Complicated parts can be made through relatively simple processes, without the need for tooling, and at low cost. When joining these parts together in assemblies, fasteners are usually the cost effective, flexible and simplest way to go.

The selection of the right hardware is often not thought through, and simple screws or bolts are picked without considering how they impact product performance, service in the field or the strength of the assembly.  If you pick the wrong fastener for a trailer, your product can shake apart on a highway. If you use the wrong grade of bolt in a structural part, it can shear off in a strong wind. If you use stainless fasteners, they can seize in a part and make it impossible to unscrew.

It’s critical to pick the right type, grade and material of fastener when approaching your sheet metal product design. We’re here to point you in the right direction with overviews of the key concerns, suggestions for different applications or needs, and defining terms and industry standards.

• Making the right choice of hardware is crucial for product performance and manufacturing speed and costs.
• Hardware choices are graded according to the materials they are made from and their strengths.
• Pop rivets are aesthetically pleasing, low cost and fast to install. They are, however, permanent, and they are less strong than other fasteners.
• Screws allow for disassembly and are relatively low cost and fast to install. They are less aesthetically pleasing and less strong than other fasteners.
• Nuts and bolts are strong and aesthetically pleasing. They are, however, expensive, they add weight, and they’re low to install.
• Galvanic corrosion is a hidden factor that you should always consider when selecting hardware. It is particularly important for parts that will be used outdoors.

• Key Takeaways
• What Hardware Options are There for Sheet Metal Parts?
• Rivets
• Screws
• Nuts and Bolts
• Other Hardware Types
• How to Identify the Best Hardware for Your Project
• Galvanic Corrosion and Hardware Compatibility
• Summary
• Komaspec

Fig. 1: Sheet Metal Hardware

There are various types of hardware used for sheet metal fabricated parts. The most commonly used, however, are rivets, screws and nuts and bolts. These are all types of hardware that most people are familiar with, and we’ll cover them in detail in this article.

As well as looking in depth at these types, we’ll also briefly touch on some alternative fastener types.

To look at a range of fasteners and variations for each type, look here.  

Ultimately, the right choice of hardware should be considered at the design stage, taking into account the type of fastener, the material (of both the sheet metal part and the fastener), tolerances and placement. We can, however, start by looking at what different types of fastener there are.

Rivets are widespread, permanent fasteners that can be used for both large and small products and in a variety of applications. They cannot be removed once applied (hence the word permanent), and they are quick and easy to apply. Often, they make a low-cost alternative to screws.

A rivet includes a cylindrical shaft, known as a tail with a head at one end. Rivets are installed by forcing the tail through two pieces of metal and then compressing it so that it expands and flattens. When the tail expands and flattens it forms a head at the other end of the rivet, and a permanent join is created.

Three common types of rivet are:

Solid Rivets

Solid rivets are made with a solid shaft that’s generally shorter than with pop rivets. They are often inserted into a pre-drilled hole, and access to both sides of the rivet is required during insertion. Solid rivets are commonly used in industrial applications requiring high strength.

• Generally seen as the strongest of all the rivet types
• Heads can be flat, coned, buttoned or countersunk
• Can be clinched using staking, spin-roller forming, orbital forming or radial forming

Fig 2: Solid Rivet

Tubular (and Semi-Tubular) Rivets

Tubular rivets are similar in appearance to solid rivets, but they have a hole in the tail end. This hole helps minimize the force needed for insertion and also the amount the tail deforms during installation.

• Riveted surfaces can rotate, creating a permanent, hinged joint
• Access to both sides of the hole is needed during insertion

Fig. 3: Tubular Rivet

Pop Rivets (Blind Rivets)

Pop rivets, also known as blind rivets, generally feature a longer tail and are installed from just one side of the hole using a specially designed tool. Pop rivets require a pre-drilled hole but are fast and convenient in installation.     

• Specialized riveter required for installation
• Available as self-drilling, self-tapping and speed fastening

Fig. 4: Pop Rivet

Design Considerations for Rivets

The permanence of rivets may be useful in some situations and a drawback in others.

Advantages

• Rivets are low cost, with no threading required prior to installation, and installing them is faster and easier than most other fastening methods.
• They’re also aesthetically pleasing in a lot of applications (especially flush rivets, which allow for a much sleeker appearance), and they create a permanent, tamper proof product.

Disadvantages

• Rivets are often less strong than other types of fastener, and they’re a less precise method of joining.
• As well as this, the application method is more inconsistent than with other fasteners. Screws, for example, can be inserted with a precise level of torque, whereas there’s much more variability in rivet application.

