Bolt-type parts are basic standard components that perform the core functions of connection and fastening, and are widely used in various types of mechanical equipment and engineering structures.
Based on differences in mechanical and physical properties, these parts can be classified into 10 grade categories.
Among them, bolts with a mechanical performance grade of 8.8 or higher are typically defined within the industry as high-strength bolts.
Structural Composition and Functions of High-Strength Bolts
Although high-strength bolts come in a wide variety of types and dimensions, their overall structural configurations share common characteristics.
Based on these commonalities, manufacturers can divide them into three major functional modules: the head, the shank, and the threaded section.
Head
The head primarily provides a fulcrum for reverse torque when tightening the nut onto the bolt, ensuring that the nut achieves sufficient preload torque.
Heads come in a wide variety of forms, with common standard types including square heads, half-round heads, and hex heads.
In addition, engineers custom-design the head shapes of some non-standard high-strength bolts to meet specific assembly space and load requirements.
Shaft
The shank primarily guides the bolt during assembly.
For guide-diameter bolts that withstand radial shear loads, the shank’s outer diameter must maintain a tight clearance fit with the assembly hole.
Therefore, manufacturers must strictly control its roundness and surface roughness.
For bolts that experience only axial tensile forces after assembly, the shank requires relatively lower precision.
Manufacturers can therefore relax the outer-diameter dimensional tolerances appropriately.
For high-strength bolts, the transition fillet between the shank and head requires strict manufacturing control.
A sufficient fillet radius helps reduce stress concentration and prevents fracture under high tensile loads. Proper control during quenching and cooling also helps prevent cracks from forming in this area.
Threaded Section
The threaded section serves as the core area through which the bolt performs its fastening function and can be further divided into three sub-areas: the effective thread zone, the taper zone (i.e., the chamfer area), and the thread end.
The three key thread elements—pitch, thread angle, and mean diameter—directly determine the precision grade of the threaded fit, making them critical factors that operators must strictly control during machining.
Analysis of High-Strength Bolt Manufacturing Processes
Manufacturers typically do not need ultra-high-precision specialized machine tools to machine high-strength bolts; they can complete all processes using conventional, general-purpose equipment.
Based on the three major structural modules mentioned above, manufacturers can divide the manufacturing process into three main sections: head forming, shank machining, and thread manufacturing.
Due to differences in dimensional specifications and technical requirements, each of these sections encompasses a variety of specific process routes.
It is important to emphasize that these three process segments do not exist in isolation; rather, they are interdependent and work in concert—they may occur simultaneously within the same process or even the same process step.
Machining of the Head
1. Blank Preparation
Selection of Blank Type:
The geometric shape of the bolt head directly determines the choice of blank:
| Head Type | Recommended Bar Stock | Selection Criteria |
|---|---|---|
| Square Head | Cold-Drawn Square Steel | Cross-sectional shape matches; high material utilization |
| Hex Head | Cold-Drawn Hexagonal Steel | Hexagonal cross-section can be used directly; low machining volume |
| Round Head | Forged Bar Stock | Complex curved surfaces require forging to form the shape |
| Special Custom Head | Forging (Recommended) | Avoids adding head machining operations; use forging first when technical conditions permit |
When the head’s maximum clearance dimension differs significantly from the shaft diameter, manufacturers should use forged blanks.
The same approach is suitable for relatively long parts because it reduces material waste and shortens machining time.
Machining Allowance for Blanks
① Structural steel blanks: Reserve the allowance primarily in the longitudinal direction; typically, 4 mm is sufficient, while maintaining material utilization at the recommended K ≥ 93%.
If a single blank has a short length, use a “multiple parts from a single blank” approach. In this case, set the allowance per part to the cutting tool width + 1 mm.
② Forged blanks: Where technical conditions permit, form the head shape directly by forging as much as possible, leaving a 1.5 mm allowance on the inner end face of the head.
Leave a 1.5 mm allowance on one side of the shank’s outer diameter.
For slender shaft-type bolts, increase the allowance to 2 mm to prevent excessive deformation during heat treatment, or normalize the blank first (hardness should not exceed HRC 32).
Establish specifications for the depth of surface defect layers and the coaxiality of the head and shank (generally ≤0.3 mm).
Unless otherwise specified, the forging should undergo normalizing treatment after forging to reduce hardness and improve machinability for subsequent cutting operations.
2. Head Forming Machining
Control the head dimensions, including the width, outer envelope dimensions, and internal and external end-face chamfers.
A standard lathe can typically perform the rough machining.
Some high-strength bolts require that the inner end face meet specifications for runout and perpendicularity relative to the shank centerline (generally controlled machining methods.
