Taps | KEVOS

§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.

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§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.

tap drill d ≈ D − P % thread ≈ 76.98 × D − dP — D major dia, P pitch (mm)

Example 1 — the standard metric tap drills

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.

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§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.

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§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.

The three chamfer lengths
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.

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§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.

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§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.

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§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

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Handbook application: from concept to controlled practice

Purpose. This expanded section turns the original page into a practical handbook. It preserves the supplied material and adds a repeatable way to apply, check and review Taps. It does not replace a contract, legislation, a controlled standard, competent engineering judgement or specialist advice.

The operating aim is to carry the subject from function and assumptions through design evidence, verification and controlled release. Read the original explanation first, then use the workflow and checks below to convert knowledge into evidence.

Apply Taps by beginning with the duty, not the component or software command. Convert the key ideas—taps, cutting, thread, percentage, forming—into measurable requirements and interfaces. Record operating and non-operating environments, duty cycle, expected life, loads, energy sources, human interaction and reasonably foreseeable abnormal conditions. When a value is not a project requirement or verified supplier datum, identify it as an assumption or illustrative value.

Create a calculation and evidence trail that another competent person can audit. Every input should carry a source, unit, revision and uncertainty or tolerance where relevant. Every model should state its boundary conditions and limitations. Keep nominal capacity separate from design capacity, and keep verification margin separate from an arbitrary safety factor. If a code or standard governs the work, confirm the applicable edition and contractual status rather than copying a number from a secondary summary.

Design for manufacture, assembly, inspection, operation and maintenance at the same time. A technically valid geometry can still fail because it cannot be fixtured, measured, cleaned, guarded, reached or replaced. Review process capability, datum or reference strategy, tolerance accumulation, access, error-proofing and changeover. Where people interact with plant, apply the hierarchy of controls and consult those who will operate, clean, maintain and recover the equipment.

Plan verification before release. Define the characteristic, method, equipment, sample or test condition, acceptance criterion, record and responsible person. Validation then asks a different question: whether the resulting system is effective and suitable in the intended use context. A passed drawing check or analysis does not by itself validate usability, maintainability or production performance.

Step-by-step operating method

Define the duty. Capture the required function, interfaces, operating environment, life, loads and unacceptable outcomes.
Establish the model. Identify governing principles, units, material or process data, assumptions and uncertainty.
Develop alternatives. Compare feasible concepts against performance, manufacturability, safety, maintainability and cost.
Verify the design. Use analysis, test, inspection or demonstration with acceptance criteria defined before execution.
Release and learn. Baseline the design, control changes, retain evidence and feed operating results into the next revision.

Illustrative design review record

Illustrative values only. Build a one-page record with the required function, input sources, assumptions, governing load or process condition, failure consequences, selected concept, verification method and acceptance criterion. Mark every numerical input as project requirement, verified supplier data, measured value, calculation output or assumption. Review the weakest evidence first. If an assumption can change safety, compliance, interchangeability or capacity, it must be resolved before release rather than buried in a calculation note.

Evidence class	Question	Release expectation
Requirement	What must the design do and under which conditions?	Approved and traceable
Input	Where did the load, property, tolerance or process limit come from?	Source, unit and revision recorded
Analysis	Which model and assumptions connect input to result?	Checkable calculation or simulation
Verification	How will conformity be demonstrated?	Method and acceptance criterion agreed
Validation	Will the solution work for intended users and conditions?	Representative use evidence

Common failure modes and recovery actions

1. Watch for

Starting detailed design before interfaces and operating limits are agreed.

Recovery: Return to the governing definition or requirement and restate the decision in one sentence.

2. Watch for

Using catalogue or typical values as though they were certified project inputs.

Recovery: Separate evidence from assumption, assign an owner and set a date for validation.

3. Watch for

Checking nominal performance while ignoring tolerances, degradation and foreseeable misuse.

Recovery: Run a small counterexample, boundary test, pilot or independent check before proceeding.

4. Watch for

Confusing verification of requirements with validation of user need.

Recovery: Record the consequence, decision and rationale, then update the controlled baseline.

5. Watch for

Releasing drawings or procedures without configuration, inspection and change controls.

Recovery: Escalate when the issue affects safety, compliance, acceptance, material value or an agreed tolerance.