# Spiral-flute taps

Chip evacuation in blind-hole tapping, distinctions from spiral-point tools, material-specific geometry and a bounded thread-depth calculation.

Reviewed: 2026-11-02

Tags: tooling, tapping, threads, blind-holes

An example **1.25 mm-pitch** blind thread needs **15.00 mm full profile**. With **2.5 pitches** from tap tip to full profile, tip depth is **18.125 mm**. Adding a **1.00 mm reserve** gives **19.125 mm cylindrical depth**. A **6.80 mm, 118° drill** adds **2.043 mm** to the point for **21.168 mm** total. Verify actual tip geometry, chip space and reversal.

## Function and tool identity

Helical flutes carry chips back toward the shank. OSG describes the benefit in blind holes and the risk of chips trapped against the new thread. [OSG, Chip shape during tapping, published September 5, 2025; updated December 24, 2025](https://faq.osg.co.jp/faq/show/18102?category_id=636&return_path=%2Fcategory%2Fshow%2F636%3Fpage%3D1%26site_domain%3Den%26sort%3Dsort_access%26sort_order%3Ddesc&site_domain=en)

OSG's product overview distinguishes **spiral-flute taps** for blind holes and coiling-chip materials from **spiral-point taps** for through holes. It separately lists hand taps and forming taps. These are different tool categories; a name containing “spiral” does not establish identical geometry or chip handling. A spiral-flute cutting tap is also distinct from a form tap, which generates the thread by plastic displacement. [OSG, Taps overview](https://www.osg.co.jp/en/products/tap/index.html).

| Term | Geometric or functional meaning |
| --- | --- |
| Flute helix | Orientation of the chip-carrying channel |
| Thread helix and lead | The thread's axial advance per turn |
| Cutting hand | Required cutting rotation for the specified tool |
| Lead/chamfer section | Entrance region that progressively generates the full profile |
| Full-thread depth | Length of the required complete thread, distinct from deepest tool travel |

The table is explanatory terminology. Flute helix, thread hand and lead length describe different features; one angle or a photograph does not specify all three.

## Chip shape, material and geometry

OSG associates narrow-curl, fine-pitch chips with easier evacuation and thick, long chips with entrapment. Its FAQ calls tapping beyond twice tool diameter deep-hole tapping and links depth to evacuation difficulty. This classification is not a universal maximum, and its illustrated speeds belong to named test conditions. [OSG, chip-shape discussion](https://faq.osg.co.jp/faq/show/18102?category_id=636&return_path=%2Fcategory%2Fshow%2F636%3Fpage%3D1%26site_domain%3Den%26sort%3Dsort_access%26sort_order%3Ddesc&site_domain=en).

A bounded product example is OSG's **A-SFT-SUS**, described for stainless-steel applications where tough, ductile chips entangle. The page identifies stainless-oriented flute geometry, a shank length intended to suppress entanglement and VI coating; its named size lineup is **M3–M12**. These features belong to this product family. A shared nominal thread size does not make every spiral-flute tap equivalent in stainless steel, and advertised chip-control improvements do not establish freedom from entanglement in every setup. [OSG, A-SFT-SUS](https://osg.co.jp/en/products/tap/spec/a-sft-sus.html).

Explanatory synthesis: flute space and the remaining tool core are competing geometric considerations. Lubrication must reach sliding and cutting contacts, while chips need a return path around the engaged tool. The presence of coolant, an appropriate coating or a helical channel alone cannot prove that the complete arrangement provides both.

## Prepared hole and synchronized motion

OSG's Japanese-language SFT guidance relates pre-hole size, cutting conditions and feed control to thread accuracy. It explains that larger holes within the permitted range reduce chip volume, while incorrect spindle/feed synchronization can over-advance the freely cutting tap and enlarge the thread. This is a process relationship, not an instruction to enlarge holes beyond a thread's limits. [OSG, SFT tapping points, published May 10, 2021; updated May 13, 2026](https://faq.osg.co.jp/faq/show/9050?site_domain=default).

A pre-hole diameter, final thread minor diameter, pitch diameter and nominal thread diameter are different quantities. Likewise, machine motion must follow thread lead, rather than flute count. For a single-start thread, lead equals pitch; a multi-start thread has a larger lead. These are explanatory geometric relationships. A holder's compliance and the machine's reversal behavior affect the real motion, so nominal programmed feed does not establish absence of axial loading or profile error.

## Worked example: full threads, tool tip and drill point

Use the hypothetical single-start thread, **2.5-pitch** tip-to-profile distance and **1.00 mm axial reserve** from the same entry plane.

```text
Tip-to-full-profile distance = 2.5 × 1.25 = 3.125 mm
Deepest tip position = 15.00 + 3.125 = 18.125 mm
Required ideal cylindrical pre-hole depth = 18.125 + 1.00
                                        = 19.125 mm
```

The 1.00 mm reserve is an arbitrary model input, not a validated clearance for chips or reversal. Any additional non-cutting nose, tool tolerance or dynamic travel would change the result.

Now assume that pre-hole has **6.80 mm diameter**, produced by an ideal drill with **118° included point angle**. The point extends below the cylindrical portion by:

```text
Point extension = (6.80/2) / tan(118°/2) = 2.043 mm
Depth to theoretical drill point = 19.125 + 2.043
                                 = 21.168 mm
```

A depth to the point of 19.125 mm would leave less cylindrical clearance. Check the actual drill point, nose and bore size.

For an unrelated kinematic illustration at **400 rpm**, the single-start 1.25 mm lead requires `400 × 1.25 = 500 mm/min` axial feed. A second flute or third flute does not multiply that value. This is thread synchronization arithmetic, not a cutting-speed choice or complete cycle-time estimate.

## Finished-thread and source limits

Explanatory synthesis: an intact tap and a completed cycle do not prove full thread depth, pitch diameter, minor diameter, surface condition or absence of trapped chips. A blind hole's deepest conical point is not usable cylindrical space, and a nominal thread length is not identical to total tool travel. Thread acceptance and assembly strength depend on the actual specified geometry and application.

The two FAQ pages display the publication and update dates cited above. The live OSG overview and A-SFT-SUS page reviewed on November 2, 2026 PST display no separately verified technical issue dates. This reference adopts no universal flute angle, tap-drill size, lead length, coolant recipe, speed, torque limit or depth rating.

## Related reference

[Form taps](form-taps.md), [center drills](center-drills.md), and [Cutting tools and tooling references](../index.md).