Threaded Inserts: Features, Materials, and CNC Machining Guide

For vehicle designs, the type of insert used may influence both the assembly process and the lifespan of the component itself. Choosing the correct insert will not only help in making the assembly process more efficient but will also ensure longevity of the part. This article provides an overview of the considerations required in the selection of an insert.

What Are Threaded Inserts?

Threaded inserts are small metal fasteners that fit into holes that have been drilled or molded. They are considered when threads formed directly in the base material are not strong enough. Threaded Inserts prove to be highly beneficial in soft metals, plastic, composites, thin parts, and assemblies that need repeated assembling and disassembling.

When to Used Threaded Inserts?

Your part may need threaded inserts in the following situations:

• The base material cannot provide sufficient thread strength.
• The assembly requires frequent disassembly and reassembly.
• The original thread is damaged or stripped.

How Do Threaded Inserts Work?

Different types of inserts require different insertion methods. The hole may be drilled, milled, or tapped prior to inserting the insert. The insert may be inserted using pressure, screws, expansion, molding, and other ways. It transmits the load of the fastener to the substrate using external threading, knurling, interference fits, and molded joints.

Features of Threaded Inserts

Threaded inserts can vary in design depending on how and where they are installed, as well as the loads they need to carry. If you are choosing a threaded insert for your parts, it helps to understand the most common design features first. However, some special features are typically supplied as custom threaded inserts.

Internal Threads

The internal thread engages with the mating screw or bolt and provides the fastening interface of the threaded insert. Tuofa commonly machines internal threads from M2 to M12, with pitch, class, and engagement matching the fastener specification. Otherwise, it may result in stripping of either the inserts or the screws under load.

External Threads

The exterior threads on the body of the insert act as the connecting mechanism between the insert and the host material(the existing part). Different from the internal threads, the external thread is designed to achieve strong engagement with the parent material(the existing part). This design can spread the load over a larger area, improving the insert's resistance to pull-out and rotation.

Steps and Shoulders

The stop shoulder prevents the insert from going further into the hole. In addition, the correct depth is ensured using this feature. Due to it, the flushness of the insert can be provided for all pieces manufactured.

Knurls

Knurling on the outer surface of the insert gives more grip and resists the rotation of the insert after it has been installed. It is called knurled inserts. Some of the common types of knurl patterns include diamond knurl and straight knurl.

Grooves and Undercuts

The grooves and undercuts allow the melted or displaced material to fill up in the grooves, thereby making it mechanically locked.

Slots and Cutting Features

Slots formed in the insert body during the insertion of the body into the host material provide resistance to rotation and removal. This design permits the insertion of the self-tapping insert without the need for preheating of the pilot hole.

Thin-Wall Sections

A thin wall insert allows for a smaller boss diameter in the case of compact housing, hence saving materials and space. Due to the thin wall thickness, it becomes important to ensure roundness in CNC turning.

Benefits of Threaded Inserts

Threaded inserts create a reliable interface between an existing part and a screw, allowing the fastener load to be transferred into the existing part through the insert. If the original thread becomes worn or damaged, the connection can often be restored by repairing or replacing the insert instead of replacing the entire part. Different inserts can also be used to accommodate different screw sizes or thread specifications, giving the assembly greater flexibility. Understanding these benefits will help you see why choosing the right threaded insert is important to the performance and reliability of your part.

Improved Thread Strength in Soft Materials

Aluminum and plastic threads wear very fast under continuous torque. The use of a steel or stainless steel insert creates strong threads which will endure wear caused by frequent usage of the part over its lifetime.

Better Resistance to Thread Wear

As the insert absorbs all the stress and friction involved in the assembly process, the other component stays safe. This is especially true for components that require servicing frequently.

Easy Repair of Damaged Threads

If the stripping happens, an insert can be put in place to restore the hole back to its previous condition without having to re-machine the housing.

Choose Standard or Custom Threaded Inserts

Choosing the proper insert is not simply based on finding an insert that matches thread size. Consider these four things before you finalize the material bill for a production run.

• Thread fit of the insert and fastener
• Size of the insert relative to the loading requirements
• Hole size and depth availability
• Installation method

When Are Custom-Threaded Inserts Needed?

Standard inserts can be easily obtained in most cases of fastener sizes and can be in stock. Customized inserts might be considered for cases where there are special considerations regarding loading, spacing, or material types.

Unusual Thread Combinations

When selecting a threaded insert for an existing part, you need to consider both the thread in the parent component and the external thread of the mating screw. Standard threaded inserts are usually available only in predefined internal and external thread combinations. If none of those standard combinations fit your assembly, a custom insert may be required.

