNC sheet bending uses a CNC press break or a CNC folding machine to bend metal into two or three-dimensional shapes.

This sheet metal fabrication process can be used to bend large or small sheets of metal, and high-quality pieces can be fabricated quickly. CNC sheet bending can be used in the early stages of fabrication, or it can be used to adjust a finished or near-finished product. Dies are available for standard angles and operations, meaning custom tooling may not be required.

Advantages:

• Highly accurate process, ideal for parts requiring high precision.
• Can produce large volumes in a short time.
• Low-cost basic process with minimal tooling costs.
• Suitable for high or low volume production.
• Can create multiple, custom shapes through a series of bending processes.
• Standard punches and dies are available, including in V and U shapes.

Disadvantages:

• The process can cause indentations or marks on the product linked to the point at which the bend is made.
• While bending can be roboticized, it can be labor-intensive, which makes is less suited for high volume production.
• Custom tooling is required for specialized bending projects.
• Bends need to be positioned on the sheet metal part to allow it to fit into the equipment without slipping.
• Fractures can occur when hard metals are bent parallel to the rolling direction of the sheet metal.
• Holes, slots or other features close to the bend can become distorted.

Bending Rollers (Barnshaws)

In roll forming a long strip of sheet metal is passed through a series of paired rollers. Each pair of rollers performs a partial bend on the strip of sheet metal. As it passes through the rest of the rollers, it is continuously bent until it meets the final, specified shape. The strip of sheet metal can then be cut into the required lengths to provide a finished part.

To create a unique shape through roll forming, it’s necessary to create a unique set of rollers. Custom roll sets are often needed, which can be costly and complicated to produce. This form of production is, however, useful for large-scale production runs because of the fast production speeds and low ongoing production costs.

Better results can also be obtained than they can through extrusion, which is a rival process. Thinner and stronger profiles are possible, for example. Often, no post-processing is required with roll forming as well, and the sheet metal part is ready to as soon as it’s cut to size. By performing bending at multiple stages (at each roller), it’s also possible to produce highly complex shapes through roll forming. Roll forming may, in fact, be able to produce shapes that aren’t possible through other production methods.

Advantages:

• Fast production speeds.
• Low cost over large production runs.
• Minimal post-processing required.
• Secondary processes, such as punching or cutting, can be incorporated into the rolling process.
• Improved results compared to extrusion for certain sheet metal parts.
• Highly complicated shapes may be possible through a relatively simple production process.
• Can provide an alternative where stamping is not possible due to engineering constraints.

Disadvantages:

• Complicated and sometimes expensive production set up process.
• Metal springback may create difficulties during production.
• Certain shapes cannot be formed through rolling.
• The end pieces in a strip of sheet metal can flare out and be unusable or poor quality.
• A large amount of floor space is required at the production facility.
• Can be inappropriate for narrow or short parts.

Stamping is the process of using a mechanical or hydraulic stamping press to form metal. When metal is formed in stamping press, a die is used to bend, punch, blank, coin, emboss, stretch, curl or otherwise process the piece of metal. Customized dies are created for individual production runs. Dies are highly specialized and need to be manufactured very precisely before being used in production.

Simple examples of the use of stamping are for forming shapes, letters or images in sheet metal through bending. For these productions, more simple dies can usually be used. In more complex projects, where processes like stretching, hemming, flanging or curling occur, dies are more complicated. Stamping presses can even mold metal into highly complex forms, such as when four-slide stamping is used to create a four-sided shape.

While creating dies can be time consuming and expensive, stamping machines work very quickly and very accurately. Stamping can be a highly cost effective and fast production method for large scale production runs.  

Advantages:

• Fast and cost-effective for medium to high volume production.
• Accurate and precise for complex shapes, producing identical pieces.
• Once the die is created, thousands or tens of thousands of identical parts can be produced very quickly.
• Well suited to small parts requiring micro-precision (±05mm linear tolerances or better).
• Versatile process that can include punching, blanking, embossing, flanging, bending or coining.
• Leftover material can be recycled.

Disadvantages:

• A long pre-production time is needed to make the custom die – production of dies can take upwards of 45 days.
• Changes needed once the die is created are costly and impact the production schedule.
• Stamping tooling is relatively fragile – repeated changes can result in shortened tool lifespan.
• Errors in the die will result in errors with an entire production.
• Pieces may require additional sheet metal fabrication processes, including deburring, tapping, reaming or counterboring.
• Less cost effective for smaller order production (generally thousands to tens of thousands of pieces minimum).
• Stamping tooling undergoes tremendous forces – it must be regularly maintained and monitored.
• Must use materials that are malleable enough to form without breaking.