Table 1: Comparison of Rivet Types and Features

Screws are high strength fasteners that can be removed for repair or maintenance. They also do this without the need for a nut, as is the case with bolts.

Fig 5: Metal Screws

The most common types of screw are machine screws, self-tapping screws and self-drilling screws. Different screws come with different head types, including pan head, hex head, round head, oval head, truss head and flat head.

Machine Screws

Fig. 6 Machine Screws

Machine screws are screws that are installed into a pre-drilled hole with a pre-installed internal thread.

Machine screws are strong and visually appealing, but they are more time-consuming and expensive to install than other types of screw. They are particularly useful in applications where metal is joined to metal. As well as this, the consistent torque and clamping force they provide makes them useful in application such as electronics, where reliability is important.

Self-Tapping Screws

Fig. 7: Self Tapping Screws

Self-tapping screws are screws that are inserted into a pre-drilled hole without a pre-installed internal thread. As the screws is wound into the hole, it creates its own thread.

Self-tapping screws allow for fast installation, but the pre-drilled holes also make it possible to install them accurately and consistently. Time and machinery is not need for installing tapping in the holes, making production much simpler. Self-tapping screws are also useful in applications where metal is joined to another material, such as wood or plastic.

Self-Drilling Screws

Fig. 8: Self Drilling Screw

Self-drilling screws are installed without the need for a pilot hole. The screws feature an extra-durable, drill tip like head that penetrates the material as it is driven in.

Self-threading screws are the fastest to install, saving more costs and time in production. As with self-tapping screws, self-drilling screws make a good choice where metal is joined to another material, such as wood or plastic. Seld drilling screws are typically used in situations where aesthetics are less important. They’re also usually used on thin materials.

Table 2: Comparison of Screw Types

Screw Installation

Screws are installed by being driven through two pieces of material using a hand drill or power tools. As the screw is driven into the two pieces of material, its thread grips and pulls the two parts together.

• Most screws require pre-drilled holes with internal threads (thread tapping) in the part before screws can be inserted. This creates processing costs and requires time
• Self-tapping screws do exist, which create their own thread as they are driven in
• Some screws can be driven into material with no pre-drilled hole
• Screws are generally lower cost than nuts and bolts, and installation is often faster
• In some applications, screws can be time consuming to install. Although, if automation is possible, this won’t be the case
• Screws are generally more expensive than blind rivets

Design Considerations for Screws

Advantages

• Screws are highly versatile, with a wide range of types available to suit different materials (structural steel, brass, bronze, zinc, aluminum all suit screws) and applications.
• They also offer the possibility of making fine adjustments during installation and of creating very tight seals. They can also be removed if a component needs maintenance or repairs.

Disadvantages

• Incorrect installation can cause threads to strip, which decreases the strength of the joint.
• There’s the potential for screws to loosen if vibration occurs; this may require the addition of locking compound, spring or lock washers, or nyloc nuts in order to ensure the screw stays fastened.
• Inconsistent sheet metal thickness can result in inconsistent torque, which impacts hardware strength.
• Screws may also be unaesthetically appealing in some circumstances.

Bolts are long threaded metal bars that are used in conjunction with a nut that is screwed onto the bolt.

Common bolt types include carriage, hex head, machine, shoulder, socket cap, socket set and square head.

Common nut types include cap, castle, coupling, flange serrated, hex, keps-k lock, thumb, structural and wing.

Fig. 9: Sheet Metal Hardware

Nut and Bolt Installation

Nuts and bolts are installed by threading a nut through a hole that is drilled between two pieces of sheet metal. The nut is then tightened to bring the two pieces of sheet metal tightly together. Usually, nuts and bolts are used in conjunction with washers to distribute loads more evenly.

• A pneumatic wrench is often required for installation and removal
• Installation requires access to the back of the joint

Design Considerations for Nuts and Bolts

Advantages

• Nuts and bolts are the best option when a high strength joint is required.
• They’re also very easy to remove, and so they make a good choice when dismantling will be necessary.
• Nuts and bolts are also very versatile, and there’s a wide range of types to choose from.
• Lastly, they are easy to inspect, making them a good choice in safety critical applications where joint condition needs to be inspected.

Disadvantages

• Nuts and bolts can be more expensive than alternative fasteners, and more manual labor is often required during installation
• Nuts and bolts may also be unaesthetically appealing in some applications
• Size and weight can be a problem in some applications
• Corrosion can occur with nuts and bolts
• Over-tensioning can cause strength problems, if it occurs

Table 3: Comparison of Hardware Types for Sheet Metal Projects

Two other common types of hardware are threaded inserts and threaded rods.

