Which taper is my spindle? CAT vs BT vs SK vs HSK
CAT40, BT40 and SK40 all use a 7:24 taper with the same 44.45 mm gauge diameter, but they are not interchangeable. CAT40 has a 2.500 in (63.5 mm) flange and a 5/8"-11 pull stud, BT40 a 63 mm flange and an M16 stud, and SK40 a 63.55 mm flange, an M16 stud and a longer taper (68.4 mm vs 65.4 mm). Measure the flange and the stud thread below to tell them apart.
7:24 steep tapers: CAT, BT and SK
A value is shown only where the cited catalogues print the same number (an inch figure and its mm conversion count as the same). Rows with one source are marked.
| Taper | Flange Ø | Gauge Ø (D1) | Taper length L1 | Pull stud thread | Drive slot B1 | Sources | PTP holders |
|---|---|---|---|---|---|---|---|
| BT30JIS B 6339 / MAS 403 (ISO 7388-2) | 46 mm 1.811 in | 31.75 mm 1.250 in | 48.4 mm 1.906 in | M12 | 16.1 mm | 2 makers agree [1][2][3] | 84 holders |
| BT40JIS B 6339 / MAS 403 (ISO 7388-2) | 63 mm 2.480 in | 44.45 mm 1.750 in | 65.4 mm 2.575 in | M16 | 16.1 mm | 2 makers agree [1][2][3] | 149 holders |
| BT50JIS B 6339 / MAS 403 (ISO 7388-2) | 100 mm 3.937 in | 69.85 mm 2.750 in | 101.8 mm 4.008 in | M24 | 25.7 mm | 2 makers agree [1][2][3] | 147 holders |
| CAT30ASME B5.50 | 1.812 in 46.02 mm | 1.250 in 31.75 mm | 1.875 in 47.63 mm | 1/2"-13 | - | 1 source: check your manual [5] | - |
| CAT40ASME B5.50 | 2.500 in 63.5 mm | 1.750 in 44.45 mm | 2.687 in 68.25 mm | 5/8"-11 | 0.646 in [Haimer] / 16.38 mm [Falcon] | 2 makers agree [4][5][6] | - |
| CAT45ASME B5.50 | 3.250 in 82.55 mm | 2.250 in 57.15 mm | 3.250 in 82.55 mm | 3/4"-10 | - | 2 makers agree [5][6] | - |
| CAT50ASME B5.50 | 3.875 in 98.43 mm | 2.750 in 69.85 mm | 4.000 in 101.6 mm | 1"-8 | 1.02 in [Haimer] / 25.91 mm [Falcon] | 2 makers agree [4][5][6] | - |
| SK30DIN 69871 (ISO 7388-1) | 50 mm 1.969 in | 31.75 mm 1.250 in | 47.8 mm 1.882 in | M12 | 16.1 mm | 1 source: check your manual [7] | 84 holders |
| SK40DIN 69871 (ISO 7388-1) | 63.55 mm 2.502 in | 44.45 mm 1.750 in | 68.4 mm 2.693 in | M16 | 16.1 mm | 2 makers agree [7][6] | 149 holders |
| SK50DIN 69871 (ISO 7388-1) | 97.5 mm 3.839 in | 69.85 mm 2.750 in | 101.75 mm 4.006 in | M24 | 25.7 mm | 2 makers agree [7][6] | 148 holders |
Labels D1, L1 and B1 follow HAIMER's drawings. Kennametal's CV table prints "2.500 (63,05)" for the CAT40 flange; 2.500 in is 63.5 mm, which HAIMER (2.5 in) and Falcon (63.5 mm) also give, so we use the inch value. CAT drive slot widths differ slightly between HAIMER and Falcon and are shown per source. BT35 and BT45 are left out: the catalogues disagree on the BT35 taper length and only one prints BT45.
HSK hollow taper shanks
HSK size is the flange diameter in mm. The letter is the form: A, E and F with the same flange can have different tapers, so HSK-A63 and HSK-F63 are not interchangeable.
