It is very common for us to hear questions from clients whose production process we are supporting, while they make decisions about fastener types that are best suited for their product. We have a wealth of experience in this and we have seen the consequences of poor choices too often!
An often neglected corner in engineered solutions is concealed in the options of screw-thread form, when threaded fasteners are demanded. Our experience in this is extensive and we want to share it with you, to try to make your process towards production a little smoother.
Selecting the most appropriate type of thread profile/action is crucial in ensuring structural integrity, durability, and ease of assembly of any connection. The wrong choice typically delivers weakened connection, increased effort and/or scrap, or even failure of the joint in the market.
Various screws, while ostensibly identical, are in reality carefully targeted to specific materials and receptacle approaches, each with characteristic benefits and assembly/performance issues.
In this blog, we’ll explore the commonly misinterpreted formats of tooth/thread: self-tapping, self drilling, thread-cutting, and thread-forming screws. Understanding the subtle differences between these will help you make wise decisions in selection, ensuring strong and reliable connection outcomes.
As with all screws intended for single use, removing and reinstalling thread-forming and thread-cutting screws will weaken the formed threads, making subsequent connections less secure. Our experience in mass production and product maintenance suggests an approach that solves this issue. When reintroducing a fastener to an already thread-cut or thread-formed hole, a short ‘reverse’ turn as a starting point will allow the user to feel the engagement with the existing thread. When the threads of the fastener engage with the threads in the receptacle, there will be a small drop-in that is virtually always detectable. After this, the fastener can be driven home normally, with no degradation resulting from a second cutting/forming.
What are Self-tapping screws?
Self-tapping screws is an often misinterpreted term that defines a broad family of fasteners that create their own internal threads as they are driven into appropriate materials, eliminating the need for prior thread cutting or forming in a receptacle hole. ‘Tapping’ is the correct term for the cutting of a thread into material, removing the material required to form the female thread. In reality, this cut type of thread imposition is a subclass that is typically lumped-in with thread-forming or thread-rolling screws in the catch-all term ‘self tapping’. It also typically includes self-drilling screws, which differ only in tip-profile in that they integrate a drilling tip that obviates the need for a pre-existing hole.
This very wide classification of screws is used across virtually all sectors and are offered in a bewildering variety of thread shapes/sizes, specific to the intended receiver material and the required assembly process.
Thread-forming screws
Thread-forming screws displace material as they are driven in, creating internal threads without cutting the material. They are generally used in softer materials like plastic, wood, and softer metals.
Commonly used in plastic assemblies and lightweight materials where cutting threads might weaken the material, the forming of threads reduces the high stress concentrations that occur at the thread tips, in cut profiles.
The most common method of thread forming is a progressive tip that expands the diameter of the contact point in easy steps to allow material to displace. This type lacks an edge to cut the engagement, using progressive action to displace or compact material to make a place for the threads to bite. The approach of thread forming, forcing the material to flow around the threads as the screw is driven in, tends to result in a tight fit.
This distinction makes thread-forming screws more suitable for certain applications, particularly in softer materials where maintaining material integrity is crucial and for resilience in high vibration applications. The threads formed by compression provide a tight, vibration-resistant connection, as the material is not weakened by material removal. Because the screw does not cut into the material, there is less risk of stress concentration and crack propagation that otherwise weakens the material during installation.
Thread-forming screws are particularly well-suited for use in softer materials, including:
- Plastics: Used in electronic enclosures, automotive components, and other plastic assemblies where cutting could lead to weakening in the receptacle, where polymer components may be more fracture prone.
- Thin Sheet Metal: Employed in applications like HVAC ducts, where the metal thickness may not support thread cutting, but the thin cut-edge of a hole can easily displace along the screw axis to form a one-turn thread profile.
- Composite Materials: Suitable for fastening in composites where not degrading local material properties can greatly enhance the quality of connection.
These screws typically require more torque during installation compared to thread-cutting screws, as the material displacement process involves higher local forces to displace material.
Trilobe screws
Trilobe thread-forming screws are a less commonly specified type of screw designed with a unique three-lobed (trilobe) cross-sectional shape. They are a standalone subclass of thread forming screws that are often referred to as thread-rolling. This shape allows the screw to more progressively form threads through deformation with more capacity for spring-back. This makes them particularly effective in soft or more ductile materials, where the torque required will be lower than for a circular profile.
The defining feature of these screws is their rounded triangular cross-section. Unlike the more typical types of screw, trilobe screws have three distinct lobes, which help in distributing the load more evenly across the material.
They can also work in harder materials (even steels) when the screw hardness is sufficient relative to the receptacle material. The application torque available must be sufficient to allow the process to proceed without excessive difficulty.
The trilobe concept is generally less disruptive of the pillar material, reducing the risk of tearing during thread forming as the action is essentially intermittent.
