# Tapped holes

Internal thread requirements: callouts, cutting versus forming, pilot geometry, usable depth and inspection.

Reviewed: 2026-11-01

Tags: parts-and-design, threads, tapping, holes

Specify the internal thread and full usable engagement separately from pilot-hole depth. Cutting taps remove chips. Forming taps displace suitable material. [Sandvik tap selection](https://www.sandvik.coromant.com/en-gb/knowledge/threading/tapping/how-to-choose-tap-type)

**Original depth-budget illustration:** assume complete usable thread must run from 1 to 13 mm below the entry face, giving 12 mm usable length. Assume the chosen process separately requires a further 3 mm axial region before the start of the drill point. Full-diameter pilot depth is then `13 + 3 = 16 mm`, not 12 mm. The deepest drill-point position lies farther still and depends on its geometry. These assumed allowances are not tap recommendations and do not establish a safe remaining wall.

A drawing's functional requirement may instead be an internal thread without prescribing tapping. Thread milling is another manufacturing route: Protolabs' cited threading service uses CNC thread mills. Do not describe that service as evidence of tap travel, tap lead or forming-tap capability. Preserve a required manufacturing route when it matters; otherwise confirm that the proposed process meets the finished thread requirements. [Protolabs threading process](https://www.protolabs.com/en-gb/services/cnc-machining/threaded-holes-guidelines/)

## Carry a complete thread requirement

A hole diameter alone does not specify a thread. Identify the thread system, nominal size and pitch, tolerance class, handedness where required, location/orientation, and complete usable thread depth or through-thread requirement. Keep pilot dimensions and entry chamfers distinct from the finished thread designation.

For ISO general-purpose metric threads, **ISO 965-1:2026** specifies the tolerance system for M threads conforming to ISO 261, with profiles according to ISO 68-1. Its publisher identifies this as edition 5, April 2026, replacing the 2013 edition and its 2021 amendment. State the contractual edition rather than assuming an older handbook is current. [ISO 965-1:2026 scope and history](https://www.iso.org/standard/87889.html)

For example, an illustrative **M6×1–6H** internal-thread requirement identifies a metric nominal size, 1 mm pitch and an internal tolerance class. It does not specify a 6 mm pilot hole, 6 mm usable engagement or a universal tapping tool. Vermont Gage distinguishes the finished product thread class from tool limits and explains that a 6H internal product thread is inspected with the corresponding 6H working thread-plug gauges. [Vermont Gage thread classes](https://www.vermontgage.com/support/detail/thread-classes-and-setting-plug-gages-quality-magazine)

| Requirement | What to resolve |
|---|---|
| Thread | System, size, pitch, class and direction |
| Extent | Through or blind, complete usable thread depth and reference face |
| Pilot and entry | Process-specific pilot limits, chamfer and remaining wall |
| Material and finish | Grade/condition, coating state and any insert requirement |
| Inspection | Applicable gauge, depth method, geometry and acceptance stage |

## Select the tap and pilot together

Choose the tap for the hole type and material, not only its nominal thread size. Sandvik distinguishes straight-flute, spiral-point, spiral-flute and forming tools and relates selection to blind versus through holes. Its chamfer discussion explains that the lead distributes cutting over several teeth: a shorter lead may work closer to the bottom but has different cutting conditions. “Bottoming” does not mean zero bottom allowance. [Sandvik tap types and chamfers](https://www.sandvik.coromant.com/en-gb/knowledge/threading/tapping/how-to-choose-tap-type)

Use the selected toolmaker's pilot diameter and tolerance for the actual process and material. A cutting-tap drill chart is not automatically suitable for forming. Avoid treating nominal thread size minus pitch as a complete manufacturing instruction; the accepted minor diameter, tool behavior and required thread form still need verification. Forming's lack of cutting chips also does not remove the need for lubrication, preparation or final inspection. [Sandvik forming prerequisites](https://www.sandvik.coromant.com/en-gb/knowledge/threading/tapping/how-to-choose-tap-type)

The required hole is the **produced** pilot, not merely the drill's marked diameter. Assess the actual bore before threading when its size or form is critical. Confirm the effects of prior operations, material condition and coating on the final accepted thread rather than selecting a tool solely by a matching label.

## Separate thread depth, tap travel and drill depth

A blind feature can contain an entry chamfer, complete threads, incomplete threads made by the tap lead, additional clearance and the pilot drill's point. These axial regions serve different purposes. Specify minimum complete thread depth from a clear datum and ensure the manufacturing plan provides the necessary space beyond it.

Protolabs explicitly shows a blind-hole pilot point as separate from the region its thread mill can thread, and reports the offered thread depth on its quotation. This reinforces the depth distinction, but the service's numerical limits belong to its thread-milling process. [Protolabs blind-hole and depth examples](https://www.protolabs.com/en-gb/services/cnc-machining/threaded-holes-guidelines/)

Do not derive fastener strength from usable length alone. Required engagement depends on the mating fastener, materials, load case, thread geometry and joint design. Check fastener penetration and clamped stack so that the screw seats as intended without bottoming first. Inserts or other reinforcement require their own preparation and acceptance requirements.

## Feed synchronization and chip control

A tap follows the thread helix. For an ideal single-start thread of pitch P, axial advance per revolution is P, so synchronized feed is `F = nP`. **Illustrative arithmetic:** for P = 1 mm and an assumed 500 rev/min, the corresponding advance is 500 mm/min. This is a kinematic relationship, not a recommended speed or machine program.

Actual implementation depends on the machine, holder and tapping method. Sandvik's operation page gives a different feed recommendation specifically for floating tapping attachments; do not transplant it into a rigid-tapping cycle. Confirm the control's feed mode, spindle synchronization and reversal behavior using the relevant machine instructions. [Sandvik tapping-operation scope](https://www.sandvik.coromant.com/en-gb/knowledge/threading/tapping/operation-tips)

Sandvik also identifies coolant supply as affecting chip evacuation, thread quality and tool life. Plan evacuation for the chosen tap and hole, especially when chips cannot exit through the far end. Use material/tool-specific fluid recommendations and define cleaning before gauging or assembly. This page establishes no universal speed, fluid concentration or depth-to-diameter limit. [Sandvik coolant guidance](https://www.sandvik.coromant.com/en-gb/knowledge/threading/tapping/operation-tips)

## Inspection and finished condition

Select gauges for the finished thread standard, class and processing state. Vermont Gage explains that a GO thread plug is a cumulative functional check excluding minor diameter; it does not measure the internal thread's major and minor diameters. Its guidance calls for separate internal minor-diameter inspection. Apply the actual governing gauge rules rather than inventing a universal NO-GO turn limit. [Vermont Gage internal-thread inspection](https://www.vermontgage.com/support/detail/internal-part-thread-inspection-quality-magazine)

Thread acceptance also does not certify location, perpendicularity, complete thread depth, remaining wall or cleanliness. Define those checks separately. Preserve the drawing revision, material/condition, final coating state, tool/process selection and inspection record. A screw that can be installed is not proof of every specified thread or joint requirement.

## Source versions and related reference

Reviewed 2026-11-01, America/Los_Angeles. ISO's record identifies **ISO 965-1:2026**, edition 5, April 2026. The cited Sandvik, Protolabs and Vermont Gage live technical pages have no independently verified technical revision dates; historical standard references in the inspection explanation are not presented as current gauge-standard editions. Original arithmetic above uses explicit assumptions and is not supplier cutting data.

[Part features](index.md) · [Blind holes](blind-holes.md)