Machine Taper Reference — Morse, R8, Jacobs, CAT / BT
Dimensions and taper ratios for Morse (MT0-6), R8, Brown & Sharpe, Jacobs, and V-flange CAT / BT / NMTB tool holders used on drill presses, mills, and CNC spindles.
Reference
Machine tapers transfer torque and provide self-centering. Self-holding tapers (Morse, Brown & Sharpe, Jacobs) grip on friction alone — knock-out slot and drawbar release them. Steep / self-releasing tapers (R8, CAT / BT / NMTB / HSK) need a drawbar to hold the tool and come free with a rap once the bar is loosened. Taper per foot (TPF) is the included diameter change — a 0.500 TPF means the diameter grows 0.500" over 12" of axial length (i.e., each side angles 1.19°).
Morse taper (MT) — self-holding
| Taper | Small end (D₁) | Large end (D₀) | Length | Taper /foot | Use |
|---|---|---|---|---|---|
| MT0 | 0.252" / 6.401 mm | 0.3561" / 9.045 mm | 2.000" / 50.8 mm | 0.6246 | Very small drills, micro lathes |
| MT1 | 0.369" / 9.370 mm | 0.475" / 12.065 mm | 2.125" / 54.0 mm | 0.5986 | Small hand drills, #6 to 3/8 drill shanks |
| MT2 | 0.572" / 14.529 mm | 0.700" / 17.780 mm | 2.5625" / 65.1 mm | 0.5994 | Drill-press spindles, 3/8 - 1/2" drill shanks |
| MT3 | 0.778" / 19.760 mm | 0.938" / 23.825 mm | 3.1875" / 81.0 mm | 0.6024 | Lathe tailstocks, small mills |
| MT4 | 1.020" / 25.909 mm | 1.231" / 31.267 mm | 4.0625" / 103.2 mm | 0.6233 | Mid-size lathes, Bridgeport spindle nose (older) |
| MT5 | 1.475" / 37.465 mm | 1.748" / 44.399 mm | 5.1875" / 131.8 mm | 0.6315 | Large engine lathes, big drill presses |
| MT6 | 2.116" / 53.746 mm | 2.494" / 63.348 mm | 7.250" / 184.2 mm | 0.6257 | Heavy industrial lathes, horizontal boring |
| MT7 | 2.750" / 69.850 mm | 3.270" / 83.058 mm | 10.000" / 254.0 mm | 0.6240 | Very heavy machinery (rare) |
R8 — Bridgeport-style mill spindle (self-releasing)
- Taper angle
- 16°51' (a steep self-releasing taper — drawbar required)
- Small end Ø
- 0.687" / 17.45 mm (nominal)
- Large end Ø
- 0.950" / 24.13 mm
- Length
- 0.912" / 23.16 mm
- Drawbar thread
- 7/16-20 UNF (imperial) · M12 × 1.75 on most metric R8 spindles
- Keyway
- 5/32" wide × 1-1/8" long (indexes on spindle key to transmit torque)
- Origin
- Bridgeport Series 1 milling machines from ~1936; now the de-facto standard on knee mills and small VMCs
Jacobs taper (JT) — chuck arbors (self-holding)
| Taper | Small end | Large end | Length | Taper /foot | Typical use |
|---|---|---|---|---|---|
| JT0 | 0.2500" | 0.2284" | 0.43" | 0.5915 | 0-3 mm chuck |
| JT1 | 0.3840" | 0.3334" | 0.8125" | 0.9228 | 1/4" drill chuck |
| JT2 | 0.5590" | 0.4876" | 0.875" | 0.9782 | 3/8" drill chuck (most common benchtop) |
| JT33 | 0.625" | 0.561" | 1.000" | 0.7660 | 1/2" drill chuck (Albrecht, precision) |
| JT3 | 0.811" | 0.7565" | 1.000" | 0.6340 | 1/2" - 5/8" chuck |
| JT4 | 1.124" | 1.0401" | 1.375" | 0.6233 | 3/4" chuck, lathe dead centers |
| JT5 | 1.4163" | 1.3196" | 1.4375" | 0.6396 | 1" chuck (heavy) |
| JT6 | 0.676" | 0.624" | 1.000" | 0.6240 | Morse-like alternate for lathe tailstocks |
| JT7 | 0.750" | 0.6980" | 1.375" | 0.6240 | Extra-small keyless chucks |