A significant advantage of this form, irrespective of the thread profile, is that they have a stronger tendency towards self locking. This is one of the reasons why they are commonly used in automotive components, especially where plastics and composites are involved, such as in dashboards, interior panels, and engine components. This more than compensates for their higher initial cost.
Thread cutting screws
Thread-cutting screws are the most commonly specified type of self-tapping screw designed to create internal threads in a material through the cutting and removal of material as they are driven in. Unlike thread-forming screws, limited displacement of material occurs during insertion, because a sharp cutting edge actively removes material, enabling greater facility in creating precise threads in harder or more brittle materials.
As these screws cut and remove material to form threads, their behavior is more akin to the separate tapping of a hole that is involved in typical cutting of machine screw threads. This makes them more suitable for harder materials like metals, hard plastics, and rigid composites such as GRP and CFRP. The cutting action imposes precise threads without the risks associated with material deformation, which is impractical in harder materials.
The defining feature of thread-cutting screws is their sharp, cutting edge, usually located at the tip or across the initial turns of the thread. This edge is designed to expose a sharp, hard edge akin to a machine tap flute, which cuts material as the screw is driven in.
Thread-cutting screws are widely used in applications where strong engagement with harder materials is required, particularly where the material of the screw receptacle is excessively hard or brittle, excluding the use of thread-forming screws.
Thread-cutting screws can be used in a wide range of materials, including metals, harder and reinforced plastics, composites and brittle materials like cast Iron. Cutting rather than displacing material typically reduces the stress on the surrounding material beyond the screw engagement, reducing the hoop stress in the receptacle, minimizing the risk of cracking or damage during installation. However, it can increase the installation torque required, making over-torquing a higher risk, when larger forces become harder to control with finesse. These types often achieve the highest pull-out resilience.
Because thread-cutting screws remove material, there is a higher risk of stripping the threads by over tightening, if the material is too soft. Stripping is the linear shearing of the inter-thread material that remains un-cut at installation. The cutting process can increase the risk of this, as probability of crack propagation from the tips of the cut profile is increased. In particular, this stripping tendency requires careful torque control when screwing into softer materials.
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CTO
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Comparing the three types of screws
Understanding the differences in application suitability, performance factors, and cost considerations of the various screw options is essential to effective design. Factors such as material hardness, screw hardness, receptacle material ductility, shear strength, hoop stress resilience in the hole and operator/process proficiency are critical design drivers.
Application factors
Thread-cutting screws:
- More appropriate for harder materials like metals, hard plastics, and composites. They are typically selected where localized deformation of the hole material is undesirable or impossible.
- They are most suitable for metal fabrication, electrical components, and heavy-duty assemblies, offering high holding power under higher load/vibration.
Thread-forming screws:
- Ideal for softer materials such as plastics, softer metals, and some natural materials.
- Commonly used in lower stress automotive, electronics, and consumer goods applications where softer materials are more common and a secure, vibration-resistant connection is demanded.
Thread-rolling screws:
- These are typically used in softer metals and ductile materials where forming threads through displacement is relatively easy and does not create collateral structural issues.
- They are increasingly common in high-volume manufacturing environments requiring moderate resilience and good vibration resistance, such as automotive assembly, where consistent thread formation and swarf-free, moderate torque assembly is preferred.
Performance factors
Ease of installation:
Thread-cutting screws: This class typically requires higher torque during installation and provides precise, clean threads, which is beneficial for applications where accuracy is paramount.
Thread-forming screws: These are viewed as easier to install in softer materials, due to the lower driving torque and reduced the risk of damage both to the screw head and the receptacle.
Thread-rolling screws: These require an intermediate level of torque and are considered relatively easy to install. The rolled threads are robust and self locking and this type is widely viewed as suitable for automated insertion.
Holding power:
- Thread-cutting screws: This type is viewed as having strong pull-out strength in harder materials, with threads that are clean and sharp.
- Thread-forming screws: This class provides good holding power in softer materials, with the formed threads offering a secure fit that is resistant moderately self locking and vibration resistant.
- Thread-rolling screws: These provide excellent pull-out resilience in ductile and softer materials, with stronger and more durable threading than those created by either cutting or forming.
Durability:
- Thread-cutting screws: This class is considered highly durable in hard materials under high stress or vibration.
- Thread-forming screws: These are more durable in softer materials than thread cutting types, with reduced cracking or progressive material failure.
- Thread-rolling screws: These are extremely durable, particularly in metal-to-metal applications. The threads created by rolling are often more resistant to relaxation and can be durable under repeated use.
Cost considerations
- Thread-cutting screws: These typically have a higher cost per screw, as the extra process in introducing the cutting edge is high precision. They also demand harder materials, which reduces the potential range of low cost options and increases processing costs.
- Thread-forming screws: These are typically less expensive than thread-cutting screws, as they can perform well with lower strength materials, making them easier to manufacture.
