Screw threads are made by cutting, forming, or pressing a helical groove into metal (or other materials) using tools ranging from a simple handheld tap to a CNC milling machine. The method you choose depends on whether you need internal threads (inside a hole) or external threads (on a rod or bolt), what material you’re working with, and how many pieces you need to produce.
Internal Threads With a Tap
A tap is a hardened steel tool that cuts threads inside a pre-drilled hole. You drill a hole slightly smaller than the final thread diameter (called a tap drill), then twist the tap into it to carve the spiral grooves. Taps come in three types, and you’ll often use them in sequence:
- Taper tap: Has a gradual taper at the tip, making it the easiest to start. Use this first to guide the thread into the hole.
- Plug tap: Less taper, so it cuts threads deeper into the hole. Use this for through-holes or when you need to extend beyond where the taper tap stopped.
- Bottoming tap: Almost no taper at all, allowing you to thread all the way to the bottom of a blind hole (one that doesn’t go all the way through).
To use a tap by hand, secure the workpiece in a vise so it won’t move. Insert the tap into a T-handle or tap wrench, align it straight with the hole, and turn clockwise while pressing down firmly. After every half turn or full turn, reverse the tap about a quarter turn to snap off the metal chips forming inside the hole. This “back off to clear chips” rhythm prevents the tap from jamming or breaking. Skipping this step is the most common reason taps snap, and a broken tap lodged in a hole is genuinely difficult to remove.
Choosing the right tap drill size is critical. Too large and the threads will be shallow and weak. Too small and the tap will bind and break. Tap drill charts, which come with most tap and die sets, list the correct drill size for each thread. For example, a 1/4-20 thread (one of the most common sizes) calls for a #7 drill bit.
External Threads With a Die
A die does for rods and bolts what a tap does for holes. It’s a round cutting tool with internal threads that you rotate around the outside of a workpiece to cut matching grooves. Clamp the rod vertically in a vise, fit the die into a die stock (the handle that holds it), and align it squarely on the end of the rod. Turn clockwise with steady downward pressure, backing off periodically to clear chips, just like tapping.
Before starting, chamfer the end of the rod with a file or grinder. A small bevel on the tip helps the die bite into the metal and start cutting straight. If the die starts at an angle, the threads will be crooked for their entire length, and there’s no fixing that without starting over.
Cutting Fluid Makes a Real Difference
Threading without lubrication produces rough threads, dulls your tools faster, and dramatically increases the chance of breaking a tap. For steel, a sulfur-based cutting oil is the standard choice. It reduces friction and heat at the cutting edge, giving you cleaner threads and longer tool life. For aluminum, lighter options work better. Many machinists use WD-40 or even kerosene on aluminum, which prevents the soft metal from gumming up and welding itself to the tap flutes.
Apply cutting fluid before you start and add more as you go. On deep holes especially, the fluid also helps flush chips out of the cutting zone.
Threading on a Lathe
A metal lathe gives you far more control over thread quality than hand tools. The technique, called single-point threading, uses a triangular cutting tool mounted on the lathe’s tool post. The lathe’s lead screw synchronizes the carriage movement with the spindle rotation so the tool traces a perfect helix along the workpiece.
For a standard 60-degree thread (which covers UNC, UNF, and metric threads), you set the compound rest to 29 or 30 degrees. This angles your cuts so the tool shears mostly on one side of the groove, producing cleaner threads and better chip flow. You make many light passes, advancing the compound rest a few thousandths of an inch each time, rather than trying to cut the full thread depth in one go. For the final pass, switch to feeding with the cross-slide instead of the compound. Just a thousandth or two of depth on that last cut cleans up both flanks of the thread evenly.
A thread dial indicator on the lathe tells you exactly when to engage the half-nut for each pass so your tool drops back into the same groove every time. Misreading this dial means cutting a second groove next to your first one, ruining the workpiece.
