ISO, BT and CAT interfaces: the 7/24 taper and its regional variants
One shared principle born in 1927, three incompatible regional standards, and a fourth for conventional machines. Here is how to find your way, with an equivalence table.
A shared origin, three incompatible regional standards
TLDR: In the 1960s the 7/24 taper gained gripper grooves and a retention knob for automatic tool change. Three standards with incompatible dimensions followed: ISO/DIN in Europe, BT in Asia, CAT in North America.
The principle is the same everywhere: a 7/24 ratio taper, purely conical contact, a retention knob for the automatic changer. But flange dimensions and retention knob threads differ from region to region, which makes toolholders non-interchangeable despite an identical nominal diameter.
It is the most frequent source of error in the field: ordering a BT 40 for an ISO 40 spindle because "it's a 40". The taper goes in, but gripping and clamping do not work properly.
SA and DIN 2080: the conventional machine standard
TLDR: The SA standard (DIN 2080, equivalent to ISO 2583) equips conventional manual-load machines, without gripper grooves for an automatic retention knob.
An SA taper does not necessarily carry the gripper grooves an automatic retention knob requires. On a conventional SA 40 machine, fitting a BT 40 toolholder is generally possible, provided the retention knob length is compatible. Fitting an ISO 40 also remains possible in most cases, but may require reworking a notch: some SA configurations have a shallower notch than the full ISO standard.
BT and BBT: the Japanese line and its dual-contact evolution
TLDR: BT — "Bottle Grip Taper" — is the Japanese version of the 7/24 taper, in BT30, BT40 and BT50. Its BBT variant adds face contact, comparable in principle to HSK, without changing the taper geometry.
The BT standard is codified by MAS 403 (or JIS B6339) and comes in sizes by taper diameter: BT30, BT40 and BT50 are the most widespread.
To answer the same stiffness and speed limits as the conventional 7/24 taper, BIG-PLUS technology was developed in Japan by BIG Daishowa in the 1990s. It produced the BBT standard (Big Plus BT), whose American equivalent is designated BCB (Big Plus CAT). The evolution keeps taper and grooves identical to the conventional standard but adds face contact through tighter tolerances — the same benefit, in principle, that HSK brings on an entirely different taper standard.
Practical equivalence table
| Nominal diameter | Europe | Japan | North America | Conventional machine |
|---|---|---|---|---|
| 40 | ISO 40 (DIN 69871) | BT 40 (MAS 403) | CAT 40 (ANSI) | SA 40 (DIN 2080) |
| 50 | ISO 50 (DIN 69871) | BT 50 (MAS 403) | CAT 50 (ANSI) | SA 50 (DIN 2080) |
This nominal diameter equivalence does not guarantee interchangeability: a precise check of flange and retention knob dimensions remains essential before fitting across standards.
Frequently asked questions
Is a CAT 40 toolholder compatible with an ISO 40 spindle?
Generally not, without an adapter. Despite an identical nominal diameter, flange geometries differ between regional standards.
What is the difference between BT and BBT?
BBT, for Big Plus BT, adds face contact to the conical contact of standard BT, which raises stiffness. The taper geometry and gripper grooves stay identical.
Can I fit an ISO toolholder on a conventional SA machine?
Often yes, but it may require reworking a notch, depending on the exact configuration of your machine’s SA taper.
How can I identify my spindle standard if the documentation is lost?
A precise dimensional check of the taper and flange identifies the exact standard, even with no original documentation. It is routine work for us.
Do you rework a marked or worn spindle taper?
Yes. Reworking the taper seat is part of our overhaul operations — a marked taper degrades accuracy and damages new toolholders.
Unsure which standard your spindle uses?
We identify the exact standard by dimensional check, even without original documentation, and rework marked taper seats. It is often the first step in diagnosing a loss of accuracy.
Our teams are available from our sites in Beaurepaire (France) and Lussery-Villars (Switzerland).
Request a quote