# BT30 interface

BT30 steep-taper geometry, retention knobs, dual-contact variants, and limits of interface compatibility.

Reviewed: 2026-11-02

Tags: tooling, toolholders, spindle-interfaces

## Identity and scope

Command's BT30 knobs **RB2M-0100** and **RS2M-0207-C** share an **M12×1.75** thread but differ in axial dimensions and coolant provision. TAC Rockford's reference drawing uses a 31.750 mm gauge diameter and 46 mm flange identify the nominal interface. Verify the machine's drawbar and coolant route against the exact knob drawing. [Command Tooling Systems, retention-knob dimensions, undated](https://catalog.commandtool.com/online-catalog/c-class-products/retention-knobs/for-other-domestic-and-imported-machines), [TAC Rockford, MAS 403-BT reference drawing 100.226, undated](https://tacrockford.com/pdf/100.226/100226-bt-tool-holder-specifications.pdf).

Falcon lists BT30 holders with ER collet, end-mill and face-mill fronts. The BT machine-side shank uses a 7/24 taper.

BT tooling is commonly described using MAS 403 and JIS B 6339 terminology. ISO 7388-2:2007 covers 7/24 automatic-tool-change shanks in forms J, JD and JF. ISO lists its second edition as August 2007 and records confirmation in 2023. The public scope is not a complete dimensional acceptance specification. [ISO, 7388-2:2007 overview](https://www.iso.org/standard/44099.html); [TAC Rockford, MAS 403-BT reference drawing 100.226, undated](https://tacrockford.com/pdf/100.226/100226-bt-tool-holder-specifications.pdf).

## Geometry and the meaning of size 30

TAC Rockford's BT30 reference row uses drawing-specific symbols.

| Feature | BT30 reference value |
| --- | --- |
| Gauge diameter, D1 | 31.750 mm |
| Flange diameter, D6 | 46 mm |
| Axial dimension L1 in the shank drawing | 48.4 mm |
| Retention-knob thread, D2 | M12 |

TAC marks the drawing uncontrolled and for reference only. L1 measures the shank rather than assembled cutting-edge length. [TAC Rockford, BT toolholder specifications 100.226](https://tacrockford.com/pdf/100.226/100226-bt-tool-holder-specifications.pdf).

## Retention knob and coolant path

A **retention knob**, or **pull stud**, connects the holder to the machine drawbar. It is a separate component with a finite service life. The thread connection to the holder and the gripped shape inside the spindle are different parts of that interface. [MariTool, retention knobs, undated](https://www.maritool.com/Retention-Knobs).

A shared thread does not establish complete compatibility. Command's catalog lists BT30 knobs RB2M-0100 and RS2M-0207-C with the same M12×1.75 thread, but different A and L1 dimensions and different through-coolant provision. These are distinct product geometries, not interchangeable options inferred from “BT30.” [Command Tooling Systems, retention-knob dimensions, undated](https://catalog.commandtool.com/online-catalog/c-class-products/retention-knobs/for-other-domestic-and-imported-machines).

Coolant routing is another independent feature. Falcon distinguishes central coolant delivery and flange coolant delivery within its BT family. A holder's coolant designation does not establish that the spindle, knob, seals and cutting tool provide the corresponding continuous flow path; that is an assembly-level requirement. [Falcon Toolings, BT forms and coolant routing](https://www.falcontoolings.com/bt-din-iso-7388-2-shank-detail.php).

## Conventional BT30 and BBT30

**BBT** is BIG DAISHOWA's designation for its BIG-PLUS BT interface. BIG-PLUS combines taper contact with flange-face contact. Conventional steep-taper support does not provide that same simultaneous seating. BIG DAISHOWA states that its corresponding holders can fit conventional spindles, but dual contact is not obtained there. [BIG DAISHOWA, *What is BBT and BDV?*, March 9, 2020](https://www.bigdaishowa.eu/en/news/detail/?news_id=13334).

| Spindle and matching-size holder | Contact described by BIG DAISHOWA |
| --- | --- |
| Conventional spindle + conventional holder | Taper contact |
| Conventional spindle + BIG-PLUS holder | Taper contact; no simultaneous face contact |
| BIG-PLUS spindle + conventional holder | Taper contact; no simultaneous face contact |
| BIG-PLUS spindle + BIG-PLUS holder | Taper and flange-face contact |

The table summarizes the manufacturer's compatibility explanation, with the appropriate retention knob and machine requirements still applicable. BIG DAISHOWA ties proper dual contact to controlled taper-to-face geometry and its gauging system. A generic “dual contact” description does not demonstrate the same dimensions or performance. Its comparative cutting demonstrations on other taper sizes do not establish a BT30 load rating. [BIG DAISHOWA, BIG-PLUS spindle system, undated](https://www.bigdaishowa.com/en/products/tool-holders/big-plus-dual-contact-spindle-system).

## Machine limits remain separate

Brother's S300Xd1/S500Xd1/S700Xd1 specification page illustrates the distinction: it names MAS-BT30 holders and MAS-P30T-2 pull studs, while listing BIG-PLUS as a spindle option. It also specifies a Brother-specific pull stud for through-spindle coolant. Tool mass depends on shape and center of gravity; the listed 4 kg capability requires a parameter change and excludes the 27,000 rev/min specification. These are model/configuration conditions, not universal BT30 limits or proof that an individual machine has those options. [Brother, S300Xd1/S500Xd1/S700Xd1 specifications and footnotes 1, 4 and 10, undated](https://www.brother.co.jp/product/machine/sxd1/spec/index.aspx).

## Worked example: interpreting 7/24

**Original ideal-geometry calculation.** A 7/24 taper changes diameter by 7 units over 24 units of axial length. Its half-angle is therefore arctan(7/48), approximately **8.297°**; the factor of two converts diameter change into radius change. This derives from the nominal taper ratio, not a tolerance or measured part. [ISO, 7/24 shank scope](https://www.iso.org/standard/44099.html).

For two hypothetical sections 20 mm apart along an ideal cone:

- Diameter difference = 20 × 7/24 = **5.833 mm**.
- Radius difference = 5.833/2 = **2.917 mm**.

This arithmetic explains why radial and diametral interpretations give different answers. It does not identify a seating datum, predict clamped axial position, define a face-contact gap or establish interchangeability between holders that share the taper angle.

## Coverage limits

Sources were inspected on November 2, 2026. The ISO edition and BIG DAISHOWA article dates are identified above; the other cited product pages and TAC drawing show no issue date. An undated current webpage is not a frozen catalog revision.

This article describes the machine-side BT30 interface. It does not supply full tolerances, drive-key and gripper details, installation torque, drawbar force, balance acceptance, maximum spindle speed or allowable cutting moment. Those depend on the exact spindle, holder, retention knob and assembled tool. BT30, CAT30, SK30 and HSK designations do not become synonyms merely because a size number or intended machining task is similar.

## Related reference

[Cutting tools and tooling references](../index.md)