What is a self-tapping screw?

In the usual sense, a self-tapping screw is a threaded fastener that does not require a pre-cut internal thread. When the screw is driven into an untapped (plain) hole, it cuts its own internal thread. This operation demands high torque; the mating part is normally a relatively soft material such as plastic, aluminum, or magnesium.

Self-tapping joints offer high connection strength, low cost, and easy lightweighting, so they are widely used in automobiles. To guarantee the robustness and reliability of these joints, vehicle manufacturers place great emphasis on controlling clamp torque and on developing tightening processes.

The torque required for thread-forming is strongly influenced by the plain-hole diameter. For screws pre-coated with adhesive, the torque during the engagement phase is also affected by the adhesive, and the two cases differ noticeably. Because extra torque is consumed while the screw forms its own thread, the snug torque (the torque at which the joint first becomes tight) for a self-tapping screw is usually higher than that for a standard threaded fastener. Consequently, the target tightening torque specified in the design is also set higher.

If you are still struggling with the complex self-tapping tightening process, Danikor intelligent tightening tools can help. Our tools include a dedicated self-tapping tightening strategy, suitable for self-tapping joints in plastic, aluminum, and similar materials.
The self-tapping strategy consists of five phases:

1. Soft-start

2. Fast thread-forming

3. Continuous run-in

4. Seating / snug

5. Final tightening

These five steps can be combined flexibly to match the required cycle time and other customer needs.

Phase-by-Phase Breakdown

1. Soft-Start Phase

• Purpose: provides the motor with a gentle entry transition.

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

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

• Torque ceiling: ≤ target torque.

• Time ceiling: ≤ 5 s (to preserve takt time).

2. Fast Thread-Forming Phase

• Purpose: cuts the internal thread in the plain plastic hole so that subsequent tightening can proceed.

• Angle: large (normally ≤ 720°–1080°).

• Speed:
  – Plastic parts: ≤ 400–600 rpm.
  – Aluminum parts: ≤ 80 % of the tool’s maximum rpm (exact value set by the tightening process).

• Torque ceiling: ≤ target torque.

• Time ceiling: ≤ 5 s.

3. Continuous Run-In Phase

• Follows fast thread-forming.

• Speed: same as fast thread-forming.

• Angle calculation:
  Run-in angle = (total tightening-angle limit) – (fast thread-forming angle) – (soft-start angle) – 200°.
  A margin of 180°–360° is normally reserved for the next phase to prevent torque overshoot.

• Torque ceiling: ≤ target torque.

• Time ceiling: ≤ 5 s.

4. Seating (Snug) Phase

• Speed: 100–200 rpm, ≤ tool maximum.

• Snug torque:
  – Plastic parts: normally ≤ 80 % of target torque (depends on thread-forming speed).
  – Aluminum parts: ≤ 40 % of target torque (thread-forming speed has little effect).
  A braking control at the snug point guarantees final tightening quality even if speed is increased.

• Torque ceiling: ≤ target torque.

• Time ceiling: ≤ 5 s.

5. Final Tightening Phase

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

• Target torque: ≤ tool rated capacity.

• Torque ceiling: ≤ 1.2 × tool rated capacity.

• Time ceiling: ≤ 5 s (actual limit depends on line takt).

Real-Time Monitoring & Protection

When a Danikor intelligent tightening tool is used, torque is monitored in real time. If the measured torque ever exceeds the upper limit, the tool stops immediately, ensuring a stable thread-forming process and preventing damage to the work-piece. Combined with the dedicated self-tapping tightening strategy, the tool provides superior monitoring and quality assurance for your production.