Special Thread Requirements

Special thread requirements that may need a custom threaded insert include:

• Uncommon thread pitches: For example, M6 × 0 is the standard coarse pitch, while M6 × 0.75 and M6 × 0.5 are finer, less commonly stocked options.
• Left-hand threads: A standard M8 × 25 thread is normally right-hand, but some assemblies may require an M8 × 1.25 LH thread for reverse-direction fastening.
• Special thread forms: These may include trapezoidal threads, ACME threads, pipe threads, or other application-specific thread profiles.
• Tighter thread tolerances: A common metric internal thread class is 6H, while precision applications may require a tighter class such as 5H. For inch threads, an internal thread may similarly be specified as 3B instead of the more common 2B.

Nonstandard OD or Length Requirements

Due to thin wall sections, small bosses, or shallow installation depths, it may be necessary to have an insert with a nonstandard OD or length.

Special Material Compatibility Requirements

The chosen material for the insert should match both the host material(existing part) and the surrounding environment. Issues like corrosion, high temperatures, galvanic effects, wearing off, and chemical interaction need to be addressed. Surface finish of parts should also be considered.

What Materials Are Used for Threaded Inserts?

The material of a threaded insert affects its strength, wear resistance, corrosion resistance, and compatibility with the parent part and mating screw. The common materials of inserts below can help you choose the right option for your custom threaded insert.

Brass Threaded Inserts

Brass offers excellent machinability and thermal conductivity, making it easy to turn, drill, tap, and knurl for threaded inserts with more complex features. It also provides good corrosion resistance and generally does not require additional surface treatment. Brass threaded inserts are commonly used in plastic parts.

Steel Threaded Inserts

Steel is widely used for threaded inserts because it is relatively easy to machine and performs well in applications that require higher loads and stronger threads. Compared with brass, steel offers greater strength and wear resistance, although it is generally less machinable. Steel threaded inserts often need surface finish to improve corrosion resistance.

Stainless Steel Threaded Inserts

Stainless steel is also widely used for threaded inserts because of its high strength and good wear resistance. It offers better corrosion resistance than carbon steel, making it more suitable for environments where rust is a concern. However, stainless steel is generally more difficult to machine than brass or carbon steel. During CNC machining, it can work-harden quickly and usually requires harder, more wear-resistant cutting tools.

Aluminum Threaded Inserts

Aluminum is lightweight and easy to machine, and it is often anodized to improve surface hardness and wear resistance. Aluminum threaded inserts are better suited to lightweight, low- to medium-load applications, such as aerospace components.

Titanium Threaded Inserts

Titanium thread inserts are highly recommended for applications where strength is a priority but weight is of paramount importance. Titanium thread inserts have been widely used in both aerospace and automotive industries while the cost of titanium is higher.

Key Considerations for CNC Turning Threaded Inserts

The accuracy provided by CNC turning ensures that the dimensions are uniform and properly installed. This is particularly crucial in the manufacture of many inserts because even the slightest difference could make a huge difference. The list below contains aspects you should consider before starting production.

Control Concentricity Between Internal and External Features

Both internal and external threads should have concentricity. Or else, the thickness of the wall could be different for each insert. This would cause variation in the torque applied during the installation process.

Prevent Deformation in Thin-Wall Inserts

Thin-wall inserts may deform while being machined due to improper setup of fixturing or cutting process. Proper machining ensures roundness of all pieces.

Control Knurling Quality

Deepness and evenness of the knurling affect the pulling out resistance of the insert. In case the knurling tool becomes worn out, the pattern becomes shallow.

Minimize Burrs on Threads and Edges

Thread burrs can cause damage to either the mating bolt or the threads. Perform deburring after the turning is complete.

Turn-Mill Machining for Special Features

It is not possible to have all the features manufactured by a CNC Turning machine to make an insert. For instance, some of the common ones are keyways, cross holes, flats, slot, and offset holes. Some of these features might require extra milling or drilling work. A turn-mill machine can do several of these processes in one setting.

Are Threaded Inserts Expensive?

The cost of inserting will depend upon the material, quantity, and if the standard or custom insert will be utilized. The table below represents typical cost comparison among different types of inserts:

Custom threaded inserts through CNC machining usually involve a higher upfront cost, but they can reduce expenses over the product's service life by improving durability and making repairs or replacement easier. Manufacturing cost can also be controlled by simplifying unnecessary design features or selecting a more cost-effective material when the application allows.

