Tool holder tapers: BT, CAT, SK and NT compared

BT40 and CAT40 have the same taper angle and the same gauge diameter. They are still not interchangeable — and the reason is the part most people never look at.

The key idea: all of these are 7:24 taper systems (3.5 in/ft, roughly 8.3° per side). The taper will accept any holder of the same size. What differs is the flange geometry, the drive keys and the retention thread — and those are what stop you mixing them.

Taper sizes and gauge diameters

The number in the designation does not directly give a diameter, which surprises everyone once. The gauge diameter is standardised per size:

7:24 taper gauge-line diameters, agreed across holder manufacturers and taper references.
Taper size	Gauge diameter (mm)	Gauge diameter (in)	Typical machines
30	31.75	1.250	Small VMCs, drill-tap centres, Robodrill class
35	38.10	1.500	Less common intermediate size
40	44.45	1.750	The default VMC size worldwide
45	57.15	2.250	Heavier 40-class machines
50	69.85	2.750	Large VMCs and horizontal machining centres
60	107.95	4.250	Heavy horizontal boring mills

BT — MAS 403 / JIS B 6339

The Japanese standard, and the one you'll see most often on Asian machines. Symmetric flange, metric retention knob.

Size	Gauge Ø (mm)	Flange Ø (mm)	Shank length (mm)	Retention thread
BT30	31.75	46.0	48.4	`M12`
BT35	38.10	53.0	56.4	`M12`
BT40	44.45	63.0	65.4	`M16`
BT45	57.15	85.0	82.8	`M20`
BT50	69.85	100.0	101.8	`M24`
BT60	107.95	155.0	161.8	`M30`

CAT / V-flange — ASME B5.50

The American standard. Same taper and gauge diameters as BT, imperial retention thread, and a different drive-slot layout.

Size	Gauge Ø (mm)	Flange Ø (mm)	Shank length (mm)	Retention thread
CAT30	31.75	46.02	47.63	`M12`
CAT40	44.45	63.55	68.25	`M16`
CAT45	57.15	82.55	82.70	`M20`
CAT50	69.85	98.42	101.75	`M24`

Retention knob threads are listed here as the metric sizes used on the knob itself; the drawbar thread on a CAT40 spindle is commonly 5/8"-11 UNC. Confirm which thread you are actually measuring before ordering.

SK / ISO — DIN 69871 & ISO 7388

The German standard, frequently written SK40, ISO40 or DIN 69871 interchangeably in catalogues. Same 7:24 taper and the same gauge diameters again; the flange form and retention-knob standard (DIN 69872) differ from both BT and CAT. If you are buying for a European machine, this is usually the one you need.

NT — JIS national taper (DIN 2080 pattern)

The older, simpler pattern: asymmetric, key-driven, and intended for manual tool change — there is no ATC-friendly flange because there's no automatic changer involved. Common on conventional milling machines and on older Japanese equipment.

Size	Gauge Ø (mm)	Overall length (mm)	Drawbar thread	Whitworth alternative
NT30	31.75	70	`M12`	W 1/2
NT40	44.45	95	`M16`	W 5/8
NT50	69.85	130	`M24`	W 1
NT60	107.95	210	`M30`	W 1¼

The Whitworth thread column is not decoration. NT tooling is listed with either a metric or a Whitworth drawbar thread depending on the maker and the machine's vintage. Sizes 30 through 60 are all affected. If an NT holder won't thread onto your drawbar, check whether you're holding a Whitworth part before concluding anything is wrong.

Identifying your spindle in two minutes

Read the drawbar thread. On a #40 machine, an M16 × 2.0 thread points to BT-40; a 5/8"-11 UNC thread points to CAT-40. This is the fastest test.
Count and measure the drive slots. Two slots about 16.3 mm wide, 180° apart = CAT-40. Four slots about 10 mm wide, 90° apart = BT-40. No slots on the flange, just a keyway in the bore = an NT-style tang-driven spindle.
Measure the gauge diameter at the mouth of the taper to confirm the size class: 44.45 mm is a 40 taper, 69.85 mm is a 50.

Used machines occasionally turn up with a mix — a BT-style drive key pattern combined with an imperial drawbar. Trust the thread you measure, not the badge on the machine.

FMB face mill arbors

FMB is not a taper — it's the arbor that carries a face mill cutter, and it's specified by the pilot bore in millimetres. The number after FMB is that bore: 16, 22, 27, 32 or 40.

Published dimensions for SK40 and NT-series FMB arbors. `d` = pilot bore, `D` = cutter seating face, `L` = arbor length, `M` = locking screw.
Model	d (mm)	D (mm)	L (mm)	Locking screw
SK40-FMB16	16	38	45	`M8`
SK40-FMB22	22	48	45	`M10`
SK40-FMB27	27	58	50	`M12`
SK40-FMB32	32	78	55	`M16`
SK40-FMB40	40	88	55	`M20`
NT40-FMB40	40	88	30	`M20`
NT50-FMB32	32	78	40	`M16`
NT50-FMB40	40	88	40	`M20`

The pilot bore has to match the cutter's mounting hole exactly. An arbor that is one size out will either not clamp or will leave the cutter off-centre, which shows up immediately as vibration and a poor finish.

Extension holders and extension rods

Extension holders and collet-chuck extensions exist in all of these shank forms and solve one problem: reaching into a deep pocket or cavity that the standard holder can't get to.

Rigidity drops fast with overhang. Every extra millimetre of extension costs you stiffness, and the practical result is chatter long before it's a measurable accuracy problem.
Use the shortest extension that reaches. Buying one long "just in case" extension and using it for everything is the usual mistake.
An extension plus a long collet is worse than either alone. If you need reach, prefer a slim extended-nose collet chuck over stacking an extension rod onto a standard holder.

We deliberately don't publish deflection figures here — they depend entirely on the holder's own geometry and material. The guidance above is qualitative and safe; a specific number without that context would be misleading.

Not covered: HSK

HSK is a different system entirely — a 1:10 hollow shank with internal clamping and no retention knob, designed for high spindle speeds. It shares nothing with the 7:24 family above and is out of scope for this page.

Holders, arbors and extensions

Taper identifier

Found an unmarked arbor? Measure the two ends and the length between them and the identifier shortlists Morse, Jacobs, 7:24 and R8.