With precision progressive stamping, a coiled strip of metal is passed through a stamping machine that contains a series of dies arranged in a sequence. Rather than having a single die processing the metal in one stage, as in manual stamping, processing occurs over several stages through the sequence of dies. As the strip of metal passes through the machine, bending, punching, blanking, coining, embossing, stretching, curling or another form of processing happens as the strip of metal passes each die. At the final stage, the strip of metal is cut to provide a finished part.

Progressive stamping is used where it isn’t possible to process metal with one die or where other production methods aren’t suitable. The big benefit with progressive stamping is that it can be used to create highly complex parts at fast speed. Dies can be created to produce a huge variety of forms with a high degree of accuracy, and stamping machines operate extremely quickly.

As with manual stamping, a great deal of care is needed in the creation of the dies, and they can also be costly to produce. Where complicated parts need to be created in large volume, however, progressive stamping can be very useful.

Advantages:

• Can be used to create more complex shapes than manual stamping or other production methods.
• Cost-effective for high volume production (tens to hundreds of thousands of parts) - due to very low levels of labor involved, the per piece cost for high volumes is very low.
• Accurate production of complex shapes.
• Fast production speeds.
• Well suited to small parts requiring micro-precision.
• Versatile process that can include punching, blanking, embossing, flanging, bending, coining or other processes.
• It is possible to reduce scrap production through careful design of parts and dies.

Disadvantages:

• Custom dies can be time consuming to produce – in many cases, more than 50 days lead time is needed before production can start.
• Dies and other tooling is also costly to produce, requiring a high upfront investment.
• If changes are needed after the die is created, this will be costly and disruptive.
• Stamping tooling is relatively fragile – repeated changes can result in shortened tool lifespan.
• Any errors in the die will result in errors with an entire production.
• Pieces may require additional sheet metal fabrication processes, including deburring, tapping, reaming or counterboring.
• Stamping tooling undergoes tremendous forces – it must be regularly maintained and monitored.
• Must use materials that are malleable enough to form without breaking.

The selection of cutting and forming processes for sheet metal parts depends on both design requirements and production limitations.

Different cutting and forming processes produce sheet metal parts with slightly varying physical characteristics. First of all, it’s important to choose a process that will produce a sheet metal part that meets physical design requirements. Different processes also come with different production costs, production times and levels of difficulty in manufacture.

Usually, the best thing is to pick the processes that deliver what’s needed in terms of the design requirements at the minimum cost, on time and with as little hassle as possible.     

The most important things to consider are:

• Process Cost
• Precision requirements (tolerance)
• Custom tooling requirements
• Bend thickness requirements
• Minimum order quantity
• Necessary lead time

Process Best Used For Precision/Tolerance Level Custom Tooling Required? Thickness Minimum Order Quantity Typical Lead Time Laser Cutting Small to large parts with every geometry possible Linear ±0.10mm
Angular - No 0.5mm to 20.0mm 1 Unit – prototype
100s to 1ks - MP ≤5 days for sampling, 10 days for MP CNC Sheet Bending Small to large parts with straight angle geometry, multiple bend possible ±0.18mm No, unless special form required 0.5mm to 20.0mm 1 Unit – prototype
100s to 1ks - MP ≤5 days for sampling, 10 days for MP CNC Punching Small to large parts with most geometry available, good for parts with multiple holes and embossed ±0.10mm No, unless special form required 0.5mm to 4.0mm* 1 Unit – prototype
100s to 1ks - MP 5 days for sampling, 10 days for MP Manual Stamping Medium volume production with tight tolerances, restricted geometry ±0.05mm to 0.10mm Yes; from 250 USD to 50k USD+ 0.5mm to 4.0mm* 3 to 5k units and above 30 to 40 days for tooling; 15 days for production Progressive Stamping Medium volume production with tight tolerances, restricted geometry ±0.05mm to 0.10mm Yes; from 10k USD to 100k USD+ 0.5mm to 4.0mm* 10k units and above 45 to 55 days for tooling; 15 days for production Shearing Thin material with simple geometry (straight lines) and low tolerances requirements ±0.50mm No 0.5mm to 4.0mm* 1 Unit – prototype
100s to 1ks - MP ≤5 days

Table 1: Manufacturing Processes Overview

* Higher gauge may result in deformation or reduced precision

Process cost is often the most important factor to consider when selecting the right sheet metal fabrication process.

