A tapped hole is simply a hole with internal threads cut into it, allowing a bolt or screw to be threaded directly into the material without needing a nut on the other side. The process of cutting those threads is called “tapping,” which is where the name comes from. If you’ve ever screwed a bolt into a metal bracket and felt it grip into the hole itself, you were using a tapped hole.
A plain drilled hole has smooth internal walls. A tapped hole starts as that same drilled hole, then gets internal threads added using a cutting tool called a tap. The terms “tapped hole” and “threaded hole” mean the same thing when referring to a hole that accepts a fastener. The distinction that matters: tapping creates threads inside a hole, while threading typically refers to creating external threads on a bolt, screw, or pipe.
How a Tapped Hole Is Made
Creating a tapped hole is a two-step process. First, you drill a smooth hole to a specific diameter, called the “tap drill size.” This hole is slightly smaller than the finished thread size, leaving just enough material for the tap to cut into. Second, you run the tap into the hole to cut the spiral threads.
Before tapping, the edge of the drilled hole is usually chamfered (given a small bevel) so the tap can start cleanly. When tapping by hand, the standard technique is to turn the tap forward one full rotation, then back it up half a turn to break the chips loose. Cutting fluid or oil is applied between the tap and the workpiece to reduce friction and prevent the tap from binding. Chips are cleared frequently, especially in deeper holes.
Machine tapping follows the same basic principle but uses a drill press, milling machine, or dedicated tapping center to keep the tap perfectly aligned and control the speed. High-speed tapping centers with rigid tap holders can thread holes much faster than manual methods and are standard in production manufacturing.
Through Holes vs. Blind Holes
The two basic hole geometries you’ll encounter are through holes, which pass completely through the material, and blind holes, which stop partway through. Tapping a through hole is the easier of the two. Chips from the cutting process can fall out the bottom, and you can run the tap all the way through to ensure full threads along the entire length.
Blind holes are trickier for two reasons: chip removal and thread depth. Since chips can’t fall out the bottom, they accumulate in the hole and need to be directed upward against the cutting direction. Taps designed for blind holes use spiral flutes that act like an auger to pull chips out. The trade-off is that these spiral flutes can create a corkscrew effect under too much pressure, leading to thread misalignment.
Thread depth is the other challenge. In a through hole, you can keep turning the tap until its tapered lead section passes all the way through, giving you complete threads from top to bottom. In a blind hole, there’s no room for that, so the last few threads at the bottom won’t be fully formed. Shorter tap chamfers minimize this dead zone but wear out faster because fewer cutting edges share the load.
A useful rule of thumb: a tap designed for blind holes will often work fine in a through hole, but using a through-hole tap in a blind hole almost always causes problems.
Types of Taps
Three main chamfer styles cover most tapping situations: taper, plug, and bottoming. They share the same basic shape but differ in how gradually they start cutting.
- Taper taps have about nine threads of gradual taper at the tip. The long lead-in makes them the easiest to start and produces the cleanest threads with the longest tool life. They’re ideal for through holes where the tapered section can pass completely through the material.
- Plug taps have a shorter chamfer of roughly four threads. They balance cutting efficiency with the ability to thread closer to the bottom of a hole, making them the most popular general-purpose choice.
- Bottoming taps have only one or two chamfered threads, letting them cut nearly to the very bottom of a blind hole. The short chamfer concentrates cutting forces on fewer edges, so they wear faster and work less well in tough materials like stainless steel or high-temperature alloys.
For difficult blind holes in hard materials, many machinists start with a plug tap to establish most of the threads, then follow up with a bottoming tap to finish the last few turns at the bottom.
Reading Thread Callouts
When you see a tapped hole specified on a drawing or in a product listing, it follows a standard naming convention that tells you the diameter and how tightly the threads are spaced.
In the imperial system (common in the U.S.), a callout like “1/4-20 UNC” means the major diameter is 1/4 inch, there are 20 threads per inch, and it follows the Unified National Coarse standard. A finer version of the same diameter would be “1/4-28 UNF,” with 28 threads per inch. Coarse threads (UNC) are the default for most general-purpose fastening. Fine threads (UNF) offer more precise adjustment and better resistance to vibration loosening.
Metric callouts work slightly differently. “M6x1.0” means a 6 mm major diameter with a 1.0 mm pitch (the distance from one thread crest to the next). A smaller pitch number means finer threads. Metric threads are standard outside the U.S. and increasingly common everywhere.
Choosing the Right Drill Size
The drill bit you use before tapping has to be the right diameter. Too small, and the tap has to remove too much material, increasing the chance of breakage. Too large, and the threads will be shallow and weak. The correct size, called the tap drill size, is specific to each thread specification and is found in standard reference charts.
For example, a 1/4-20 UNC thread requires a tap drill of about 5.35 mm, while the finer 1/4-28 UNF uses a slightly larger 5.50 mm drill. The difference is small but matters: the finer thread has shallower grooves, so less material needs to be removed. Most tap manufacturers include recommended drill sizes on the packaging or in their catalogs.
Why Taps Break and How to Avoid It
A broken tap stuck inside a hole is one of the most frustrating problems in metalworking, and it’s almost always preventable. The three main causes are using the wrong tap for the material, misalignment between the tap and the hole, and worn or dull tools.
Chip packing is the most common culprit. If chips can’t clear the flutes fast enough, they jam and the tap seizes. This is especially likely in blind holes with straight-flute taps (which push chips downward with nowhere to go) or when cutting fluid isn’t applied generously enough. Running the tap too fast, or failing to back it up periodically to break chips, compounds the problem.
Misalignment causes uneven loading on the tap’s cutting edges. Even a small angle error means one side of the tap does more work than the other, and taps are brittle tools with very little tolerance for bending forces. Using a tap guide block for hand tapping, or a rigid holder for machine tapping, keeps the tap square to the hole.
A dull tap requires more force to cut, and that extra force can exceed what the tool can handle. If a tap starts producing rough threads or requires noticeably more effort than when it was new, replace it before it snaps.
Tapped Holes vs. Other Fastening Methods
Tapping directly into a part is the fastest and most compact way to create a threaded connection. There’s no nut to hold on the back side, no extra hardware, and the resulting assembly sits flush. This makes tapped holes the standard choice in machined metal parts, engine blocks, structural frames, and anywhere space is tight.
The limitation is material strength. Tapping works well in steel, aluminum, brass, and cast iron, but soft materials like thin sheet metal or plastics may not hold threads reliably under load. In those cases, threaded inserts (small metal sleeves pressed or screwed into the hole) add durable threads to materials that can’t hold them on their own. Helicoil-style inserts are a common example, often used to repair stripped tapped holes in aluminum.
The simplest alternative is a clearance hole: a plain, unthreaded hole sized larger than the bolt, with a nut on the back side. This works when both sides of the joint are accessible and when you don’t need the compactness of a tapped connection.