Skills · skill
Tapping a Thread
Metalwork · Beginner
- Difficulty
- Beginner
- Safety
- Small taps snap under side load and the stub is very hard to remove; keep it square and never lean on it. Cutting fluid on hands and floor.
- Done When
- The bolt runs in by hand smoothly to the bottom with no wobble, and takes torque.
- Tools
- Tap set (taper, second, plug), tap wrench, correct tap drill, centre punch, cutting fluid, a square.
- Category
- Metalwork
- Summary
- Cutting an internal screw thread into a drilled hole with a tap — the skill behind almost every bolted joint.
Steps
Centre-punch, drill the tap-drill size square to the surface; countersink the mouth slightly.
Start the taper tap in the wrench, oil, and turn it in with light downward pressure, checking square from two sides.
Half a turn forward, quarter back to break the chip; feel for the click.
If it binds, back out and clear the chips — never force.
Follow with the plug tap for full threads to the bottom of a blind hole.
Blow out chips and test with the bolt.
The Science
The tap drill leaves the right amount of metal for the thread: for an M6 × 1 thread the hole is 5 mm — major diameter minus pitch. The tap cuts on its flutes and the chip has to go somewhere; that is why you back off every half turn to break it. Steel is threaded to about 60–75 % engagement; more only makes tapping harder, not the joint stronger.
How It Fails
A snapped tap in the hole. Torn threads from no lubricant or dull tap. Threads that lean because the start was not square. A stripped hole from a drill too big.
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