What is a self-tapping screw?
In the usual sense, a self-tapping screw is a threaded fastener that does not need a pre-tapped internal thread. When the screw is driven into a smooth, untapped hole, it cuts its own internal thread, so a relatively high torque is required. The mating part is normally a soft material such as plastic or an aluminum/magnesium alloy.

Self-tapping joints offer high strength, low cost and easy lightweighting, so they are widely used in automobiles. To guarantee robust, reliable assembly, vehicle makers place great emphasis on clamp-torque control and on developing a sound tightening strategy for self-tapping screws.

The torque required to form the thread is influenced by the pilot-hole size; for screws coated with adhesive, the torque during the rundown phase is also affected by the adhesive, so the values can differ markedly. Because extra torque is consumed while the thread is being cut, the seating torque of a self-tapping screw is higher than that of an ordinary threaded fastener. This means that, for the same overall tightening torque, the clamp force obtained from a self-tapping screw is lower. Consequently, the target tightening torque for self-tapping screws is usually set higher at the design stage.

If you are still struggling with the complex tightening process for self-tapping screws, Danikor intelligent tightening tools can help. Our controllers contain a dedicated “self-tap” strategy intended for plastic, aluminum and similar self-tapping applications. The strategy is divided into five phases: soft-start, rapid thread-forming, continued rundown, thread seating and final tightening. These five steps can be combined flexibly to match the customer’s required cycle time.

Phase breakdown

1. Soft-start phase

• Purpose: provide a gentle motor ramp-in.

• Forward speed: ≤ 100 rpm (typically 50–100 rpm).

• Forward angle: ≤ 100° (commonly 60–90°).

• Torque ceiling: ≤ target torque.

• Time limit: ≤ 5 s (for takt considerations).

2. Rapid thread-forming phase

• Purpose: cut the internal thread in the plastic pilot hole so that tightening can proceed.

• Angle: ≤ 720–1080°.

• Speed: high; for plastics ≤ 400–600 rpm, for aluminum ≤ 80 % of the tool’s maximum (process-dependent).

• Torque ceiling: ≤ target torque.

• Time limit: ≤ 5 s.

3. Continued rundown phase

• Immediately follows rapid thread-forming.

• Speed: same as previous phase.

• Angle: calculated as
  total self-tap angle – rapid-form angle – soft-start angle – 200°,
  leaving 180–360° for the next phase to avoid torque overshoot.

• Torque ceiling: ≤ target torque.

• Time limit: ≤ 5 s.

4. Thread seating phase

• Speed: 100–200 rpm (≤ tool maximum).

• Seating torque:
  – plastics: ≤ 80 % of target (depends on forming speed);
  – aluminum: ≤ 40 % of target (little influence from forming speed).

• Brake control at seating point ensures final quality even if speed is increased.

• Torque ceiling: ≤ target torque.

• Time limit: ≤ 5 s.

5. Final tightening phase

• Speed: ≤ 100 rpm; the higher the target torque, the higher the allowed speed (commonly 10–50 rpm).

• Target torque: ≤ tool rated torque.

• Torque ceiling: ≤ 1.2 × tool rated torque.

• Time limit: related to overall takt; normally ≤ 5 s.

When a Danikor intelligent tool is used, torque is monitored in real time. If the torque exceeds the programmed ceiling, the tool stops instantly, ensuring a stable thread-forming process, reducing work-piece damage caused by fastener problems, and—together with the self-tapping strategy—providing superior monitoring for your tightening operations and safeguarding your production quality.