As a basic approach, the best thing is to choose the cheapest production method that will produce parts to at least the minimum quality standards. There is no reason to over-manufacture parts at additional cost. The chosen production method needs to be able to manufacture parts to design specifications (dimensions, material use, material thickness, correct finish, etc.) and within tolerance ranges at the best price possible.

Making the right choice cost-wise can, however, become more complicated because different production methods will vary in cost according to various factors related to the manufacturing process. The main factors to consider are the volume of production and the capabilities of your manufacturer.

• Production volume – production volume is a critical factor because some methods of production are cheap over high volumes but expensive over small volumes. Stamping and progressive stamping, for example, require a high initial investment for tooling. This makes them unsuitable for small-scale production runs. Over high volumes of production, however, these production methods are far faster and more cost effective than others.
• Manufacturer capabilities – Manufacturers that specialize in certain production methods may be able to reduce costs for using them. This can be useful where using a particular production method will have benefits in terms of output quality or speed of production.

The table below presents a basic guide to manufacturing cost for different production methods:

Process Tooling Cost Part Cost Laser Cutting None Medium CNC Sheet Bending None to Low Medium CNC Punching Low Low to Medium Manual Stamping Medium Low Progressive Stamping High Very Low Shearing None Medium

Table 2: Manufacturing Processes Cost Overview

One of the most important things to consider when choosing a cutting or forming process is the level of precision that is required of the finished sheet metal part. The levels of precision that are achieved during production are important when it comes to the performance of the finished part as well as further assembly and alignment.

Tolerance is a measurement of the acceptable variation between an initial part design and an actual finished part. This measurement is used to determine precision requirements.

Most importantly:

• It’s essential to choose a cutting or forming process that will produce a part within the minimum tolerance levels.
• It may be beneficial to choose a process that is less precise but still within tolerance levels.

Achieving higher levels of precision is usually more expensive, and it can mean using more complicated and time-consuming production methods. As a result, the best approach is usually to choose processes with lower levels of precision wherever possible. Unnecessary precision may add no benefit at extra cost, time and effort.

Where high levels of precision are required, with certain processes, it’s possible nowadays to achieve precision levels of ±0.05 mm.

As well as the choice of cutting or forming process, there are other things that also influence precision levels:

• Material selection –thickness and shape accuracy
  • Metals are supplied to manufacturers within thickness tolerance ranges because precision varies in the initial production of sheet metal. This is even the case with high precision methods of production.
  • Some materials will have greater variability in flatness or material consistency – this can impact angular tolerances and springback.
• Equipment precision
• Fabrication company capabilities

With some sheet metal cutting and forming processes, custom tooling is required before manufacturing gets underway. It may also be necessary to buy additional equipment for certain production runs.

Examples of customized tools are dies, punches or molds of specific sizes and dimensions. These are made to match the specific requirements of a part being created in a production run.

Stamping is one process where custom tools are almost always required. They are used in other processes, however, with punching and bending being other examples of where they may be used.

Custom tools can be expensive to make in some cases, and a great deal of care is needed to ensure they are accurate. For this reason, there are often lengthy qualification processes, CMM measurement of tooling, 3D and 2D scanning of parts, and part measurement before tools are certified for use in production.

In some production processes, such as shearing, tools are rarely used, and in other processes standardized tools are often available. Where standardized tools are available, this negates the need to create custom tools. Custom tools do allow for highly individualized modes of production, but they take time to create and can add cost to the production process.  

Roll forming is another example of a sheet metal forming process that may require custom tooling and equipment.

The thickness of the sheet metal being processed in the production of a part is something that needs consideration. The main consideration is whether or not you will need to process thick pieces of sheet metal during production.

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Thick sections of sheet metal can only be processed using certain methods. Of the methods we’ve discussed above, the only two processes that are capable of being used with sections of sheet metal over 4 mm are laser cutting and CNC bending.

You may need to discuss minimum order quantities with your manufacturer before deciding on a cutting or forming process. It may be that your manufacturer is only able to use certain processes on large volume production runs.

