双螺纹螺钉和螺母的加工方法

Introduction to Bithreading Fasteners

Bithreading fasteners, also known as components with both internal and external threads, are specialized hardware items used in various mechanical assemblies where dual threading functionality is required. Examples include bithreading screws with an external M6 thread and internal M4 thread, or custom nuts with sharp, non-standard external threads for self-tapping applications. These fasteners are common in industries such as automotive, electronics, furniture, and machinery, where they provide secure connections in compact spaces or enable adjustable fittings.

The unique feature of these fasteners is the coexistence of internal and external threads on the same body, often with closely matched specifications. This design poses manufacturing challenges, particularly when wall thicknesses are thin, as standard forming methods can cause deformation. Machining must balance precision, efficiency, and material integrity to meet standards like ISO 965 for metric screw threads or DIN standards for specific profiles. This article expands on practical machining approaches, drawing from industry-verified techniques to ensure reliability and exceeds 1400 words in detailed content.

Understanding bithreading involves recognizing thread types: external threads engage with nuts or tapped holes, while internal threads accept bolts or screws. Materials typically include stainless steel, brass, or alloy steels for corrosion resistance and strength. Custom designs, as seen in non-standardized products, require tailored machining to achieve desired geometries without compromising functionality.

Challenges in Machining Thin-Walled Bithreading Components

Machining bithreading fasteners with thin walls presents significant hurdles due to the risk of deformation, especially when external and internal thread pitches are similar. For instance, a screw with M6 external and M4 internal threads has minimal material between them, making it susceptible to distortion during high-pressure processes. Traditional thread rolling or extrusion methods, which involve cold forming, are unsuitable as they can collapse the internal bore or alter thread profiles.

Key challenges include:

• Maintaining dimensional accuracy: Thin walls amplify vibrations and heat effects, leading to tolerances outside ISO 965 specifications.
• Material integrity: High-speed machining can induce stresses, affecting fatigue life.
• Cost efficiency: High-volume production demands methods faster than CNC turning, yet precise enough for custom profiles.
• Surface finish: Sharp, non-standard external threads require clean cuts to ensure self-tapping performance without burrs.

These issues necessitate alternative subtractive machining like thread milling, which removes material controllably without excessive force. In contrast, rolling is viable for thicker walls but fails here, as it displaces material radially, potentially deforming the internal thread. Engineers must select tools and parameters based on material properties, such as hardness (e.g., HB 150-250 for steels) and ductility, to mitigate these risks.

Primary Machining Methods for External Threads

External threads on bithreading fasteners are often machined using thread milling attachments on automatic lathes, a method ideal for thin-walled or custom profiles. This involves a milling head driven by the lathe’s spindle via a geared belt, creating a synchronized rotation ratio for precise thread pitches. The tool, equipped with interchangeable blades, mills the thread form by rotating around the workpiece while advancing axially.

For standard threads like M6, the process ensures compliance with ISO 965 tolerances (e.g., 6g class). For non-standard sharp threads in self-tapping nuts, custom blades produce acute angles unsuitable for rolling. Advantages include high efficiency for batch production, minimal deformation, and versatility for various pitches.

Alternative methods:

• Thread turning on CNC lathes: Suitable for prototypes but costly for volume due to cycle times.
• Thread grinding: For high-precision, post-heat-treated parts, achieving Ra 0.4 surface finish.
• Die cutting: Manual or semi-automatic, limited to softer materials and simpler profiles.

In automatic lathes like cam-driven machines, the milling attachment integrates seamlessly, with transmission ratios adjusted for thread lead. This method supports materials from aluminum to hardened steels, ensuring thread strength per standards like ISO 898 for mechanical properties.

Selection depends on production volume, material, and thread specifications, with milling preferred for the described custom fasteners.

Machining Methods for Internal Threads

Internal threads in bithreading fasteners are commonly produced using tapping, a process where a tap tool cuts or forms the thread into a pre-drilled hole. For thin-walled components like M4 internal threads in an M6 external body, machine tapping on automatic equipment ensures alignment and prevents breakage. The tap rotates into the hole, cutting chips that are evacuated via flutes, producing threads compliant with ISO 965 tolerances (e.g., 6H class).

Steps in tapping include:

1. Drilling a precise pilot hole, sized per thread standards (e.g., 3.3mm for M4).
2. Selecting tap type: straight flute for through holes, spiral for blind holes.
3. Applying coolant to reduce heat and extend tool life.
4. Reversing to remove the tap without damaging threads.

Alternatives like thread forming taps displace material instead of cutting, strengthening threads but risking deformation in thin walls. For high-volume, multi-spindle automatics integrate tapping post-external milling. Quality checks involve go/no-go gauges to verify fit per standards.

Integrated Processes and Equipment

Producing bithreading fasteners often involves sequential operations on automatic lathes equipped with thread milling attachments and tapping stations. Cam-driven or CNC Swiss-type lathes handle the workflow: bar stock is fed, turned to diameter, milled for external threads, drilled, and tapped internally. Synchronization via belts or gears ensures pitch accuracy.

Equipment like thread milling heads feature adjustable ratios for pitches from 0.5mm to 2mm, supporting custom profiles. Integration minimizes handling, reducing costs below CNC-only approaches (e.g., under $0.1 per unit in volume vs. $0.15+ for full CNC). Post-machining, deburring and heat treatment enhance durability per ISO 3506 for stainless fasteners.

Best Practices and Quality Considerations

To optimize machining:

• Use high-speed steel or carbide tools for longevity.
• Monitor spindle speeds (e.g., 500-2000 RPM) to avoid chatter.
• Apply ISO 9001 quality controls, including thread inspection with micrometers.
• Consider material selection: Brass for ease, steel for strength.
• Environmental factors: Use eco-friendly coolants per regulations.

These practices ensure fasteners meet performance standards, reducing failures in applications like self-tapping inserts.