> 1. Turning the Outer Cylindrical Surface
When the dimensional accuracy and surface roughness requirements for the shaft’s outer cylindrical surface are not particularly stringent, turning can serve as the final machining operation:
| Machining Stage | Accuracy Grade | Ra Value (μm) |
|---|---|---|
| Rough Machining | IT12 ~ IT11 | 50 ~ 12.5 |
| Semi-Finishing | IT10 ~ IT9 | 6.3 ~ 3.2 |
Rough turning employs a strategy of deep cutting depth, high feed rate, and low spindle speed; semi-finish turning, on the other hand, involves correspondingly reduced cutting depth and feed rate.
During turning, the bolt head serves as the clamping reference.
If the head is narrow, a center hole must be drilled on the opposite end face to aid in positioning. The specifications for the center hole are determined based on the bolt dimensions and material;
refer to relevant standards.
If subsequent precision grinding is required, leave a 0.2–0.45 mm allowance on both sides after turning.
For bolts with a high length-to-diameter ratio or those requiring multiple grinding passes, use the upper limit of the allowance.
> 2. Cylindrical Grinding
Cylindrical grinding is a key process for achieving high-precision cylindrical surfaces. It is typically performed after heat treatment and can achieve an IT6 precision with a Ra value of 0.4–0.2 μm.
For bolt-type parts, manufacturers often prefer centerless cylindrical grinding because it provides high production efficiency and simple operation.
However, setting up the machine can be relatively cumbersome, and proper grinding wheel dressing is essential for achieving consistent machining results.
In particular, when there are runout and perpendicularity requirements for the inner end face of the head, geometric and positional tolerances depend entirely on the precision of the grinding wheel;
therefore, the grinding wheel must undergo rigorous dressing.
For precision die-forged parts, the turning process may also be skipped in favor of direct grinding.
Thread Forming
Thread forming methods include a variety of processes such as turning, milling, grinding, and thread rolling.
For high-strength bolts, thread rolling is the optimal choice—it is a chip-free cold forming process that relies on the plastic deformation of the blank’s surface to create the thread profile.
Advantages of thread rolling:
① Extremely high productivity;
② Precision up to Grade 4h, with a surface roughness as low as Ra 0.2 μm;
③ The process does not sever the workpiece material’s fibers; instead, work hardening strengthens them and significantly improves fatigue strength.
④ The blank diameter is smaller than that required for cut threads, resulting in material savings of 16% to 25%.
Precautions: Thread rolling imposes strict requirements on the dimensional accuracy of the blank diameter.
When turning the blank diameter on a conventional lathe, operators must control the tolerance within 0.04 mm.
When machining on a grinder (a centerless grinder is recommended), operators must control the tolerance within 0.02 mm.
Refer to the process manual for specific blank diameter values.
Manufacturers must conduct process trials before full-scale production begins and may proceed with mass rolling only after the trials pass inspection.
The thread end form must comply with relevant standards, and operators must leave a 2–3 mm clearance at the end for tool withdrawal.
Heat Treatment
The fundamental difference between high-strength bolts and ordinary bolts lies in their superior tensile strength, surface hardness, and overall mechanical properties—all of which stem from differences in material selection and heat treatment processes.
Manufacturers typically heat-treat high-strength bolts through quenching and tempering (quenching + high-temperature tempering).
During routine quality control, technicians can estimate the material’s approximate tensile strength by measuring hardness values (refer to relevant manuals for conversion relationships).
At the same time, technicians should conduct tensile tests periodically and use the actual test data to verify whether the tensile strength meets the specified standards.
Surface Treatment
There are numerous metal surface treatment processes, including oxidation, Dacromet, galvanizing, and phosphating.
For high-strength bolts, manufacturers recommend phosphating or oxidation because these methods offer excellent protective performance and process compatibility.
Conclusion
High-strength bolts are critical fastening components that require careful control throughout the entire manufacturing process.
Their performance depends on the coordinated design and machining of the head, shank, and threaded section, as well as proper blank preparation, heat treatment, and surface treatment.
Among these processes, precise dimensional control, sufficient fillet radii, accurate cylindrical grinding, and thread rolling play important roles in achieving reliable mechanical performance.
Manufacturers can improve the strength, fatigue resistance, dimensional accuracy, and service reliability of high-strength bolts by choosing suitable manufacturing methods.
They should also control machining allowances and tolerances, apply appropriate heat treatment, and use compatible surface treatments.
Consistent process control and inspection are therefore essential for producing high-strength bolts that meet demanding mechanical and engineering requirements.