What Is Tapping in Machining? | Types, Tools & Process

Tapping in machining cuts internal threads into a pre-drilled hole so screws and bolts can fasten securely into metal or machined plastic parts.

A drilled hole is just a hole until it gets threads — and without threads, screws spin uselessly in place. Tapping in machining is the process that cuts or forms those threads into the walls of a pre-drilled hole, turning a blank opening into a precise fastener point. Whether you work on a manual mill or a CNC machine, the same principle applies: the tap rotates into the hole and either removes material or displaces it to create a thread profile that matches a screw or bolt. This guide covers the two main methods — cut tapping and form tapping — the types of taps available, the step-by-step manual process, and the speeds and feeds that keep taps from breaking.

Tapping in Machining: Definition and Key Differences From Drilling

Tapping and drilling serve different purposes, though they often happen in sequence. Drilling creates a smooth cylindrical hole of a specific diameter. Tapping then cuts a helical ridge — the thread — into that hole’s inner surface. The two operations use different tools and different cutting parameters. A drilled hole without threads can only accept a screw if you add a nut on the other side; a tapped hole lets the screw thread directly into the workpiece, making it a self-contained fastener point.

Cut Tapping vs. Form Tapping: Two Ways To Create Threads

Cut tapping uses a tap with sharp cutting edges that slice material away, producing chips (swarf) that must be evacuated. It works on most metals and machinable plastics and is the more common method.

Form tapping, also called roll tapping or extrusion tapping, uses plastic deformation to displace material into the thread shape without cutting. It produces no chips and creates stronger threads because the material grain structure is compressed rather than cut. Sandvik Coromant notes that form tapping works best on softer, ductile materials like aluminum, brass, and mild steel.

How Do You Tap A Hole By Hand?

Manual tapping follows a straightforward sequence that rewards patience and good setup. Xometry’s documentation provides the standard procedure.

1. Mount the workpiece securely in a vise or work holder so it cannot shift.
2. Select the right tap for your thread specification and hole type. A taper or plug tap starts the thread; a bottoming tap finishes a blind hole.
3. Mount the tap in a tap wrench, making sure it is clamped straight and firmly.
4. Apply cutting compound to the tap. Lubrication reduces friction and extends tool life.
5. Align and engage the tap taper with the hole. Apply gentle axial pressure while rotating the wrench clockwise until the cutting edges catch.
6. Cut and break chips by advancing a half turn, then reversing a quarter turn. This fractures the chips so they clear the flutes instead of jamming.
7. Complete the first pass with the starter tap. Switch to a bottoming tap if the hole is blind and you need threads close to the bottom.
8. Remove and clean. Back the tap out fully, then clear all swarf from the hole and the tap. On the final pass the tap turns smoothly without binding — that is your success cue.

For production runs, an automatic tapping machine handles this sequence much faster and with consistent depth control, eliminating the guesswork of manual feed.

How Does CNC Tapping Differ From Manual Tapping?

CNC tapping replaces the manual wrench with a programmed spindle and Z-axis. The critical difference is synchronization. In rigid tapping, the spindle rotation and Z-axis feed are locked together so the tap advances exactly one thread pitch per revolution. This requires the feed rate to match the tap’s pitch precisely and is the standard on modern CNC machines.

In flexible tapping, used on older equipment or when synchronization is uncertain, the tool holder includes a compensating mechanism. The feed rate is set slightly slower than the pitch — between 0.95 and 0.98 pitches per revolution — and the holder absorbs the mismatch. Forcing a rigid tap without proper synchronization is a common cause of breakage, as noted by Haas Automation. Sandvik Coromant’s tapping guide covers the full equipment compatibility details.

Common Tapping Mistakes And How To Avoid Them

• Misalignment — Starting the tap off-center produces weak, angled threads prone to cross-threading. Use a tap guide or center the tap carefully in the hole.
• Undersized pilot hole — Drilling too small forces the tap to cut more material than designed, which can snap the tool. Always check the recommended drill size for your tap specification.
• Too much pressure — After the first 3–4 threads have engaged, the tap self-feeds. Applying continued downward pressure at that point distorts the thread form and can break the tap.
• Chip accumulation — Not backing out periodically (every half turn) lets chips pack into the flutes, increasing torque and risking breakage. Blind holes demand even more frequent clearing.
• Wrong tap for the hole type — Using a straight flute tap in a blind hole traps chips at the bottom, usually snapping the tap when it hits the packed debris. Use a spiral flute tap for blind holes.

Cutting Speeds, Feeds, And The “10 Rule”

The right cutting speed and feed rate prevent tap breakage and produce clean threads. Speeds vary significantly by material.

For rigid tapping, the feed rate is always one pitch per revolution — this is not adjustable. For flexible tapping, set the feed between 0.95 and 0.98 pitches per revolution and let the holder compensate. The drill size for a cutting tap is the major diameter minus the pitch; for a forming tap, it is the major diameter minus half the pitch.