Thread-Forming Screws for Metal金属用自挤螺钉

How thread-forming screws work自挤螺钉的原理

A thread-forming (thread-rolling) screw has a slightly triangular — trilobular — cross-section. As it is driven into a plain pilot hole, the lobes press the material outwards and form a mating thread by cold flow, instead of cutting it. No chips are produced. Metric thread-rolling screws to DIN 7500 form a standard metric internal thread, so the joint can later be serviced with an ordinary machine screw.自挤螺钉（三角牙螺钉）的横截面略呈三角形。拧入光孔时，三个凸起部位把材料向外挤压，通过冷挤压形成内螺纹，而不是切削，因此不产生切屑。按 DIN 7500 生产的公制自挤螺钉挤出的是标准公制内螺纹，日后维修时可以直接换用普通机螺钉。

Advantages优点

• No separate tapping operation — lower assembly cost省去攻丝工序，降低装配成本
• No chips — important for electrical and electronic assemblies不产生切屑，对电气电子装配尤为重要
• The formed thread is work-hardened and fits the screw with no clearance, giving strong threads and good resistance to vibration loosening挤压形成的螺纹经过加工硬化，且与螺钉无间隙配合，螺纹强度高、抗振防松性能好
• A wide gap between drive torque and strip torque makes assembly easy to control拧入扭矩和失效扭矩差距大，装配过程容易控制

Where to use them适用场合

• Steel sheet and profiles钢板和型材
• Aluminium and zinc die castings (cored holes)铝、锌压铸件（铸出孔）
• Aluminium extrusions铝型材

They are not suitable for brittle materials that cannot flow, or for plastics — use screws designed for plastics塑料专用螺钉 instead.不适用于无法塑性流动的脆性材料，也不适用于塑料（塑料请用screws designed for plastics塑料专用螺钉）。

Pilot hole size底孔尺寸

The pilot hole is the key to a good joint. It depends on screw size, material thickness and hardness: too small and the drive torque becomes too high; too large and the thread engagement is weak. Start from the hole sizes recommended in DIN 7500 and confirm with assembly tests. For die castings, remember that cored holes have draft, so the diameter changes along the depth.底孔是保证连接质量的关键，取决于螺钉规格、材料厚度和硬度：孔太小，拧入扭矩过高；孔太大，螺纹咬合不足。可先按 DIN 7500 推荐的孔径选取，再通过装配试验确认。压铸件的铸出孔有拔模斜度，孔径沿深度方向是变化的，需要注意。

Thread-forming, tapping or self-drilling?自挤、自攻还是自钻？