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.
Sheet metal fabrication processes can be classed as two main groups:
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
Forming
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.
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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.
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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.
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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.
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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.
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With shearing, force is applied with either a die, punch or blade to sheet metal, and this causes it to fail and separate in the location of the desired cut. Shearing is typically used for straight line cuts but can also produce angled cuts.
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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.
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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.
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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.
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With CNC punching, a punch press is used to push sheet metal into a die to create holes or cut-outs. CNC punch machines can also be used for secondary processes like tapping, blanking, forming, extruding or deburring.
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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:
Table 1: Manufacturing Processes Overview
* Higher gauge may result in deformation or reduced precision
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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.
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 MediumTable 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.
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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:
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.
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 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:
It is possible to weld other metals, although it can be more complicated and expensive.
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A Close-up View of Two Rivets
Another popular sheet metal joining process is riveting. With riveting, two sections of sheet metal are joined together with a rivet. A hole is drilled through both sections, and a rivet is then installed to join them together. Rivets have a flat, dome-shaped head at one end, and after they&#;re installed, the other end is flattened to prevent the rivet from moving. This creates a semi-permanent bond between the two sections of sheet metal.
The exact method of installation varies according to the type of rivet being installed and the type of joint being created. The two types of rivet joint are:
As well as butt joints with a single cover plate, it&#;s also possible to create a double butt-joint with two metal cover plates.
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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.
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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:
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.
The total size of your order may impact the choice of joining or assembly method. Large volume orders are likely to better suit automated processes or processes where initial set up costs may be higher. The costs associated with planning and setting up specialist equipment will be negated over the course of the entire production run.
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 15 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.
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