Process Optimization in Fastener Manufacturing Through Die Technology

This paper focuses on improving processes and reducing costs to enhance efficiency. We gathered examples of bolt and nut fasteners that have the potential for improvement. Through thorough research on cold heading forming solutions, we identified several typical cases where process optimization could be achieved through modifications to the dies. Additionally, this paper introduces a one-step cold heading forming solution for dovetail nuts, hollow bolts, shouldered stud bolts, and scraper bolts. The implementation of this solution has significantly improved production efficiency, reduced production costs, and maintained stable product quality.

 

1 Introduction

In modern manufacturing, fasteners play a crucial role in connecting and securing mechanical components across various industries, including automotive, aerospace, construction, and electronics. Cold heading is a process that involves the plastic deformation of metal materials using a die at room temperature. This technique offers several advantages over traditional hot working methods, such as higher material utilization, increased production efficiency, greater dimensional accuracy, and superior surface quality. As a result, cold heading has become a widely adopted process in fastener manufacturing.

With advancements in the intelligence of multi-station cold heading equipment, it is now possible to produce complex structural parts—such as dovetail nuts, hollow bolts, shoulder stud bolts, and scraper bolts—without relying heavily on subsequent mechanical processing that was necessary in traditional methods. By innovating mold structures and optimizing process parameters, fastener production can effectively integrate multiple processes, leading to greater efficiency.

 

2 Dovetail nut forming

The structure of the dovetail nut product is illustrated in Figure 1. The dovetail portion of the nut has a thin wall thickness, making it challenging to form. The conventional processing method typically involves cold heading, followed by machining the dovetail, and then tapping. However, after optimizing the process, a new approach using two-die cold heading for hexagonal shapes and a three-die floating extrusion for dovetails (as shown in Figure 2) allows for a one-time cold heading forming. The updated processing method now consists of only cold heading followed by tapping. The main challenges associated with this process are as follows.

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1) The alloy used in the three-die floating die is prone to breakage. It is essential to strictly control the punch position and perform dynamic balancing to ensure uniform force distribution across the die.

 

2) There is a risk of thin-wall fracture. Cold heading dies often suffer from “oil and air trapping” during operation. If coaxiality is not precisely controlled during machine adjustments, excessively thin dovetail walls may easily break. Therefore, it is crucial to optimize the die cavity venting structure and design appropriate dovetail wall dimensions. The goal is to minimize the wall thickness to facilitate subsequent riveting of the nut and gasket while ensuring the dovetail does not break.

 

3) Surface folding defects can occur during the design and verification processes. It has been observed that folding is more likely when the corner radius of the second die is too small. Additionally, after the flange formation, folding can extend to the nut support surface. To prevent this, it is important to appropriately increase the transition corner radius of the second die and optimize the blank volume distribution ratio.

 

3. Hollow Bolt Forming

The structure of the hollow bolt product is illustrated in Figure 3. Its internal bore is narrow, deep, and thin, which complicates the achievement of the necessary dimensional specifications for the axial inner hole through a single cold forging process. The typical production process involves cold forging, followed by machining and drilling, and then thread rolling.

However, by employing a reverse extrusion process (as shown in Figure 4), the axial hole of the hollow bolt can be formed in one cold forging operation. The improved process consists of cold forging followed by thread rolling. The primary challenge lies in the design of the mold structure.

First, we need to calculate the deformation rate for the reverse extrusion. The formula is:

\[ \text{Deformation Rate} = \left( \frac{\text{Hole Diameter}^2}{\text{Outer Diameter}^2} \right) \times 100\% \]

For a hole diameter of 6.63 mm and an outer diameter of 10.91 mm, the calculated deformation rate is 36.9%, which satisfies the reverse extrusion forming condition of 25% to 75%.

Secondly, the depth of the extruded inner hole is determined to be three times the hole diameter, fulfilling the requirement of a depth-to-diameter ratio of ≤3.5 for reverse extrusion. Finally, the thickness from the bottom of the hole to the bottom of the hexagonal socket in the head after extrusion is set to be 1.2 times the hole wall thickness, which is within the acceptable range of the bottom thickness to wall thickness ratio of 1 to 1.5.

After analyzing the feasibility of the forming process, the cold heading deformation process is designed as follows: pre-forming with chamfering in one die, initial extrusion of the inner hole in two dies, reverse extrusion of the inner hole in three dies, and finishing of the hexagonal socket in four and five dies.

Throughout the cold heading process, various factors such as die material compatibility, die machining precision, wire plasticity, surface treatment quality, and equipment lubrication directly influence the outcomes of the reverse extrusion forming.

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4 Shoulder Stud Bolt Forming

The structure of a shoulder stud bolt is illustrated in Figure 5. This type of product is typically produced using a five-station cold heading process. However, due to the large wire-to-wire ratio at the die end, a single cold heading process is often not feasible. The conventional production flow includes cold heading, custom aluminum machining the wire blank, and thread rolling.

Recently, a multi-stage forward extrusion process (see Figure 6) has been adopted, allowing for a single-stage cold heading. The improved production flow now consists of cold heading followed by thread rolling.

One of the main challenges is the proper layout of the multiple forward extrusions. First, the total product volume and head volume are calculated to determine the necessary material length. Then, based on the material’s diameter and length, the head upset ratio and forward extrusion ratio are calculated to establish how many times the rods are bundled. It is essential to keep the bundle ratio at or below 30% each time to ensure that the final product dimensions are achieved through multiple bundles.

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5. Scraper Flute Bolt Forming

The structure of a scraper flute bolt is illustrated in Figure 7. During the tapping or precision CNC turning process, the scraper flute creates a channel for chip removal, which helps prevent chip accumulation in the thread or tool groove. Accumulated chips can lead to defects such as thread rot and burrs.

Forming scraper flutes in a single step using conventional cold heading processes is challenging because of the excessive depth of the scraper flute and the high risk of burr formation at the tail end. The typical manufacturing process involves cold heading, groove milling, and then thread rolling.

However, by employing a three-die main die in combination with a specially shaped through-die, it is possible to achieve a single-step cold heading process. The improved manufacturing process now consists of only cold heading followed by thread rolling.

The primary concern is still addressing the burrs formed at the tail end. This can be effectively managed by designing an appropriate clearance between the three-die main die and the specially shaped through-die, utilizing a tapered transition structure to optimize material flow, and refining the die cutting edge to reduce the likelihood of burrs.

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6 Conclusion

This paper focuses on four product categories: dovetail nuts, hollow bolts, shouldered stud bolts, and scraper bolts. It provides an in-depth discussion of the design and improvement of their cold heading dies. During the research process, we carefully analyzed key aspects such as die structure, material selection, and the calculation of forming parameters. Through actual production verification, we continuously optimized the product forming solution, ultimately achieving a single-step cold heading process. This advancement significantly improves production efficiency, reduces costs, and minimizes material loss, providing solid and reliable technical support for the company’s production process.

 

 

 

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