Threaded Inserts

Threaded inserts are inserted into a piece of sheet metal, and they feature an internal thread. Another sheet metal part (usually, with a pre-drilled hole) can then be joined by inserting a bolt or screw into the threaded insert. Threaded inserts are useful in materials that are too soft to suit thread tapping for screws. They are also useful when joining a thin sheet metal part to one that is too thick for nuts and bolts. There are other situations in which they offer an advantage as well.

Fig. 10: Threaded Insert

Threaded Rods

Threaded rods are attached to a piece of sheet metal. Two pieces are then joined by attaching a nut to the threaded rod, with the second piece of sheet metal having a hole through which the threaded rod goes. Threaded rods are particularly useful when there needs to be space between two pieces of sheet metal that are being joined. A simple method is to use a threaded rod in conjunction with a spacer.

Fig. 11: Threaded Rod

Not all fasteners of the same type are equal in terms of strength, durability and resistance to environmental conditions. Fasteners are made with different materials, and some will perform better in some environments than others. They’re also made to different strengths and levels of durability. Grades are given to fasteners according to the materials they’re made from and their strengths.

The most important metric in grading a fastener’s strength is the tensile strength. Tensile strength is the amount of stretching stress or load that the fastener can withstand before it breaks or deforms. Two other important strength metrics are yield strength and proof load.

Fig. 12: Metal Nuts

Markings are placed on fasteners to distinguish the grade. Bolts and screws, for example, are marked with lines, numbers or codes on the head. Nuts are given markings on the face of the nut.  

Usually, grading applies to fasteners over a certain size and to fasteners made for specialist purposes.

Fig. 13: Custom Hardware Kit from Komacut

Here are some questions you can ask to help you narrow down your hardware choices.

Is the joint permanent, or will you be taking it apart for maintenance or repairs?

• If the part is going to be taken apart, opt for screws or bolts
• If the joint is permanent, choose rivets
• Hardware with self-locking threads will not be self-locking if they are reused

Will the joint be exposed to vibration?

• Joints that need to withstand vibration should use a conventional or blind rivet and avoid fasteners like tapping screws
• Threaded fasteners will often loosen with vibration
• Tapping screws can resist some vibration with 16-gauge material (over 1.5 mm (0.0579 inches) in width).

What strength requirements do you have for the fastener?

• You’ll get the most tensile strength, shear strength and twisting resistance from nuts and bolts
• Conventional rivets offer the next best option with respect to strength, followed by tapping screws and blind rivets
• Use blind rivets if you’re working with thin material and the joint must not loosen

Is it possible to bring all the components to an assembly machine?

• If the parts can’t be moved to an assembly machine, opt for blind rivets because the riveting tool can be brought to the part
• Other choices of hardware may be possible, depending on the manufacturing set up

Can you access the back of the joint?

• Blind rivets and tapping screws are the best option when the back of the joint is not accessible
• Conventional riveting machines can accommodate a distance of no more than 15 to 30 cm (6 to 12 inches) between the hole for the fastener and the edge of the joint
• Conventional rivets or nuts and bolts can work for joints with access to the back

How much backup space is available?

• Tapping screws must extend 9.5 to 15.9 mm (0.375 to 0.625 inches) beyond the back of the joint to fasten properly
• Less backup space than this requires blind rivets

How hard is the material of the part?

• Hardware should be at least as hard as the sheet metal material used to fabricate the part
• With self-clinching hardware, there should be at least a 20-point difference on the Rockwell B scale between the part and the hardware

How thin is the material being joined?

• Threaded screws will require sheet metal material thicker than 3.2 mm (0.125 inches) for proper thread engagement
• Self-clinching inserts can be used to overcome limits for thread engagement
• Tapping screws work for materials in the range of 0.5 to 12.7 mm (0.02 to 0.50 inches)

Do you have any special requirements that might limit your options?

• Consider surface finishes, materials, aesthetic requirements, watertight seals or anything else you may need to consider that is unique to your product

What is the best option with respect to cost?

• Don’t compromise function for cost. Only consider cost once you’ve identified the hardware that will function properly for your joint
• Some types of hardware, like nuts and bolts, are expensive to buy and also require more manual labor in installation. Other types, like self-clinching hardware, have cheaper parts and fewer assembly steps. A range of factors impact the cost of hardware.

