What Is Drill And Tap – Creating Strong Threaded Holes In Metal

ByJim Boslice Metalworking & Fabrication

Drill and tap is a two-step machining process used to create internal threads inside a hole, allowing a bolt or machine screw to be threaded directly into a material. First, a specific “tap drill” size is used to create a hole, and then a “tap” tool is rotated into that hole to cut the matching threads.

This technique is essential for DIYers working on automotive repairs, custom metal fabrication, or heavy-duty woodworking projects where nuts and bolts are impractical.

You have likely been there: working on a project where a simple nut and bolt just will not cut it. Maybe you are mounting a bracket to a thick steel plate or repairing a stripped-out hole on an engine block.

Mastering the skill of what is drill and tap allows you to create professional-grade fasteners exactly where you need them. It is a fundamental skill that transforms how you approach assembly and repair in your home workshop.

In this guide, we will break down the tools you need, the math behind sizing, and the step-by-step technique to ensure you never snap a tap. Let’s get your workbench ready for some precision metalwork.

Understanding what is drill and tap for DIY Projects

At its core, the process is about creating a mating surface for a threaded fastener. Unlike a wood screw that displaces material, a tap actually cuts or forms threads into the walls of a pre-drilled hole.

This is a game-changer for the garage tinkerer because it eliminates the need to reach the backside of a workpiece to hold a nut. It allows for “blind holes,” where the fastener does not go all the way through the material.

Understanding what is drill and tap starts with recognizing that it is a precision operation. You are working with tolerances measured in thousandths of an inch, so patience and the right technique are your best friends.

The Anatomy of a Tap

A tap looks like a hardened steel bolt with grooves, or “flutes,” cut into the sides. These flutes serve two purposes: they create the cutting edges and provide a path for metal chips to escape.

The top of the tap is usually square. This allows a specialized tap handle to grip it securely so you can apply even, rotational pressure without slipping.

Most taps are made from High-Speed Steel (HSS) or carbon steel. For most DIY shop tasks, HSS is the gold standard because it stays sharp longer and handles heat better during the cutting process.

The Essential Tool Kit for Tapping Threads

You cannot just grab any drill bit and a pair of pliers and expect good results. Having the right kit is the difference between a successful thread and a broken tool stuck in your project.

  • Tap Sets: These come in various sizes, both Metric and SAE (Imperial).
  • Tap Handle: A T-handle or a long bar-style wrench used to turn the tap manually.
  • Cutting Fluid: Essential for lubricating the cut and preventing the tap from overheating or seizing.
  • Center Punch: Used to mark the exact spot for your hole so the drill bit doesn’t “walk.”
  • Drill Bits: Specifically sized “tap drills” that correspond to the tap size.

Types of Taps You Should Know

Not all taps are created equal, and using the wrong one for a specific hole can lead to frustration. There are three main types you will encounter in a standard set. Taper Taps have a significant lead-in, with the first 7 to 10 threads ground down. These are the easiest to start and are perfect for through-holes where the tap can pass all the way through. Plug Taps are the most common “all-purpose” taps. They have a shorter taper of about 3 to 5 threads, making them versatile for both through-holes and deeper blind holes. Bottoming Taps have almost no taper at the end. These are used to cut threads all the way to the very bottom of a hole that doesn’t go through the material.

The Critical Step: Choosing the Right Drill Bit

One of the biggest hurdles in learning what is drill and tap is realizing you cannot just use a 1/4-inch drill bit for a 1/4-inch tap. If you do, there will be no material left for the tap to cut threads into.

The hole you drill must be slightly smaller than the outer diameter of the tap. This “tap drill” size ensures that just enough material remains for the tap to form 75% to 80% thread depth, which is the industry standard for strength.

Most tap sets come with a drill and tap chart. For example, if you are using a 1/4-20 UNC tap (a common 1/4-inch size), you would typically use a #7 drill bit, not a 1/4-inch bit.

Metric vs. Imperial Sizing

Always double-check your fasteners before you drill. A 6mm bolt might look like a 1/4-inch bolt, but the thread pitches are entirely different. Trying to force one into the other will ruin the hole.

Imperial taps are defined by diameter and “threads per inch” (TPI), such as 5/16-18. Metric taps use diameter and “pitch” (the distance between threads), such as M8 x 1.25.

Keep a digital caliper on your workbench. It is the fastest way to verify the diameter of the bolt you are trying to match before you start the what is drill and tap process.

Step-by-Step Guide: How to Drill and Tap Like a Pro

Now that you have your tools and your sizes sorted, it is time to get to work. Follow these steps to ensure your threads are straight and strong.

Step 1: Mark and Punch

Precision starts with a mark. Use a center punch and a hammer to create a small dimple in the metal. This gives the drill bit a place to “seat” so it doesn’t wander across the surface.

If you are working on a finished surface, you can put a piece of masking tape down first to prevent scratches. Accuracy here is vital for the bolt to line up later.

