Your machine spindle decides the taper — you cannot choose it when buying holders. BT, CAT and SK all use the same 7/24 steep taper but differ in the flange and the pull stud, so they do not interchange. BBT adds contact on the flange face to the BT taper. HSK works on a different principle: a short, hollow 1:10 taper clamped from the inside, with taper and face contact at the same time.
| Taper | Standard | Contact | Flange and pull stud | Where you find it |
|---|---|---|---|---|
| BT | JIS B 6339 (originally MAS 403) | Taper only | Symmetric flange; metric pull stud (M16 on BT40) in 45°, 60° or 90° forms | Japanese, Taiwanese and many Asian-built machines |
| BBT | BT taper with dual contact | Taper and flange face | Same flange and pull stud as BT | Machines with a dual-contact spindle |
| CAT | ANSI/ASME B5.50 | Taper only | V-flange; inch pull stud thread (5/8-11 UNC on CAT40) | North American machines |
| SK | DIN 69871 / ISO 7388-1 | Taper only (face-contact versions exist) | V-flange; metric pull stud to DIN 69872 or ISO 7388 | European machines |
| HSK | DIN 69893 / ISO 12164 | Short 1:10 taper and face, at the same time | No pull stud — gripped from inside the hollow shank | High-speed machining, mill-turn, 5-axis |
| BT in a CAT spindle | No. Same taper angle, different flange and pull stud thread. |
|---|---|
| CAT in an SK spindle | No. The V-flanges look alike but the pull stud thread and dimensions differ. |
| BBT holder in a BT spindle | Yes, as a normal taper-only connection. You do not get the face contact, but it works. |
| BT holder in a dual-contact spindle | Yes, taper only. The gain needs both a dual-contact spindle and a dual-contact holder. |
| HSK-A and HSK-E or -F | No. The forms differ in drive slots and flange, even at the same nominal size. |
On a taper-only holder, the spindle mouth opens slightly at high speed and the holder can be drawn deeper into the taper, so the tool length changes and the radial stiffness drops. Contact on the flange face fixes the axial position and adds support against bending. The result is better Z repeatability and stiffer milling. Mongtec BBT holders are dual-contact holders on the BT taper.
HSK uses a short, hollow taper with a 1:10 angle. The clamping mechanism grips from inside the shank and the centrifugal force at speed actually helps it hold. Taper and face are in contact together, so HSK keeps its accuracy at speeds where a 7/24 taper starts to lose it. The forms are not interchangeable:
Two BT40 machines can need different pull studs — different angle, length or thread form. A holder with the wrong stud will not clamp, or worse, will clamp badly. When you order, give the machine model or the stud drawing, not only the taper size.
Mongtec builds tool holders on BT30, BT40 and BT50, BBT40 and BBT50, SK30, SK40 and SK50, CAT40 and CAT50, HSK-A32 to HSK-A100, HSK-E25 to HSK-E63, HSK-F40 and HSK-F63, and HSK-T63 and HSK-T100 for mill-turn. BT and BBT holders are cut to JIS B 6339 with a taper tolerance tighter than AT3, hardened to HRC 56–60. CNC mill tool holders →
Mongtec confirms fit and quotes from four details. Email them to info@mongtec.com or use the quote form — we reply within one working day.