Some processing methods are unsuitable for smaller production runs. Typically, this is where there are large costs or time requirements at the set-up stage of production. Stamping is one process that generally isn’t well suited to smaller production runs. This is because the time and expense that is needed in creating custom tooling and planning production procedures is often extensive. Other methods of processing may require expert labor or specialist equipment that can only be justified for a large order. This is why progressive stamping is the process of choice for high-volume sheet metal applications like automotive or consumer electronics.

On the other hand, if you have a relatively small production volume, semi-automated processes like laser cutting, or even manual processes like mechanical cutting may be better suited and faster.

Note that the exact minimum order quantities will vary between suppliers.

For designs which have not been fully prototyped or tested, it is not recommended to proceed with tooling or a long lead time process. It’s better to take the fastest possible approach to create physical samples and to be able to conduct field testing and certification. This is why initial samples and pilots are often done using laser cutting or similar processes - in case there are issues with the design, iteration is as simple as updating the drawing and recutting new pieces.

Once the design is proven and the product design and performance are stable, tooling can be cut for larger volume orders to reduce the product cost.

Another thing to consider, relevant to production requirements, is the lead time your manufacturer will need. This is the time it takes between a manufacturer receiving design requirements and them being able to begin production.

Where customized tools aren’t required and set up procedures aren’t complicated, lead times are reduced. With processes where designs can be fed straight into a production machine, such as laser cutting, lead times can be very short.

Other processes take much longer to set up. Where planning is needed, where custom tools are needed, where specialist equipment needs to be found or where staff need to be trained, for example, lead times will be longer. Large orders may also need a longer lead time because the manufacturer will need to plan to be able to free up resources.

Stamping is an example of a process that usually has long lead times. It typically takes 25 days to set up a stamping production run because of the need for custom tooling and careful production planning.

When it comes to prototype creation, it’s often possible to produce sheet metal part prototypes very quickly with the majority of cutting and forming processes. With laser cutting, bending, stamping and shearing, prototypes can often be produced in as little as an hour.

Sheet metal joining and assembly processes are processes in which separate sheet metal parts are put together to form a final assembly. The parts that are put together have usually already been subject to other forms of processing, such as cutting or forming.  

Finishing processes can be applied to parts before they are assembled, or they can be applied to complete assemblies.

In this section, we’ll look at the following joining and assembly processes:

• Welding
• Riveting
• Fasteners

Welding is a process whereby two or more parts are joined together through heating, pressure or both. As the parts cool, following the application of heat and/or pressure, a permanent bond is formed. Most people are familiar with the use of welding with metal, but it can also be used with thermoplastics and wood.

MIG welding and TIG welding are usually the best for sheet metal parts. This is because they occur at reduced temperatures, and this limits the effect high heat levels have on the metal. Where welding isn’t appropriate, brazing and soldering may make an alternative.

Welding is often used with sheet metal parts and is particularly effective with the following metals:

• Low carbon mild steel
• Aluminum
• Stainless steel

It is possible to weld other metals, although it can be more complicated and expensive.

Advantages:

• High strength joints with excellent durability and a long lifespan.
• Fast production speeds.
• Low cost in comparison to other joining processes.
• High precision production standards are possible.
• Low maintenance requirements.
• It is possible to join dissimilar metals.

Disadvantages:

• Can be visually unappealing and may require extra processing for aesthetic reasons.
• Limited to certain types of metal.
• Creates permanent joints that can’t be disassembled.
• Can require specialist workers or extra equipment for some types of welding.
• Heat can distort or discolor some metal parts and lead to reduced tensile strength.

Fasteners are external pieces of hardware that are used to join pieces of metal together. Screws and nuts and bolts are the most common examples, although there are other types of fastener available.

The big benefit with fasteners is that they create non-permanent joins between sheet metal parts. Fasteners can usually be removed at any time without the need for special tools and without damaging either the fastener of the parts that are joined together. 

The exact configuration and instalment method will depend on the parts being joined and the type of fasteners being used. However, the process is usually relatively straightforward. If the sheet metal parts being joined together need to be adapted to accommodate the fasteners, then this will take place first. The parts will then simply be fitted together with the fasteners.

Most fasteners used in sheet metal manufacture are made from metal. Steel, aluminum and titanium are the most common choice of metal for fasteners, although copper, brass, nickel and alloys can be used as well.

Advantages:

• Creates non-permanent joins between sheet metal parts that can easily be disassembled.
• Can be coated with other materials to improve corrosion resistance.
• Can be used to join dissimilar metals or different materials together.
• Suitable for thin materials.
• Issues related to heat during application that occur with other methods are avoided.
• Specialist labor and equipment usually isn’t needed during manufacture.