Thread Milling on a CNC Machine
Thread milling uses a rotating cutter that follows a helical toolpath programmed into a CNC machine. Unlike a tap, which must match the exact thread size, a single thread mill can produce multiple thread sizes by adjusting the toolpath. The same tool can cut left-hand or right-hand threads, internal or external, which makes it extremely flexible for low-volume or custom work.
Thread milling really shines in three situations: hard materials, blind holes, and large diameters. Taps generate high cutting forces concentrated on one tool, and in materials like titanium, Inconel, or hardened steel, they frequently snap. Thread mills distribute cutting forces more evenly and allow chips to evacuate freely instead of packing into flutes. In blind holes, where a broken tap would be a nightmare to extract, thread milling is the safer option. And for large thread diameters, a small thread mill is far cheaper than a large single-purpose tap.
Thread Rolling: Stronger Than Cutting
Thread rolling doesn’t cut material at all. Instead, hardened steel dies press against a blank rod or bolt under enormous pressure, displacing the metal into a thread shape. This cold-forming process is how most commercial fasteners are made, and it produces threads that are measurably stronger than cut threads.
The reason is grain flow. Every piece of metal has an internal grain structure, similar to wood grain. Cutting threads slices through that grain, creating stress points where cracks can start. Rolling realigns the grain to follow the thread profile, which increases tensile, shear, and fatigue strength. Rolled threads also have a smoother surface finish, which means less friction during assembly and better resistance to corrosion.
Thread rolling requires specialized equipment and is primarily an industrial production process rather than a shop technique. But if you’re choosing fasteners for a high-stress application, rolled threads are worth specifying.
Understanding Thread Standards
Before you make any thread, you need to know which standard you’re working with. The three most common systems are:
- UNC (Unified National Coarse): Fewer threads per inch, deeper grooves. Common sizes include 1/4-20 and 3/8-16. Best for general assembly, construction, and softer materials like wood, plastic, or aluminum. Coarse threads are more forgiving of dirt, minor damage, and imperfect alignment.
- UNF (Unified National Fine): More threads per inch, shallower grooves. Sizes like 1/4-28 and 3/8-24 are typical. Fine threads resist vibration better and have greater clamping force for the same bolt diameter, making them standard in automotive and aerospace work.
- Metric (ISO): Pitch is measured in millimeters rather than threads per inch. An M6 x 1.0 bolt, for example, has a 6mm diameter and 1.0mm between each thread crest. Metric is the global default for engineering and machinery.
Mixing these up is a common and costly mistake. A UNF bolt will seem to start threading into a UNC nut, then jam and strip. If you’re uncertain what thread you’re dealing with, identify it before cutting.
How to Identify an Existing Thread
A thread pitch gauge is the fastest identification tool. It’s a set of thin metal blades, each stamped with a pitch value, that fan out like a pocketknife. Clean the threads, hold a blade against them, and look for a snug fit where the blade’s teeth nestle into every groove with no gaps. When you find a blade that matches perfectly, the pitch is stamped right on it.
If you don’t have a pitch gauge, a ruler works for imperial threads. Place it along the threaded section and count the number of thread peaks within one inch. That gives you the TPI (threads per inch), which you can cross-reference with standard thread charts.
Dealing With a Broken Tap
Even experienced machinists break taps. When it happens, you have a few options depending on how deep the tap is stuck and what the workpiece is worth. Penetrating oil applied around the break and left to soak can sometimes loosen things enough to back the tap out with pliers. A tap extractor, which is a cylindrical tool with small claws that grip between the tap’s flutes, is purpose-built for this situation and is standard equipment in most machine shops. If the tap is hardened and truly stuck, drilling it out with a slightly smaller carbide bit can push the fragments free, though this risks enlarging the hole. For high-value parts, electrical discharge machining (EDM) can vaporize the broken tap without touching the surrounding workpiece, but that requires sending the part to a shop with EDM capability.
The best approach is prevention: use plenty of cutting fluid, back off frequently to clear chips, and don’t force a tap that’s binding.