Cutting ToolsCutting tool
Tap and Die Set
Taps cut an internal thread in a drilled hole; dies cut an external thread on a bar or a bolt. Together they make the threads this database dimensions.
What it does
- Cutting a thread in a drilled hole
- Repairing a damaged internal or external thread
- Making a stud from a length of bar
Materialswhat it is used on
All materialsWhat to look forthe judgement calls, before the spec sheet
- The tap drill comes first. Too small a hole breaks the tap; too large a hole produces a thread that strips under load. The drill size for each thread is the nominal diameter minus the pitch.
- Sets are sold as taper, second and plug taps. The taper tap starts the thread, the plug tap finishes it in a blind hole — using only the plug tap in a deep hole is how taps snap.
- For anything in stainless or high-tensile steel, a spiral-flute tap or a thread-forming tap behaves very differently from a straight hand tap. The material decides which.
Misuse and safetythe ways this tool causes rework
- Back the tap out a half turn for every full turn forward. The flutes fill, the torque rises, and the tap breaks in the hole — which usually means the part is scrap.
- Use cutting fluid on steel and cutting oil on aluminium. Tapping dry is the usual cause of a torn thread and a rough finish.
Projects that call for itand what it is doing on each one
All projectsFrequently asked questions
What size hole do I drill before tapping M6?
5 mm for the standard 1 mm pitch. The rule is nominal diameter minus pitch, and the exact tap drill for every thread here is listed in the thread tables.
Can I tap a hole in aluminium or plastic?
Yes, and it is easier than steel in both — but plastic needs a larger tap drill or a thread-forming tap, because the material recovers after the tap passes and closes on it.
Why did my tap break?
Almost always a hole that was too small, no lubrication, or chips left in the flutes because the tap was never backed out. All three raise the torque until the tap fails at its weakest section.