what size drill bit for 8mm tap

3 min read 26-12-2024

Knowing the correct drill bit size for tapping is crucial for creating clean, strong threads. Using the wrong size can lead to stripped threads, a broken tap, or a poor-fitting fastener. This guide focuses specifically on determining the ideal drill bit size for an 8mm tap, covering various tap types and materials.

Understanding Tap Drill Size

Before we dive into the specifics for an 8mm tap, let's clarify the concept of tap drill size. It's not simply the same diameter as the tap itself. The tap drill size is slightly smaller to allow space for the tap to cut threads. This creates a clean, accurately formed thread.

Calculating the Right Drill Bit Size for an 8mm Tap

The precise drill bit size needed depends on several factors, including:

Thread Type: The most common thread types are metric (like your 8mm tap) and inch. Metric threads are specified by their major diameter (e.g., 8mm) and pitch (the distance between adjacent thread crests).
Material Being Tapped: Different materials require different clearances for the tap. Harder materials need a slightly larger drill bit to avoid breaking the tap.
Tap Type: Hand taps, machine taps, and spiral point taps will each need a slightly different size drill bit.

Recommended Drill Bit Size for an 8mm Tap in Common Materials:

The general rule is to use a drill bit slightly smaller than the tap's major diameter. For an 8mm tap, this translates to approximately 6.8mm to 7.0mm. However, precise sizing depends on the material and type of thread. Here's a more detailed breakdown:

Steel (Mild Steel): 6.8mm to 7.0mm
Aluminum: 7.0mm to 7.2mm
Brass: 7.2mm to 7.4mm
Plastic: Drill size will depend heavily on the plastic type and desired thread strength, possibly larger than 7.2mm.

Important Note: These are guidelines. Always consult a tap and drill size chart for precise measurements based on your specific tap type, thread pitch, and material.

Choosing the Right Drill Bit Type

Beyond the diameter, consider the drill bit type:

High-Speed Steel (HSS): A versatile choice for many materials.
Cobalt HSS: More durable and ideal for tougher materials like hardened steel.
Titanium Nitride (TiN) coated HSS: Excellent for heat resistance and increased life.

The appropriate drill bit material ensures that the bit stays sharp and provides the best cut quality.

Step-by-Step Guide to Tapping an 8mm Hole

Select the correct drill bit: Use the size determined based on the recommendations above and your specific material.
Drill the pilot hole: Ensure the hole is straight and clean. Use cutting oil or lubricant to improve cutting and reduce friction.
Tap the hole: Carefully and steadily tap the hole, using appropriate cutting fluid. Avoid excessive force to prevent tap breakage.
Clean the threads: Remove any chips or debris from the threads after tapping.

Frequently Asked Questions

What happens if I use a drill bit that's too small?

Using a drill bit that is too small can lead to a difficult tap operation, resulting in broken taps, stripped threads, or inaccurate threads.

What happens if I use a drill bit that's too large?

Using a drill bit that is too large results in loose threads that may not hold the fastener securely. It also can cause the tap to bind easily.

Where can I find a tap and drill size chart?

Many online resources and hardware stores provide comprehensive tap and drill size charts. You can easily find them by searching for "tap drill size chart".

Conclusion

Finding the correct drill bit size for an 8mm tap requires considering multiple variables. By understanding the factors discussed and referencing a tap drill size chart, you can ensure a successful tapping operation every time, resulting in clean, strong, and reliable threads. Remember to always prioritize safety and use appropriate safety equipment when working with tools.