Making a screw thread in metal comes down to two core techniques: tapping (for internal threads, like a nut) and die cutting (for external threads, like a bolt). Both can be done by hand with inexpensive tools, and the process is straightforward once you understand the setup. The difference between clean threads and a broken tool almost always comes down to preparation: drilling the right size hole, keeping things aligned, and using lubrication.
Internal Threads: Tapping a Hole
Tapping is the process of cutting threads inside a drilled hole so a bolt or screw can thread into it. You need three things: a drill bit sized for your tap, the tap itself, and a tap wrench to turn it. The drill bit size is critical. Drill too small and the tap will bind and snap. Drill too large and your threads won’t grip properly.
The standard target is 75% thread engagement, which is what most tap drill charts are based on. This gives you strong threads without putting excessive force on the tap. You’ll find these charts printed on tap packaging, in machinist references, or with a quick search for “tap drill size chart.” For a common example, an M10 x 1.5 metric tap calls for an 8.5 mm drill bit. A 1/4-20 imperial tap calls for a #7 drill (0.201 inches).
Step by Step
Clamp your workpiece securely. Any lateral movement during tapping can snap the tap or produce crooked threads. Drill your hole to the correct size, keeping the drill perpendicular to the surface. If you’re working in steel, a center punch before drilling helps prevent the bit from wandering.
Place the tap into the hole and begin turning it clockwise with the tap wrench. The single most important thing here is starting straight. If the tap enters at an angle, the threads will be crooked and the tap may break. Some machinists use a drill press (powered off, just using it as a guide) to keep the tap perfectly vertical for the first few turns.
Use the “two forward, one back” technique: turn the tap forward about one and a half to two full turns, then reverse it roughly one turn. This breaks the metal chips that accumulate in the flutes, preventing them from packing up and jamming the tap. It also lets cutting fluid flow back down to the cutting edges. Skipping this step is the most common reason beginners snap taps.
Choosing the Right Tap Style
Taps come in three chamfer styles that look nearly identical at first glance. The difference is in the tapered lead at the tip.
- Taper taps have about nine threads of gradual taper at the tip. They start the easiest and cut the straightest, making them the best choice for through holes (holes that go all the way through the metal).
- Plug taps have about four threads of taper. They’re the most popular general-purpose style and work well in both through holes and blind holes (holes with a bottom).
- Bottoming taps have only one or two threads of taper. You need these when threads must extend all the way to the bottom of a blind hole, but they’re harder to start cleanly. Many machinists start with a taper or plug tap first, then follow up with a bottoming tap.
The longer the taper, the more gradually the tap engages the metal, which means less force and better thread quality. When you have the choice, start with the longest chamfer that works for your hole type.
External Threads: Using a Die
To cut threads on a rod or shaft, you use a round cutting die held in a die stock (a handle that grips the die). The concept is simple: place the die over the end of the rod and start turning. As the die rotates, it cuts threads into the outside surface of the metal.
Alignment matters just as much here as it does with tapping. If the die starts crooked, the threads will be off-axis and won’t mate properly with a nut. The trick is to use something rigid to guide the die for the first few rotations. A drill press (again, not powered) works well as a straightening jig. You can also use a lathe tailstock or even a square block to visually check alignment.
Before starting, chamfer the end of the rod slightly with a file or grinder. This small bevel gives the die’s cutting teeth somewhere to grab and makes it much easier to start straight. The rod diameter should match the nominal thread size. For an M10 thread, use a 10 mm rod. For a 3/8-16 thread, use a 3/8-inch rod.
Use the same forward-and-back chip-breaking motion you’d use when tapping: a couple of turns forward, one turn back. Apply cutting fluid generously. Dies can also be used to clean up damaged existing threads on bolts by simply threading the die onto the bolt and turning it through, which trims off burrs and restores the thread profile.
Lubrication Makes or Breaks the Job
Cutting fluid reduces friction, carries heat away from the cutting edges, and helps chips clear the flutes. Skipping it, especially in steel, dramatically increases your chances of breaking a tap or producing rough, torn threads.
For steel (carbon steel, alloy steel, stainless), use a dedicated cutting oil or tapping fluid. These are typically sulfurized or chlorinated oils designed to handle the high pressures at the cutting edge. For aluminum, brass, and bronze, water-soluble cutting oils work well and are generally recommended for non-ferrous metals. Cast iron is an exception: it produces powdery chips rather than long stringy ones, and many machinists tap it dry or with a light mist of cutting fluid.
In a pinch for small jobs, a few drops of any light machine oil are better than nothing. But for repeated work or harder materials, proper tapping fluid pays for itself in tool life alone.
Metric vs. Imperial Thread Standards
Before you buy taps or dies, you need to know which thread standard you’re working with. The two systems measure pitch differently.
Imperial threads are described by threads per inch (TPI). A 1/4-20 bolt is 1/4 inch in diameter with 20 threads per inch. Metric threads are described by the distance between threads in millimeters. An M10 x 1.5 bolt is 10 mm in diameter with 1.5 mm between each thread crest. You can convert between the two: pitch in millimeters equals 25.4 divided by TPI. So 20 TPI works out to a 1.27 mm pitch.
The easiest way to identify an unknown thread is with a thread pitch gauge, an inexpensive fan-shaped tool with thin blades that you match against existing threads. Getting this wrong means your tap or die won’t match the fastener you’re trying to work with, and metric and imperial threads that are close in size (like M12 and 1/2 inch) will cross-thread and destroy both parts.
What to Do if a Tap Breaks
A broken tap stuck in a hole is one of the most frustrating problems in metalworking, because taps are hardened steel and extremely difficult to drill through. Prevention is the best strategy: use cutting fluid, break chips regularly, start straight, and don’t force it.
If it happens anyway, you have a few options depending on what’s available to you. A tap extractor is a purpose-built tool with thin fingers that slide into the tap’s flutes, letting you grip and reverse the broken piece out. This is the simplest method and works well when the tap isn’t deeply embedded. If part of the broken tap protrudes above the surface, you can weld a nut or small handle to it and carefully back it out.
For taps broken flush or below the surface, more aggressive methods come into play. Electrical discharge machining (EDM) can burn away the center of the broken tap without damaging the surrounding workpiece, but it requires specialized equipment. In some cases, a carbide drill bit smaller than the tap can be used to drill into the center of the broken piece, weakening it enough to pick out the fragments. As a last resort for high-speed steel taps, a nitric acid solution can dissolve the tap material without attacking most workpiece metals, though this is slow and requires careful handling.
Keeping Your Tools in Good Shape
Taps and dies are precision cutting tools, and dull or corroded ones produce bad threads and break more easily. After each use, wipe the tools clean with a dry cloth to remove chips and residual cutting fluid. For stubborn buildup, a mild detergent or solvent works, but dry the tools thoroughly afterward.
Before storing them, apply a light coating of machine oil as a rust barrier, then wipe off the excess so it doesn’t attract dust. Store taps and dies in a dry location. If your shop is prone to humidity, a container with a desiccant packet or a dehumidifier in the room will prevent the slow oxidation that ruins cutting edges over time. Taps stored loose in a drawer bang against each other and chip, so individual slots or protective cases are worth the small investment.