Table 4: Fastener Options Based on Application Requirements

The process and its terminology may be unfamiliar to some people, but galvanic corrosion is a relatively commonplace phenomenon that needs to be accounted for, where two unlike materials in contact with each other corrode rapidly. This means the material of the part needs to be considered when selecting a fastener material as well.

What is Galvanic Corrosion?

Galvanic corrosion occurs when two metals with different electrode potentials are in direct contact or electrically connected and immersed in an electrolyte solution. One metal becomes an anode, and one becomes a cathode. The metal that becomes the anode will always corrode at a much faster rate than under normal conditions.

Things to know:

• Common metals used in sheet metal manufacture have varying electrode potentials, creating the risk of galvanic corrosion
• Water is an electrolyte solution, with salt water being a particular problem

It’s particularly important not to choose a piece of hardware that will act as an anode. Being smaller in size than the part it is connected to will cause it to corrode at an even faster rate.

Fig. 10: Rusted Metal Surface

How Can You Prevent Galvanic Corrosion?

It’s best to avoid the possibility of galvanic corrosion occurring by choosing the same metal for your fastener as for your sheet metal parts.

If this isn’t possible, you should:

• Choose metals that are as close to each other on the galvanic table as possible
• Choosing a material for the fastener that will be slower to corrode than the part if you have to use dissimilar metals
• Take actions to eliminate the physical or electrical contact between the part and the hardware. This can be done with coating and non-conductive barriers 
• Protect parts from the elements using protective coatings
• Use a sacrificial anode to prevent galvanic corrosion.

Material Selection

Copper & Stainless Parts - You should generally only use zinc and aluminum coated fasteners for parts made of copper or stainless steel. Ideally, stainless steel fasteners should not be used with other stainless parts and nuts due to the risk of galling.

Aluminum parts - You should generally use aluminum fasteners with aluminum parts. Zinc plated hardware can also be used.

Hardware Material Choice and Galvanic Corrosion Help Table

This table can be used to help with your selection of fastener and part materials.

Table 2: Material Choice and Galvanic Corrosion

Table Key:

• A: Corrosion of the base metal is not increased by the fastener
• B: Corrosion of the base metal is marginally increased by the fastener
• C: Corrosion of the base metal may be marginally increased by the fastener material
• D: Plating on the fasteners is rapidly consumed, leaving the bare fastener metal
• E: Corrosion of the fastener is increased by the base metal

As well as using the above table, you can read more about galvanic corrosion and look at the galvanic table by following this link.

As you can see, making the right choice of hardware isn’t necessarily straightforward. There is a range of factors that need to be considered, and it may be necessary to trade off some considerations against others.

If you make the wrong choice, it could also be a problem. Your product might not perform as well as it should, or you may end up spending too much on manufacturing.

Don’t forget that there are also many variations of the types of hardware we’ve covered in this article.

Sheet Metal Part Design

As well as making the right choice of hardware for a sheet metal project, it’s also important to consider whether designs need to be adapted so that hardware can be fitted. Mounting holes that are too close to the edge may break, for example. Holes that are otherwise incorrectly located may mean that assembly does not go as planned. Otherwise, holes may need to be adapted to suit hardware, such as in the case of countersunk screws.

Komaspec is a Canadian owned sheet metal, plastics and electromechanical manufacturer based in China.

We manufacture over six million precision metal components every year, and we conduct a range of advanced manufacturing techniques. These include laser cutting, stamping, CNC bending, shearing and manual and robotic welding. We have advanced equipment and an expert team of engineers and well-trained manufacturing staff.  

As well as manufacturing parts, we also do an extensive amount of assembly work with sheet metal parts. This includes using all types of hardware to assemble sheet metal components and products.

What fastening methods are used in sheet metal production?

The most common fastening methods in sheet metal production are riveting, screwing and using nuts and bolts. Threaded rods and threaded inserts are used sometimes as well, and there are also other types of fastener.

How do you choose the right material for sheet metal hardware?

Hardware material choice can be complicated, and this is often something that should be considered alongside material choice for a part at the design stage. As well as galvanic corrosion, there is a range of factors related to function and appearance that need to be considered.

What are threaded inserts and threaded rods?

Threaded inserts are inserts with an internal thread that are placed inside a sheet metal part. The part is then joined to another with a threaded rod that is inserted into the insert. Threaded rods are essentially the opposite. A rod with an external thread is attached to a sheet metal part. It is then joined to another part with a nut that goes over the rod.