- Thread-rolling screws: These are typically more expensive than thread-forming screws, especially in high-strength alloys, due to increased material and processing costs.
Installation costs:
- Thread-cutting screws: Installation can be more labor-intensive as these screws require higher insertion torque and can more easily suffer head damage. This increases labor/tooling costs and makes robot insertion more problematic.
- Thread-forming screws: These are easier and faster to install in softer materials, reducing installation labor costs. Lower torque also allows tighter control of insertion conditions.
- Thread-rolling screws: This type is viewed as best adapted to automated processes, and their durability is rated highly.
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Manufacturing Business Consultant
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Self drilling screws
Self-drilling screws define any type of fastener that combines the functions of a drill bit and a screw into a single component. There are various drill tip forms offered by the market, but they all have the common intent that the pilot hole is drilled as the screw is installed.
This makes self-drilling screws time efficient, particularly in projects that require the fastening of metal-to-metal, metal-to-wood, or metal-to-plastic components where volumes of fasteners are large, installation precision is relatively low and the process is carried out manually.
Self-drilling screws are particularly well-suited low and moderate gauge metal attachments, making them ideal for tasks like metal roofing, siding, and assembly of moderate precision metal frames. It is critical that the screw is compatible with the receiver material to prevent bimetallic corrosion and ensure a secure attachment. Stainless steel or coated screws are better for environments where rust resistance is important.
While less commonly used, self-drilling screws can be used in wood applications, particularly when fastening metal components to wood. When using self-drilling screws in wood, choose a screw with a deeper thread penetration to ensure adequate pull-out holding power.
Self-drilling screws can be used with non-brittle plastics, especially when attaching metal components to plastic materials. Be very careful with thin or very brittle plastics, as the drilling action can easily induce cracking due to high axial loading at the onset of cutting. Screws with fine thread, of an appropriate diameter for the material thickness should be selected to ensure good attachment.
Applications include:
- Installation of metal roofing and siding, for fast and secure attaching of metal panels to metal or wooden structures.
- HVAC and electrical systems to fasten metal ducts, panels, and enclosures, to speed up installation and reduce labor costs.
- Construction projects with metal framing, attaching metal studs, and securing other structural components, for speed and good holding power.
Selecting the right screw
Selecting from thread-cutting, thread-forming, and thread-rolling screws demands balancing the demands of the receptacle material(s), the screw material, the installation process, and force/vibration durability demands of the application.
Material compatibility
Thread-Cutting Screws: Opt for thread-cutting screws for harder materials like metals, hard plastics, ceramics or rigid composites. They are not suited to softer materials
Thread-Forming Screws: These are suited to softer materials such as more ductile plastics, soft metals, and higher flexibility composites. Ensure that the material is sufficiently ductile to deform around the screw without inducing crack propagation.
Thread-Rolling Screws: Select thread-rolling screws for use in ductile metals or plastics where threads need to be formed without removing material. Ensure that the material has enough malleability to be shaped by the rolling action without initiating fracture or shear in surrounding material.
Installation considerations
Thread-cutting screws: Take care over installation torque to avoid over-tightening, which can result in stripping. Pre-drill pilot holes are typically larger than the screw’s cylindrical core diameter, to avoid lock-up or excessive insertion forces/hoop stress. Good alignment at installation is important, as a skewed start is easily made and hard to correct.
Thread-forming screws: These require lower torque compared to thread-cutting screws, but offer good pull-out and vibration resilience. Pilot holes can be on-size with the root diameter of the screw, ensuring maximum thread engagement.
Thread-rolling screws: Requirements are the same as for thread forming screws, but the resulting pull-out and vibration resilience tend to be higher and the pilot hole can be on size or slightly smaller than the root diameter of the screw, depending on material properties.
Conclusion
In the no simple process of selecting fasteners for an application, understanding the differences between the three types of screws is crucial for making the right choice in your journey to production. Each screw type has distinct characteristics:
- Thread-cutting screws are ideal for harder materials like metals and dense woods, where precise, strong threading is essential. They cut and remove material, providing excellent holding power but requiring careful installation to avoid stripping.
- Thread-forming screws are best suited for softer materials like plastics and soft metals. They displace material to form threads, making them a gentle yet effective option that preserves material integrity.
- Thread-rolling screws are designed for ductile metals and some plastics, rolling material to create durable threads without cutting. They offer high holding power and are ideal for applications requiring repeated use and longevity.
Choosing the right screw for your specific application is critical to ensuring optimal performance and longevity. Using the wrong type can lead to material damage, reduced holding power, or premature failure, which could compromise the safety and reliability of your project.
In your design and production transfer process, take the time to assess the material, environment, and load requirements. This careful consideration will help you select the most appropriate fastener for the job. When in doubt, consulting with our professional network can provide valuable guidance, ensuring that you achieve the best possible outcome with the most suitable screw type. Making informed decisions about fasteners will not only enhance the quality of your work but also extend the life of your connections.