Choosing the wrong taper standard costs money. Understanding the differences between HSK, CAT, and BT before you buy saves problems.

The Three Major Taper Families

CAT (V-Flange)

Dominant in North America. Uses a 7:24 taper pulled into the spindle with a retention knob. Taper-only contact, no face contact.

  • CAT40 – Standard on most mid-size VMCs and HMCs. Most common taper in US shops.
  • CAT50 – Larger machines with more horsepower. Heavy milling, boring, large-diameter work.

BT (Metric V-Flange)

Same 7:24 taper angle as CAT but metric standard. Dominant in Japan, Asia, and Europe. NOT interchangeable with CAT.

  • BT30 – Smaller VMCs, live tooling, high-speed
  • BT40 – Most common BT size worldwide
  • BT50 – Heavy-duty, comparable to CAT50

HSK (Hollow Shank Taper)

Fundamentally different. Short, hollow taper with simultaneous face AND taper contact. Better rigidity at high RPM, improved repeatability, ideal above 15,000 RPM.

  • HSK-A63/A100 – General machining standard
  • HSK-E – High-speed with through-coolant
  • HSK-F – High-speed die and mold work

High-Speed Machining: HSK Wins

HSK prevents holder pullback under centrifugal force. Above 15,000 RPM, HSK is the correct choice. CAT40/BT40 perform well at 8,000-12,000 RPM.

Heavy Cutting: CAT50/BT50

Larger shank diameter and longer taper engagement handle high torque. For heavy roughing at moderate speeds, CAT50 or BT50 may be simpler and more cost-effective than HSK.

Most Common Taper

US: CAT40 by a wide margin. Globally: BT40. HSK is growing as high-speed machining expands in aerospace, mold, and precision work.

CAT40 vs BT40

  • CAT40 uses inch-thread pull studs. BT40 uses metric.
  • Flanges differ in dimension. They will not cross-fit.
  • BT40 has slight balance advantage at higher RPM.
  • CAT40 is US standard. BT40 is standard everywhere else.

Switching Taper Standards

THE can help: we buy your existing holders and sell used holders in your new taper. You can also trade tool holders for credit.

THE Buys and Sells All Taper Types

CAT40, CAT50, BT30, BT40, BT50, HSK. Visit sell, buy, or CAT holders. Call (517) 420-5401.