Brown & Sharpe — older milling machines
| Size | Small end | Large end | Length | Taper /foot |
|---|---|---|---|---|
| B&S 1 | 0.200" | 0.239" | 0.9375" | 0.502 |
| B&S 2 | 0.250" | 0.299" | 1.1875" | 0.502 |
| B&S 3 | 0.3125" | 0.375" | 1.5" | 0.502 |
| B&S 5 | 0.450" | 0.539" | 2.1875" | 0.502 |
| B&S 7 | 0.600" | 0.719" | 2.875" | 0.502 |
| B&S 9 | 0.900" | 1.062" | 3.9375" | 0.502 |
| B&S 10 | 1.0446" | 1.2407" | 4.75" | 0.516 |
| B&S 11 | 1.250" | 1.485" | 5.6875" | 0.500 |
| B&S 12 | 1.500" | 1.797" | 7.250" | 0.500 |
| B&S 13 | 1.800" | 2.166" | 8.8125" | 0.500 |
V-flange tool holders — CAT / BT / NMTB (self-releasing, drawbar)
| Size | Gauge Ø | Taper angle | Typical spindle | Pull stud thread (CAT) |
|---|---|---|---|---|
| CAT 30 / BT 30 / NMTB 30 | 1.250" / 31.75 mm | 8° 17' 50" (7:24) | Small VMCs, high-speed spindles | 5/8-11 UNC (CAT 30) |
| CAT 40 / BT 40 / NMTB 40 | 1.750" / 44.45 mm | 8° 17' 50" (7:24) | Mid-size VMCs (most common) | 5/8-11 UNC (CAT 40) |
| CAT 45 / BT 45 | 2.250" / 57.15 mm | 8° 17' 50" (7:24) | Large VMCs | 3/4-10 UNC |
| CAT 50 / BT 50 / NMTB 50 | 2.750" / 69.85 mm | 8° 17' 50" (7:24) | Large HMCs / heavy cutting mills | 1-8 UNC |
| CAT 60 / BT 60 | 4.250" / 107.95 mm | 8° 17' 50" (7:24) | Huge HMCs (rare, aerospace, moulds) | 1-1/4-7 UNC |
HSK — Hollow-shank tool holders
- HSK-A63
- Automatic tool-change, 63 mm gauge. Common on modern high-speed VMCs.
- HSK-A100
- Heavy-cut automatic. Large moulds and aerospace.
- HSK-E
- Symmetrical high-speed (no driving slots) — spindle speeds > 40 000 rpm.
- Clamping
- Hollow taper contacts the spindle both on the 1:10 taper and the flange face — higher rigidity than CAT / BT at the same size.
- vs CAT
- HSK has ~2× the bending stiffness of same-size CAT / BT and zero axial pull-out. Downside: fussier on cleanliness, more expensive toolholders.
Self-holding vs self-releasing — how to tell
- Self-holding tapers (Morse, B&S, Jacobs) are shallow (~3° to 8° included) and grip on friction alone — a pound of force is enough to seat them, and they need a drift punch through the knock-out slot to release.
- Self-releasing tapers (R8, CAT, BT, NMTB, HSK) are steep (15-17° included). They cannot transmit torque without a drawbar and driving key / dog.
- All CAT / BT / NMTB toolholders have the same 7:24 taper ratio (8°17'50") but differ in flange OD, pull-stud thread, and retention mechanism. CAT = ANSI, BT = JIS, NMTB = older ANSI (face is pin-driven; CAT / BT use keyway drive).
- BT and CAT toolholders are not interchangeable despite identical tapers — flange is different, retention pull-stud thread differs, and balance grade may differ. Always match the spindle type.
- To ID an unknown taper: measure small-end diameter and length with calipers, look up in the table. 2.5" length + ~0.572" small end = MT2; 0.912" length + 0.687" small end + drawbar = R8.
Notes
- Dimensions per ASME B5.10 (Morse / B&S), ISO 7388 (CAT / BT), DIN 69871 (SK / BT / HSK). Commercial toolholders are typically toleranced AT-3 or AT-4 (ISO 1947).
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