Step 2: Drill the Pilot Hole

Secure your workpiece in a vise. If the piece moves while you are drilling, your hole will be crooked. Use a drill press if you have one for maximum vertical accuracy.

Apply a drop of cutting oil and drill at a steady, moderate speed. Do not force the bit; let the tool do the work. Clear the chips frequently by pulling the bit up slightly while it’s spinning.

Step 3: Prepare for Tapping

Once the hole is drilled, clean out any metal shavings with compressed air or a magnet. Apply a generous amount of cutting fluid to both the tap and the hole.

Never skip the oil. It reduces friction, prevents the metal from welding itself to the tap, and helps the chips flow out of the flutes.

Step 4: Start the Tap

Place the tap into the tap handle and set it into the hole. This is the most critical moment. You must ensure the tap is perfectly perpendicular to the workpiece.

Apply light downward pressure and turn the handle clockwise. Once the first two threads catch, stop and check the squareness from two different angles. If it’s crooked, back it out and restart.

Step 5: The “Two Steps Forward, One Step Back” Technique

As you turn the tap, you will feel resistance. This is the tap cutting into the metal. After every half to full turn, rotate the tap counter-clockwise about half a turn.

You will hear a slight “click.” That is the metal chip breaking off inside the flute. Breaking the chip prevents the tap from getting jammed and snapping under the pressure.

Pro Tips for Different Materials

The what is drill and tap process changes slightly depending on what you are working with. Different materials react differently to the heat and pressure of cutting.

Tapping Aluminum

Aluminum is soft but “gummy.” It tends to stick to the cutting edges of the tap. Use a specialized aluminum cutting fluid or even WD-40 to keep things moving.

Because it is soft, you can easily strip the threads if you apply too much torque. Be gentle and clear the chips more frequently than you would with steel.

Tapping Steel and Stainless Steel

Mild steel is generally easy to tap with good oil. However, stainless steel is a different beast. It “work hardens,” meaning the more you friction-heat it, the harder it gets.

For stainless, use a high-quality cobalt drill bit and a heavy-duty sulfur-based cutting oil. Go slow, and do not stop the drill bit mid-cut, or you might find the metal has become too hard to finish.

Tapping Wood

Yes, you can tap wood! This is a great trick for jigs or temporary fixtures. Use a hardwood like maple or oak for the best results.

Standard metal taps work fine, but you don’t usually need cutting oil. Instead, some woodworkers use a drop of thin CA glue (super glue) in the hole after tapping to harden the threads.

Common Mistakes and How to Avoid Them

Even experienced DIYers run into trouble sometimes. Knowing the pitfalls of what is drill and tap operations can save you hours of repair work. Breaking the Tap: This is the ultimate nightmare. Taps are made of very hard, brittle steel. If you force a jammed tap, it will snap. If it feels stuck, back it out, clean the flutes, and add more oil. Crooked Threads: If the tap goes in at an angle, the bolt will never sit flush. Always use a square or a guide block to keep the tap straight during the first few turns. Using the Wrong Drill Size: If the hole is too small, the tap will break. If the hole is too large, the threads will be “shallow” and will strip out as soon as you tighten the bolt.

Safety Practices for the Workshop

Tapping involves sharp metal shards and high-tension tools. Always wear safety glasses, as metal chips can fly out of the flutes when you blow out a hole.

Be careful of the “burrs” or sharp edges left around the top of the hole after drilling. Use a larger drill bit or a countersink tool to lightly chamfer the edge of the hole before you start tapping.

Finally, always secure your work. A piece of metal spinning on a drill bit because it wasn’t clamped is a serious injury waiting to happen. Use a vise or heavy-duty C-clamps.

Frequently Asked Questions About what is drill and tap

What is the difference between a tap and a die?

A tap is used to cut internal threads (inside a hole). A die is used to cut external threads (on the outside of a rod or bolt). They are often sold together in “tap and die” sets.

Can I use a power drill to tap a hole?

While “power tapping” is done in industrial settings, it is very risky for DIYers. It is very easy to snap the tap because you lose the “feel” of the resistance. For most projects, hand tapping is much safer.

What do I do if I break a tap inside the hole?

This is difficult because taps are too hard to drill out. You can try using a tap extractor, which has fingers that slide into the flutes. In extreme cases, you may need to use a carbide end mill or a specialized chemical disintegrator.

Do I need cutting oil for every material?

You should use it for almost all metals. The only common exception is cast iron, which is often tapped dry because the graphite in the iron acts as a natural lubricant.

Mastering the Craft

Learning what is drill and tap is a rite of passage for any serious DIYer or garage tinkerer. It moves you away from “making do” with whatever fasteners are at the hardware store and into the world of custom fabrication.

Remember to always reference your drill charts, use plenty of lubrication, and take your time. There is no better feeling than threading a bolt into a hole you created yourself and feeling it lock down with perfect precision.

Now, grab some scrap metal, a 1/4-20 tap, and start practicing. Your future projects will thank you for the extra strength and professional finish that only a properly tapped hole can provide. Happy building!

Jim Boslice
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