§1Cutting a thread from inside
A tap is a hardened screw with flutes cut along it, turning its thread into cutting edges. Screwed into a plain drilled hole, it cuts a matching internal thread as it advances.
The flutes do two jobs: they interrupt the tap’s thread to form cutting edges, and they give the chips somewhere to go. As the tap turns in, each edge shaves the hole wall a little deeper until a full thread is formed to match a bolt or screw. Because the tap is guided by the thread it is cutting, it pulls itself in at its own pitch — one turn advances it exactly one pitch — so it cannot be forced or held back without stripping the thread or snapping. That self-feeding, self-guiding action makes tapping simple in principle, but it also means the tap is entirely at the mercy of the hole size beneath it, which is where this page begins in earnest.
Contents§2Tap drill and percentage thread
The hole must be drilled to the thread’s minor diameter, not its major — the tap only cuts the thread depth. The tap-drill size is found by a simple subtraction, and it sets how much thread is formed.
For M10 × 1.5, the tap drill is D − P = 10 − 1.5 = 8.5 mm, which by the shop formula gives 76.98 × (10 − 8.5)/1.5 = 77 % thread. For M6 × 1.0 it is 6 − 1 = 5.0 mm (again 77 %), and for M8 × 1.25 it is 6.75 mm. The “major minus pitch” rule targets about 77 % thread engagement, and that is deliberate: 100 % thread is barely stronger than 75 % but takes far more torque to cut and breaks many more taps. Aiming for roughly 75–80 % gives nearly full strength for a fraction of the tapping effort — which is why the tap-drill charts settle there, and why over-tight holes, not weak threads, cause most tap breakage.
§3Cutting and forming taps
Threads can be made in two ways: by cutting the metal away, or by pushing it into shape. Taps come in both kinds.
A cutting tap shears the thread and produces chips — the traditional, general-purpose tap, used in most materials and essential where chips must be removed to form the thread. A forming (roll) tap has no flutes and no cutting edges; it is lobed, and it displaces the metal, pressing the thread into the hole wall much as thread rolling forms an external thread. Forming taps make no chips (an advantage in blind holes and where swarf is unwelcome), and the cold-worked thread they leave is stronger and smoother — but they need a slightly larger hole, more torque, and a ductile material that will flow rather than tear. Cutting taps suit almost anything; forming taps excel in soft, ductile metals such as aluminium and mild steel where their chipless, stronger thread is worth the extra torque.
Contents§4Taper, plug and bottoming
A hand tap is ground with a chamfered lead of a few threads, and the length of that chamfer defines three tap styles for different depths.
| Tap | Lead chamfer | Use |
|---|---|---|
| Taper | ≈ 8–10 threads | starting a thread; easiest entry, guides straight |
| Plug | ≈ 3–5 threads | general purpose; the usual first choice |
| Bottoming | ≈ 1–2 threads | threading to the bottom of a blind hole |
| The chamfer spreads the cutting over several threads, so a long taper lead cuts gently and starts squarely but cannot reach the bottom of a blind hole, while a bottoming tap reaches almost to the bottom but cuts hard because only a thread or two share the work. The usual sequence for a deep blind hole is to start and cut with a plug (or taper) tap, then finish the last threads with a bottoming tap. | ||
§5Flutes: through and blind holes
The flute form decides which way the chips go, and that choice is governed entirely by whether the hole goes through or is blind.
A straight-flute tap is the general default, but its chips are not actively cleared, so they can pack in a deep or blind hole. A spiral-point (“gun”) tap has an angled point that throws the chips forward, ahead of the tap — ideal for through holes, where the chips are pushed out the far side and the flutes stay clear. A spiral-flute tap has helical flutes like a drill that pull the chips back out of the hole toward the operator — ideal for blind holes, where there is nowhere for chips to go but back up. Matching flute to hole is essential: a gun tap in a blind hole packs chips at the bottom and snaps, while a spiral-flute tap in a through hole works but needlessly. Through hole → spiral point; blind hole → spiral flute.
Contents§6Speed, fluid and breakage
Tapping is slow, well lubricated and easily broken — the tap is fragile, buried in the work, and expensive to extract when it snaps.
Cutting speed for tapping is low, roughly a quarter of the drilling speed for the material, because the tap must cut a full thread on every edge at once and has little room for error. Plenty of tapping fluid is essential — it reduces the torque, improves the thread finish and flushes chips — and in blind holes the tap should be backed off periodically to break and clear the chips before they pack. Breakage almost always traces to one of a few causes: a hole drilled too small (too high a % thread), chips packing in a blind hole, a tap started crooked so it binds, or forcing a dull tap. A snapped tap, being harder than the work, cannot simply be drilled out, so prevention — right hole size, right flute, ample fluid, a square start and a sharp tap — is far cheaper than cure. This is the same forming-versus-cutting and thread-geometry logic that the Threads and Threading section develops in full.
Contents§7Quick reference
The working core of the page on one card rack.
Tap drill
d ≈ D − P
M10×1.5 → 8.5 mm
% thread
76.98 (D−d)/P
aim ~75–80 %
Two kinds
cutting (chips) · forming (chipless)
Chamfer
taper · plug · bottoming
Flutes
through → spiral point
blind → spiral flute