Sheet metal joining methods include both temporary and permanent solutions as well as those that require additional parts and pre-work. These methods include, mechanical fastening, welding, rivets, brazing and soldering, tongue and groove connections, adhesives, hemming, snap fits, clinching, tabs, and hinges. Each of these has their own advantages and challenges, so it is important to choose the most suitable method for your application:
1. Welding
Welding can be used to fuse two pieces of sheet metal together, typically forming a permanent join through the application of heat. When completed correctly, welding provides an extremely strong bond between the parts and, in some instances, the process can be automated to increase reliability and efficiency. There are a range of different welding techniques that can be used to join sheet metals, such as MIG and TIG welding, laser welding or friction stir welding, which is good for quickly joining sheets that are long or wide. Spot welding can also be used to create a non-permanent join, and is widely used as an automated process in the automotive industry.
2. Soldering and Brazing
Soldering and brazing tends to be used for smaller tasks, such as joining electronic parts, however they can create a strong bond between sheet metal parts through the use of a melted filler metal. These processes can be manual or automated, but brazed joints always require overlapping parts, which will impact the strength of the final joint.
3. Rivets
Rivets create robust joins for sheet metals and have been used in a great variety of applications, from vehicles to buildings and tools. They are good for assembling parts that may experience changes in pressure or temperature. Although it is possible to remove rivets by drilling them out or with a special rivet removal tool, they are generally deemed a permanent joining method. Rivets come in a range of different types, such as pop rivets, drive pin rivets, pull-mandrel rivets, solid rivets, threaded rivets, and tubular rivets. Downsides to riveting include that the process is noisy, requires special equipment, adds weight to parts and is difficult to correct should mistakes be made.
4. Adhesives
Adhesives offer a good solution for joining metal sheers, being cheaper than mechanical fasteners while also being corrosion resistant. They are best used when there are little or no tensile or shear forces. Available as single or two-compound substances in liquid or semi-liquid forms, adhesives need to be cured to reach their maximum strength. Curing can be done by applying heat and / or pressure as well as just leaving the adhesive to cure over time. Because of their liquid or semi-liquid nature, adhesives can fill small gaps or pores in the material. They are also ideal for applications where the final appearance is important and should not be marred by the use of fasteners. Adhesives also reduce vibrations but can suffer under extreme variations in temperature and when subjected to high tensile stress. As with all adhesive bonding, it is important to clean the surfaces to be joined as dirt, oxides or other substances between the surfaces will prevent proper adhesion. Solvents can be used to clean surfaces ahead of bonding. Adhesives are perform best when applied in thin layers, as thicker layers can develop cracks. When designing an adhesive joint, the width of overlap between the parts is more important than the length. Joints should be joined to withstand shear, tension and compression rather than cleavage or peel.
There are a range of different adhesives available to use when joining metals. Anaerobic adhesives cure in the absence of air and are best used for metal joins and for locking threads when used in conjunction with mechanical fastening. Acrylic and epoxy adhesives offer strong bonds with good temperature resistance,cyanoacrylate adhesives work well in shear conditions, while modified silicone adhesives are UV cured to form thin, invisible joints. Polyamide adhesives are an expensive option, but are capable of withstanding temperatures of up to around 500⁰C.
5. Mechanical Fastening (i.e. Screws, Nuts, Bolts)
Fasteners like nuts, bolts and screws offer a non-permanent solution for joining sheet metal, allowing you to easily disassemble parts without damaging them. They do require a level of precision in creating aligned holes through which the fasteners can pass, and also require the sheets to overlap or the use of plates to join them. As a result, mechanical fastening will add weight to parts and can also create points for corrosion or stress around the joining holes. However, mechanical fastening provides a quick and easy way to join sheet metals and is particularly useful where the sheets may need to be removed to expose components beneath for repair or replacing.
There are different types of screws, nuts and bolts that will offer different attributes when joining sheet metals, for example, a square nut will provide greater contact on parts with large surface areas whereas a knurled nut can be installed and removed by hand.
6. Tongue and Groove Connections
Tongue and groove joints can be used to temporarily join sheet metal parts. A ‘tongue’ is designed at the end of one part to fit into a groove that is cut into the other part. The width of the tongue should be designed to fit snugly into the groove with a slight play between them. The tongue should fit firmly inside the groove once inserted.
7. Hemming
Hemming is another technique that can be used to join sheet metals without the use of fasteners. The edges of two sheets are folded to create curved ‘hems’ that can interlock with one-another, creating a temporary join.
8. Snap Fits
Metal sheets can be joined using snap-fits, which use spring force to hold parts together. This includes spring clips that provide a fast option for sheet metal joins by holding them together under sprung force, as well as permanent and non-permanent sheet metal covers and tubular snap fittings.
9. Clinching
Clinching can permanently join two metal sheets without needing any pre-drilling of holes or the application of heat. The process works in a similar manner to hemming in that the materials are shaped so as to hold together except, with clinching, a button-like shape is created in the two sheets, holding them together. This is a time and cost-effective method for joining metal sheets that creates no sparks, fume or heat-related damage to the parts.
10. Tabs
Tabs can be used to assemble sheet metal parts. When using tabs, one of the parts has one or more tabs which can be inserted through slots in the other part before being bent over to secure them. Alternatively, tabs can be twisted instead of folded to secure them. Tabs can vary in size and shape to make assembly easier, but the process is intrinsically the same. Self-locking tabs include both the tab and the slot on the same workpiece, which is then folded back on itself to allow the tabs at one end to go through the slots to create a ring or tube. It is also possible to include redundant tabs on a workpiece, which can be aligned and bent over through a slot should they be required.
11. Hinges
Hinges come in a range of types, with each suited for different applications. With sheet metal joining, commonly-used hinges include spring-loaded pin hinges, pivot hinges and piano hinges. In all cases, the hinge should be able to cope with the weight of the part and the frequency of use. Spring loaded and pivot hinges allow for easy separation of the hinged parts while longer piano hinges are better for supporting heavier doors and parts. Hinges offer the benefit of easily being able to move the parts being joined to gain access to areas below.
Sheet metals can be assembled in a variety of different ways, depending on the application. Some of the techniques, such as welding or adhesive joining provide permanent joints while others, like mechanical fastening, provide temporary solutions. In addition, some of the processes require the use of additional parts or materials, adding weight to a finished structure, while others use the sheets themselves to create the joins.
The decision as to which technique to use will depend upon factors including ease of assembly, service conditions, requirements for the parts, costs, and the time taken to produce the join.