Tapped Holes: 5 Essential Differences Explained for Success!
Table of Contents
Introduction
When designing mechanical assemblies, it's crucial to understand the differences between various types of holes used for fasteners.
Tapped and threaded holes are common terms, but they often cause confusion. This article will clarify these concepts, explore the processes behind tapping and threading, and help you understand when to use each term correctly.
What is Tapping?
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:
| Nominal Size (Threads/Inch) | Thread Form Designation |
| Material Tap is Constructed From | Tolerance for Pitch Diameter |
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.
What is Threading?
Threading refers to the process of creating external threads on a cylindrical fastener like a screw or bolt. These external threads engage with the internal threads in a tapped hole to fasten components together.
Threading starts at the point where the fastener engages the material and runs along the cylindrical body to the shank (the unthreaded portion). Threaded fasteners, such as screws and bolts, have been used for centuries to hold components together. Screws, in particular, act as simple machines, converting rotational force (torque) into linear force, similar to the mechanism of the Archimedes screw.
Pro-Tip: When designing a threaded fastener, it’s often better to use off-the-shelf fasteners. If custom fasteners are necessary, use threading calculations to ensure proper design.
Key Takeaways:
Tapped Hole: A tapped hole is a threaded hole created by tapping to form internal threads.
Threaded Hole vs. Tapped Hole: These terms are often used interchangeably because a tapped hole is simply another term for a threaded hole.
Drilled Hole vs. Tapped Hole: A drilled hole is a smooth cylindrical hole, while a tapped hole has internal threads to accommodate a fastener.
Conclusion
Understanding the distinction between tapped holes and threaded holes is essential for mechanical design. While both terms are often used interchangeably, knowing the correct process behind each term can improve your design’s functionality and accuracy. Whether you’re drilling or tapping, our CNC machining services can produce your custom parts quickly and efficiently.
Upload your design, and experience our fast quoting, manufacturability feedback, and precision machining process. Let us help you bring your projects to life, with parts ready in as little as two days!
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