BT is a metric steep-taper (7/24) spindle interface common in Asia and Europe. Like CAT it uses a flange for the tool changer, but the flange and retention knob are symmetric and metric, which some consider better balanced for higher speeds. BT30, BT40 and BT50 are the common sizes.
The taper itself matches CAT of the same number, but pull studs and flange dimensions differ, so BT and CAT holders are not interchangeable without changing the pull stud and confirming clearance.
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