Threaded Insert vs Direct Tapping: Which Costs Less?

Direct tapping directly might appear to be the cheaper option initially, but threads formed directly on softer materials tend to wear out faster. Insert type threaded connections offer an added durability advantage, particularly if there is a requirement for repeated assembly and dismantling of the connection. Inserts may even prove to be a better economic option over the longer term if the production volume is large enough.

Standard Insert vs Custom Insert Cost

Standard inserts save money by eliminating the tooling cost, and the lead time is shorter. Costs related to tooling are incurred in the custom insert, but the unit cost may be lowered if the order exceeds 1,000 units.

Do Finishing Requirements Increase Cost?

Finishing such as plating or passivation adds expense to the production process. Check whether any specific finishing process is required at all.

Common Threaded Insert Failure

Insert failures can be prevented by using the right machining method. Few of the failure modes are mentioned below:

Insert Spinning

Insert spinning means that the bore hole has been machined larger than intended. In this situation, the insert loses the torque-out resistance. Tight tolerance of CNC machining ensures the same bore hole dimension on every piece.

Insert Pull-Out

Insert pull-out is caused by inadequate depth of the bore hole. CNC turning ensures a constant depth so that the insert can seat every time.

Internal Thread Stripping

Thread striping occurs when too much torque is applied compared to the strength of the insert. The use of harder alloys in design reviews can avoid this failure.

Insert Back-Out

When the screw is removed, the threaded insert may rotate with it and out of the parent part. The use of a locking feature or thread locking compound minimizes insert back-out due to vibrations.

Galling Between the Insert and Fastener

Galling consists of binding of two substances owing to friction. To avoid galling, dissimilar metals can be used in the fasteners and inserts. For instance, a steel insert together with a coated fastener can assist in solving this problem.

Standard Threaded Insert Brands vs Custom Manufacturers

Standard brands provide speed and convenience for various applications. The custom manufacturer adds value when engineering services are needed that go beyond the standard part from a catalog.

Common Brands for Standard Threaded Inserts

The already existing brands of fasteners provide varieties of sizes and types. They are therefore suitable for small-scale work where availability is a concern.

What Should a Custom Threaded Insert Manufacturer Provide?

An experienced custom manufacturer will do more than just machine parts. There are four aspects that should be considered when selecting a manufacturing company. These are :

Turn-Mill Machining Capability

Tuofa CNC, a custom threaded insert supplier equipped with Turn-mill machining capability manufactures inserts with high precision, strictly control the tolerance. Threads, slots, flat, high-quality surface of threaded inserts can be created through turn-mill machining technique. It will shorten lead time and make it easier to machine complex inserts.

Thread Machining Capability

An experienced thread insert manufacturer should be highly familiar with a wide range of thread standards and maintains strict control over thread tolerances during machining. Tuofa carefully avoid issues such as insufficient thread depth, incorrect pitch, and incomplete thread profiles to ensure smooth and reliable assembly.

DFM Support for Insert Design

A reliable supplier providing customization service should have mature DFM capability that supports the design process makes it easier to detect problems that may occur during production. This results in reduced changes in tooling and lowers costs in the first production run.

Insert Installation and Assembly Capability

If you need turnkey manufacturing, it is often more efficient to work with a CNC manufacturer that can manage multiple processes under one roof. Tuofa CNC, for example, provides custom machining for small precision parts, along with additional services such as surface finishing and assembly.

Conclusion

Threaded inserts provide reinforcement for softer materials, allowing multiple assemblies without harming the substrate material. The appropriate design, material, and manufacturer can increase performance while controlling costs. When working on vehicle or equipment products in large volumes, an insert vendor capable of turn-milling and DFM will keep you on time.

FAQ

How to remove threaded insert?

Inserts can generally be extracted through the reverse-threading process and with the proper removal tool. When the inserts are heat-treated to become part of the plastic material, they can also be drilled out.

Are threaded inserts stronger than screws?

An insert is not a replacement for a screw. They add strength to the material around the screw so that all its strength is utilized without damaging the host material.

Where to buy threaded inserts?

Standard threaded inserts can be purchased from Amazon, McMaster-Carr, Fastenal, Alibaba. Customized inserts could be supplied by CNC machining companies based on the specifications.

Can you recommend the best brands for threaded insert tools?

Suppliers of threaded insert tooling differ depending on the kind of inserts used in the tools. Inquire from your insert manufacturer as to which install tool will fit their unique threads and knurl configuration.