| HSK | Flange Ø D1 | Taper Ø D3 | Taper length L1 | Coolant tube thread | Sources | PTP holders |
|---|---|---|---|---|---|---|
| HSK-A32 | 32 mm | 24.007 mm | 16 mm | M10 × 1 | 1 source: check your manual [8] | - |
| HSK-A40 | 40 mm | 30.007 mm | 20 mm | M12 × 1 | 1 source: check your manual [8] | - |
| HSK-A50 | 50 mm | 38.009 mm | 25 mm | M16 × 1 | 2 makers agree [8][9] | 70 holders |
| HSK-A63 | 63 mm | 48.01 mm | 32 mm | M18 × 1 | 2 makers agree [8][9] | 103 holders |
| HSK-A80 | 80 mm | 60.012 mm | 40 mm | M20 × 1.5 | 1 source: check your manual [8] | - |
| HSK-A100 | 100 mm | 75.013 mm | 50 mm | M24 × 1.5 | 2 makers agree [8][9] | 88 holders |
| HSK-E32no drive slots in flange | 32 mm | 24.007 mm | 16 mm | M10 × 1 | 1 source: check your manual [8] | - |
| HSK-E40 | 40 mm | 30.007 mm | 20 mm | M12 × 1 | 1 source: check your manual [8] | - |
| HSK-F63same 63 mm flange as A63, smaller taper (38 mm) | 63 mm | 38.009 mm | 25 mm | - | 2 makers agree [8][9] | - |
PTP CNC Toolings' HSK-A coolant tubes use the same threads (M16 × 1, M18 × 1, M24 × 1.5 for A50, A63, A100) [10]. HAIMER's HSK-E50 row prints a 42 mm flange, which we could not confirm, so HSK-E50 is left out.
How to tell them apart in two minutes
- Is there a pull stud? A threaded knob screwed into the back of the shank means a 7:24 steep taper (CAT, BT or SK). A short hollow shank with no stud is HSK.
- Read or gauge the pull-stud thread. An inch thread (5/8"-11 on a 40 taper) means CAT. A metric thread (M16 on a 40 taper) means BT or SK.
- Measure the flange. BT40 is 63.0 mm and SK40 is 63.55 mm. Falcon Toolings also notes SK flanges are thinner than BT [6].
- Check the taper length from the flange face to the small end: BT40 65.4 mm, SK40 68.4 mm, CAT40 2.687 in (68.25 mm).
Are CAT40 and BT40 interchangeable? No. The 7:24 taper and the 44.45 mm gauge diameter are the same, but the flange, the taper length and the pull-stud thread differ (table above). A machine is built for one of them; check the builder's spec sheet before ordering holders.
BT40 vs BBT40 or "face contact". Face-contact (dual-contact) holders also seat on the flange face. HAIMER sells BT40 and CAT40 holders "with face contact" as separate ranges. Whether your spindle supports face contact is a question for your machine builder.
Next: find the pull stud for your machine or the ER collet and nut torque for your holder.
Sources and last checked
Standards cited by these catalogues: ASME B5.50 (CAT), JIS B 6339 / MAS 403 and ISO 7388-2 (BT), DIN 69871 and ISO 7388-1 (SK), DIN 69893 and ISO 12164-1 (HSK). We did not reproduce the standards themselves. Email a correction.
Want a finder like this on your own catalog? Krira Digital builds them.
Questions
Are CAT40 and BT40 interchangeable?
No. Both use a 7:24 taper with a 44.45 mm gauge diameter, but CAT40 has a 2.500 in (63.5 mm) flange and a 5/8"-11 pull stud, while BT40 has a 63 mm flange, an M16 stud and a shorter taper length (65.4 mm vs 2.687 in / 68.25 mm), per HAIMER, Kennametal and Falcon.
What is the difference between BT40 and SK40 (ISO 40)?
Both use an M16 pull stud. SK40 (DIN 69871) has a 63.55 mm flange and a 68.4 mm taper length; BT40 has a 63 mm flange and 65.4 mm. Falcon Toolings also notes SK flanges are thinner than BT.
How do I tell what taper my spindle is?
Check for a pull stud (CAT, BT, SK) or a hollow shank (HSK). Then read the stud thread: inch means CAT, metric means BT or SK. Measure the flange: CAT40 63.5 mm, BT40 63.0 mm, SK40 63.55 mm. Use the identifier at the top of this page.
What is the difference between HSK-A63 and HSK-F63?
Both have a 63 mm flange, but HSK-A63 has a 48.010 mm taper 32 mm long and HSK-F63 a 38.009 mm taper 25 mm long (HAIMER, Falcon). They are not interchangeable.
What pull stud thread does CAT50 use?
1"-8. CAT40 uses 5/8"-11 and CAT45 3/4"-10 (Kennametal, HAIMER, Falcon). BT50 and SK50 use M24.
Is BT30 the same as ISO30 or SK30?
No. HAIMER lists BT30 with a 46 mm flange and 48.4 mm taper length, and SK30 with a 50 mm flange and 47.8 mm taper length. Both use M12 pull studs.
What does the 40 in CAT40 or BT40 mean?
It is the taper size. 30, 40 and 50 tapers have gauge diameters of 31.75 mm (1.250 in), 44.45 mm (1.750 in) and 69.85 mm (2.750 in) in all three families.
Which taper is best?
That depends on the machine: the spindle is made for one interface, so the right taper is the one your machine builder specifies. This page only identifies it; it does not rank them.
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