볼트 체결 깊이, "적당히"는 없습니다

When designers only mark "Tap" on drawings without specifying depth, field workers often face difficulties. Incorrect threading depth can lead to stripped threads during assembly or catastrophic bolt failures later. Here are the golden rules learned from installing tens of thousands of bolts over 10 years.

1. Minimum Threading Depth by Material (1.5D / 2D)

The depth of bolt engagement must vary according to material strength:

• Steel, Cast Iron: Minimum 1.5× bolt diameter (D). (e.g., M8 bolt requires ≥12mm)
• Aluminum, Copper (Non-ferrous): Minimum 2× bolt diameter (D). (e.g., M8 bolt requires ≥16mm)

Why? Soft materials like aluminum have lower shear strength, making threads susceptible to stripping. Greater depth distributes forces and ensures safety.

2. Effective Thread vs Drill Depth

A common mistake: designing drill depth equal to effective thread length.

• Chip Pocket: Space is needed for chips and the tap lead.
• Practical Tip: Make drill holes 3-5mm deeper than effective thread depth. Otherwise, bolts bottom out, preventing proper fastening.

3. Through-Hole vs Blind Hole Tapping

Prefer through holes when possible. Easier to machine, better chip evacuation, reduced tap breakage risk. For sealed or cosmetic applications requiring blind holes, strictly follow the drill depth formula above.