📐 Free Gage Block Calculator — no login. Open it →

← AskMachinist · Guides · Calculator · 🔎 Search

CAT, BT, R8 and HSK Toolholder Basics

Guide · Updated October 28, 2026

Milling machines locate tools with a taper, but unlike Morse or Jacobs tapers, milling tapers are self-releasing: they need a drawbar or power drawbar to hold them in. NMTB, CAT (ANSI B5.50), BT (MAS 403 / JIS B6339) and SK (ISO 7388-1) all share the same 7/24 taper, 3.500 in per foot. R8 is the Bridgeport taper, and HSK is a short hollow taper that contacts both taper and face. Values below come from the published standards as summarized in the Wikipedia machine taper article and common toolholder catalogs.

Need an exact reference size?

Build any gage block stack with the free Gage Block Calculator. Free, no login.

Step by step

  1. Identify the spindle: Look for a V-flange (CAT, BT, SK), a plain flange with long pilot (NMTB), a keyed small taper (R8) or a short hollow shank with face contact (HSK).
  2. Check the size: Measure the gauge diameter at the large end of the taper.
  3. Match the retention: CAT and BT use pull studs gripped by a power drawbar; NMTB and R8 thread onto a manual drawbar.
  4. Use the right pull stud: Match thread, length and the angle your spindle maker specifies.
  5. Clean every change: Wipe taper and spindle; a chip on a 7/24 taper shows up as runout.
  6. Torque pull studs: Tighten to the holder maker's torque, not by feel.

The 7/24 steep taper

A 7/24 taper changes 7 in on diameter per 24 in of length, which is 3.500 in per foot. The included angle is 2 × atan(7/48) = 16.594°, or 8.297° per side. That is far steeper than a Morse taper, so the holder will not stick, and torque is carried by drive keys on the flange, not by taper friction.

Gauge diameters

CAT40, BT40, SK40 and NMTB40 share the 1.750 in taper, so the cone fits all four spindles. They differ in the flange, the V-groove, drive key slots and pull stud threads.

Pull stud and drawbar threads

CAT and BT holders are not interchangeable in a tool changer, even though the taper matches, because the flange thickness and groove differ.

R8 taper

R8 is a Bridgeport design: 16°51′ included angle, 1.250 in outside diameter at the flange end, a straight locating diameter of 0.949 to 0.9495 in, a key slot to stop rotation, and a 7/16-20 drawbar. It takes collets from 1/8 to 3/4 in and shanked tools. It is not self-holding; always snug the drawbar before cutting.

HSK sizes

HSK shanks use a 1:10 taper, about half the length of a steep taper, and pull the flange onto the spindle face. Form A is for tool changers, C for manual use, E and F for very high speed. HSK is ordered by flange diameter in mm.

Worked example: runout from a chip

A 0.0005 in chip on one side of a CAT40 taper tilts the holder. Over the 2.69 in taper length, the tilt is 0.0005 ÷ 2.69 = 0.000186 in/in. At a tool tip 4.0 in below the gauge line, runout grows to about 4 × 0.000186 × 2 = 0.0015 in TIR. Clean tapers are cheap accuracy.

Common problems

Wrong pull stud in a CAT spindle, which is a crash and safety risk; mixing CAT and BT holders in one magazine; over-torqued pull studs that swell the holder bore; R8 collets slipping because the drawbar was not snugged.

FAQ

What is a 7/24 taper?

A taper of 3.500 in per foot, 16.594° included.

Is CAT40 the same as BT40?

The taper matches; flange, groove and pull stud thread differ.

What thread is a CAT40 pull stud?

5/8-11 UNC.

What thread is an R8 drawbar?

7/16-20 UNF.

What does HSK stand for?

Hohlschaftkegel, German for hollow shank taper.

Related guides and calculators

Build your stack in seconds

The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.

Questions? Ask a machinist.

General shop information only — follow your drawing, tool maker's data and your quality system.