Threading a drilled hole, also known as tapping, is the process of cutting internal screw threads into a pre-drilled hole, allowing it to accept a bolt or screw. This is typically done manually using a specialized tool called a tap and a tap handle.
Understanding Threading (Tapping)
Tapping is a crucial process in various applications, from assembling components to creating fastening points. It transforms a smooth, round hole into a threaded one, ensuring a secure and reliable connection. The quality of the thread depends heavily on the preparation of the pilot hole and the tapping technique.
Essential Tools for Threading a Hole
Before you begin, ensure you have the right tools for the job. Using the correct tools is paramount for achieving clean, precise threads and preventing damage to your workpiece or tools.
| Tool | Purpose |
|---|---|
| Drill | To create the initial pilot hole before tapping. |
| Correct Drill Bit | Specifically sized for the tap being used; essential for proper thread engagement. |
| Tap | The cutting tool that forms the internal threads. Taps come in various types (taper, plug, bottoming) and sizes. |
| Tap Handle | Used to hold the tap securely and provide leverage for rotation. |
| Cutting Oil/Lubricant | Reduces friction, dissipates heat, and aids in chip removal, prolonging tap life. |
| Deburring Tool | To remove any sharp edges or burrs from the drilled hole. |
Step-by-Step Guide to Threading a Drilled Hole
The process of threading a hole requires precision and patience. Following these steps will help you achieve clean, strong threads.
1. Preparation: Drilling the Pilot Hole
Before you can thread a hole, you must first drill a pilot hole. The size of this hole is critical; it must be slightly smaller than the major diameter of the tap, allowing enough material for the tap to cut the threads. Refer to a tap and drill chart to find the precise drill bit size for your chosen tap.
- Select the Correct Drill Bit: Choose a drill bit that matches the recommended size for your specific tap.
- Drill the Hole: Secure your workpiece and drill the hole straight and to the correct depth.
- Deburr the Hole: After drilling, use a deburring tool or a larger drill bit to gently chamfer the edges of the hole. This removes sharp burrs that can hinder tap entry and lead to chipped threads.
2. Tapping the Hole (The Threading Process)
With the pilot hole drilled and deburred, you are ready to begin the threading process.
- Secure the Tap: Use the tap of the correct size and a tap handle to thread the hole. Insert the square end of the tap firmly into the tap handle, ensuring it is centered and tightened.
- Position the Tap: Place the tap's tapered end into the drilled hole, ensuring it is perfectly straight and aligned with the hole. Proper alignment is crucial to prevent crooked threads or tap breakage.
- Begin Cutting the Threads: Apply slight downward pressure and rotate the taper bit clockwise until you feel resistance from the threads cutting into the material. It's helpful to add a few drops of cutting oil to the tap and the hole to lubricate the cutting action and aid in chip removal.
- Clear Chips and Continue Tapping: Once the tap has begun to bite, continue rotating it. From there, for every full rotation clockwise, make a half rotation counterclockwise. This back-and-forth motion helps to break off the metal chips that are being cut, preventing them from jamming the tap and potentially causing it to bind or break.
- Complete the Tapping Process: Continue the clockwise-and-half-turn-back technique, periodically applying more cutting oil, until the tap passes through the hole (for through-holes) or reaches the desired depth (for blind holes). Once completed, carefully back the tap out of the hole by turning it counterclockwise.
Important Tips for Successful Threading
- Lubrication is Key: Always use appropriate cutting oil. It extends the life of your tap, reduces friction, and helps produce cleaner threads.
- Maintain Alignment: Keep the tap perfectly straight throughout the entire process. Using a tap guide or a drill press (with the drill off) to start the tap can help maintain alignment.
- Don't Force It: If you feel excessive resistance, back out the tap, clear the chips, add more lubricant, and try again. Forcing the tap can cause it to break inside the hole, which is difficult to remedy.
- Cleanliness: After tapping, clean the threaded hole to remove any remaining chips and lubricant. This ensures a clean fit for your fastener.
