Tapping Head: Drill Press Tap Size & Speed Guide

A drill press, a stationary power tool, achieves threading operations with the assistance of a tapping head. Emuge-Franken, a recognized manufacturer, produces high-quality tapping attachments adaptable for use in varied drill press models. Correct tap size selection, often referencing machinery’s handbook, is critical for successful thread creation. Moreover, understanding optimal speeds and feeds, in accordance with recommendations from organizations like the Precision Machined Products Association (PMPA), ensures accurate and efficient tapping when utilizing a tapping head for drill press applications.

Mastering Thread Creation: A Drill Press Tapping Guide

Creating precise, consistent threads using a drill press and tapping head requires understanding the core principles of tapping, selecting the correct tap size, and setting the appropriate speed. This guide breaks down each element, offering actionable advice for both novice and experienced machinists.

Understanding the Tapping Head for Drill Press

The tapping head for drill press is a specialized attachment that transforms your drill press into a precision tapping machine. Unlike manual tapping, which relies on operator feel and strength, a tapping head automates the process to a degree, resulting in more uniform and accurate threads. It generally consists of a clutch mechanism to prevent tap breakage, depth control, and a reverse function to back the tap out after the threads are cut.

Why use a tapping head? Here are the core benefits:

• Improved Accuracy: Reduced risk of angled or inconsistent threads.
• Increased Efficiency: Faster tapping compared to manual methods, especially for multiple holes.
• Reduced Tap Breakage: Clutch mechanism protects the tap from overload.
• Enhanced Consistency: Uniform thread depth and quality across multiple parts.

Selecting the Right Tap Size

Choosing the correct tap size is crucial for creating functional threads. The tap size dictates the diameter of the hole you need to drill before tapping. The tap drill size is directly related to the desired thread percentage. Most applications aim for 75% thread engagement, offering a good balance between strength and ease of tapping.

Here’s how to determine the correct tap drill size:

1. Identify the Tap Size: This information is usually marked on the tap itself (e.g., 1/4-20, M6x1.0). The first number represents the nominal diameter, and the second number (after the dash or "x") represents the threads per inch (TPI) or pitch in millimeters.
2. Consult a Tap Drill Chart: These charts provide the recommended drill size for each tap size, often listed for different thread percentages (e.g., 60%, 75%, 85%). Search online for "tap drill chart" to find a comprehensive list. Many machinists also keep a physical copy in their workshop.

3. Calculate Manually (Less Common): While less practical for everyday use, you can calculate the tap drill size using the following formula:

   • Imperial (TPI): Tap Drill Size = Nominal Diameter – (1 / TPI)
   • Metric (Pitch): Tap Drill Size = Nominal Diameter – Pitch

   Example: A 1/4-20 tap has a nominal diameter of 0.250 inches and 20 threads per inch.

   Tap Drill Size = 0.250 – (1/20) = 0.200 inches. A #7 drill bit is the closest standard size.

Considerations When Selecting Thread Percentage:

• Higher Percentage (85% or Higher): Stronger threads, but requires more force to tap and increases the risk of tap breakage. Generally used for softer materials or high-stress applications.
• Lower Percentage (60% or Lower): Easier to tap, but weaker threads. Suitable for brittle materials or low-stress applications.

Setting the Correct Drill Press Speed

The speed at which you rotate the tap is critical for both thread quality and tap longevity. Too fast, and you risk overheating the tap, causing it to dull or break. Too slow, and you might produce rough or uneven threads.

The optimal speed depends on several factors:

• Material Being Tapped: Softer materials (e.g., aluminum, brass) can be tapped at higher speeds than harder materials (e.g., steel, stainless steel).
• Tap Size: Smaller taps generally require higher speeds than larger taps.
• Tap Material: High-speed steel (HSS) taps can handle higher speeds than carbon steel taps.
• Lubrication: Proper lubrication reduces friction and heat, allowing for higher speeds.

Calculating RPM (Revolutions Per Minute)

While tap speed charts are useful, it’s important to understand how to calculate the appropriate RPM for your drill press. Use the following formula:

RPM = (Cutting Speed x 3.82) / Tap Diameter (inches)

OR

RPM = (Cutting Speed x 12) / Tap Diameter (inches)

Where:

• Cutting Speed: This is the surface speed of the material being cut, expressed in surface feet per minute (SFM). SFM varies depending on the material. Refer to a cutting speed chart for recommended values.
• Tap Diameter: The nominal diameter of the tap (e.g., 0.250 inches for a 1/4-20 tap).

Example: Tapping aluminum with a 1/4-20 HSS tap. Aluminum has a cutting speed of around 200 SFM.

RPM = (200 x 3.82) / 0.250 = 3056 RPM

Since drill presses have limited speed settings, choose the closest speed below the calculated value.

General Speed Recommendations (Starting Points):

Material	Recommended SFM
Aluminum	200-300
Brass	150-250
Mild Steel	80-120
Stainless Steel	30-60
Cast Iron	60-90

Important Notes:

• Always start at a slower speed and gradually increase it until you find the optimal setting.
• Listen to the sound of the tap. Excessive squealing or chatter indicates that the speed is too high.
• Use cutting fluid specifically designed for tapping. This will reduce friction, dissipate heat, and improve thread quality.

Practical Tapping Procedure with a Drill Press Tapping Head

1. Secure the Workpiece: Firmly clamp the workpiece to the drill press table to prevent movement during tapping.
2. Align the Tapping Head: Ensure the tapping head is perfectly aligned with the drilled hole. Misalignment can lead to angled threads or tap breakage.
3. Lubricate: Apply a generous amount of cutting fluid to the tap and the hole.
4. Engage the Tap: Slowly lower the tapping head, engaging the tap with the hole.
5. Apply Downward Pressure: Apply gentle, consistent downward pressure on the drill press handle. Let the tap do the work. Avoid forcing the tap.
6. Monitor the Process: Listen for any unusual noises or vibrations. Stop immediately if you encounter resistance.
7. Reverse the Tap: Once the desired thread depth is reached, engage the reverse function on the tapping head and carefully back the tap out of the hole.
8. Clean the Threads: Remove any chips or debris from the threads using compressed air or a thread chaser.

So, next time you’re tackling a tapping job with your drill press, remember to double-check your tap size and speed settings! Using a tapping head for drill press will make things a whole lot easier, but even with the right gear, precision is key. Happy tapping, and may your threads always be clean!