Tapped Holes: 5 Essential Differences Explained for Success!

Tapping is the process of creating internal threads inside a hole to accommodate a screw or bolt. A tapped hole is simply a hole with internal threads. To start the tapping process, a hole is drilled slightly smaller than the major diameter of the threaded fastener to be used. Here’s the formula for determining the correct drill size for ANSI threads:

Drill Size = Major thread diameter – (1 ÷ # threads per inch) + 0.005″ (if pitch > 10), 0.000″ (if pitch ≤ 10)

Once the hole is drilled, a tap is used to form threads on the inside. Taps come in different varieties, such as hand taps, power taps, and tapered taps, and are marked with the following information:

The nominal size refers to the thread density, and the thread form designation indicates the thread family (e.g., ACME, UNC, UNF, etc.). The material is typically durable, such as carbide, and the tolerance provides a code representing thread precision.

Tips for Tapping:

• Hand Taps vs. CNC Taps: CNC taps offer greater precision compared to hand taps, so opt for CNC when possible.
• Blind Holes: Start with a taper tap, then switch to a bottoming tap for deeper thread engagement. A bottoming tap threads almost entirely to the bottom of the hole, ensuring optimal thread engagement.
• Spiral Point Taps: Avoid spiral point taps in blind holes during CNC machining, as they push chips down into the hole. Instead, use spiral flute or interrupted thread taps.
• Thread Forming Taps: These taps form threads through chipless compression, resulting in stronger, longer-lasting threads. They require a larger tap drill diameter, so consult Machinery’s Handbook for appropriate calculations.

Many confuse clearance holes with tapped holes, but clearance holes are not tapped. They are drilled for the threaded portion of a fastener to pass through, with a nut clamping components together at the opposite end.