Disadvantages:

• Can be prone to corrosion in certain environments.
• Can be labor intensive and time-consuming during manufacture.
• Limited joint types available.
• Fasteners may loosen over time, causing issues with certain completed assemblies.
• Fasteners can be expensive, depending on the type used.
• Weight added by fastening hardware may create issues.

As with selecting cutting and forming processes for sheet metal parts, choosing the right joining and assembly processes for sheet metal parts will depend on design requirements and production limitations. You need to choose a process that will give you the results you need without adding excessive cost, time or difficulty to production.

The most important things to consider are:

• Design requirements
• Strength requirements
• Lead time and production speed
• Production volume
• Further finishing processes

The joining or assembly process used needs to fit with the design specifications of your sheet metal parts. It may be necessary to avoid using a joining process that adds weight to a finished assembly or changes its dimensions. Using fasteners, for example, will add weight and change the dimensions where the fasteners are located.

Welding is a popular choice where design requirements are strict. The use of welding joins is less likely to conflict with design specifications regarding the dimensions of the finished part or assembly. As well as this, welded joints also offer good strength. Discoloration or dimensional changes caused by heat during processing may be an issue, however. Where this is the case, rivets or fasteners are likely to be the best alternative.   

Aesthetics are another thing that needs to be considered at this point. Different joining and assembly processes have different visual results. Where the visual impact needs to be minimized, welding or riveting is often the best choice. While welded joints can be seen, they can usually be covered up with painting or other processes. Rivets can also impact the eye less.

Strength requirements are usually stipulated in design documents and are a key consideration. Strength requirements will depend on the end use of the sheet metal assembly. In some cases, they can be important, such as with safety critical components. Parts used in aviation, for example, will have very strict strength requirements.  

The best thing is to consult with your manufacturer when making this consideration, as they will have knowledge of the different strengths different joining and assembly types offer. The different methods are tested, and the strength provisions are usually relatively well known.

In general, welding usually provides a stronger join between sections of sheet metal than rivets and fasteners. This does depend on the assembly in question, however, as well as the types of riveting system or fasteners being used. Some fasteners can offer very strong bonds, for example.

You should consult with your manufacturer about the amount of lead time that will be needed to set up production as well as the speed with which production will take place. Where skilled labor or specialist equipment needs to be sourced, this will increase the setup time before production can begin. If managing the assembly or joining process will be complicated, this will also mean set up time will be increased.

The amount of time an entire production run will take place will depend on the level of complexity of the chosen joining or assembling procedure. Where production takes longer, costs are likely to increase.  

Welding in particular is a very cost and labor intensive process – the longer and more complex the weld, the more time spent. For complex parts with large amounts of welding, takt times can be as long as 30 minutes to an hour or more. As welding laborers are generally highly skilled, this labor is costly. While welding can be roboticized, there is cost involved in programming and welding jigs.

If your sheet metal assembly will undergo further finishing processes after joining or assembly takes place, these will need to be taken into consideration. The type of assembly or joining process used will obviously need to fit with the finishing stage of the product’s manufacture. The most important thing to consider is that, where new materials are introduced, these will need to be compatible with the chosen finishing process.

As well as new materials being introduced, assembly or joining processes may also influence the uniformity and texture of a sheet metal assembly. Welded joins, for example, will have a different texture where the weld is located. In this case, additional steps may need to take place before finishing processes can be applied. Grinding, filling or smoothing may need to be employed, for example.

The best thing is to consult with your manufacturer because they will have expert knowledge.

Choosing sheet metal cutting, forming, joining and assembly processes is far more complicated than it looks at first glance. Different processes offer different advantages and disadvantages. These need to be considered carefully against design specifications, and an appropriate process needs to be chosen.

Expert guidance is nearly always essential when choosing the right processes.

Our team of engineers and technicians here at Komaspec have more than 20 years’ experience in sheet metal fabrication. We have experience with everything from simple production processes to progressive stamping to deep drawing. We also have experience of complimentary processes, such as the incorporation of fasteners, optimization for welding, and so on.

We provide professional design for manufacturing and design review services as part of our service to our customers. We’ll walk you through the process of selecting the best manufacturing strategy for your product’s needs. That’s whether it’s a forming or cutting process like laser cutting, bending or stamping or an assembly or joining procedure like welding or riveting.

Feel free to get in touch with us here.

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