BT Tool Holders - BT30, BT40, BT50

A BT tool holder is a steep-taper toolholder built to the Japanese JIS B 6339 standard (formerly MAS 403). It uses the 7:24 taper and a metric pull stud to seat in the spindle. The three common sizes are BT30 (31.75 mm gauge diameter), BT40 (44.45 mm), and BT50 (69.85 mm). Tool Holder Exchange buys and sells used BT holders in all three sizes nationwide, so the specs below come from holders we handle every week.

This page is the reference: full BT30, BT40 and BT50 dimensions, the taper and pull-stud details, how the sizes differ, and how BT compares to CAT. If you have surplus BT holders to move or you need inspected used holders at a lower price than new, the buy and sell sections at the bottom explain how that works.

What Is a BT Tool Holder?

A BT tool holder is a steep-taper holder that connects a cutting tool to a CNC milling spindle using a 7:24 taper and a threaded pull stud. BT is the Japanese tooling interface defined by JIS B 6339, which replaced the older MAS 403 standard. The taper does the centering, the spindle drive keys transmit torque, and the pull stud lets the spindle drawbar clamp the holder for automatic tool changes.

BT holders are built in metric sizes and accept both inch and metric tooling through collets, end mill holders, shrink-fit bodies, and similar bodies. The size number refers to the taper: BT30, BT40, and BT50 are the #30, #40, and #50 steep tapers. BT40 is the most common size in general machining, with BT30 on smaller and high-speed machines and BT50 on large, heavy-cut machines.

What Does BT Mean? (BT Full Form)

BT is the designation for the Japanese steep-taper tooling shank standardized under MAS 403 and the current JIS B 6339. It is commonly written as “BT” or “MAS-BT,” and the number after it (30, 40, 50) is the ISO taper size, not a separate code. So “BT40” means a #40 steep taper built to the BT (JIS B 6339 / MAS 403) standard.

The practical point matters more than the letters: BT tells you the flange style, the metric pull-stud thread, and the drive-key arrangement that a Japanese-standard spindle expects. A BT40 holder fits a BT40 spindle. It does not fit a CAT40 spindle, even though both are #40 steep tapers, because the flange and pull stud differ.

What Taper Does a BT Tool Holder Use?

BT tool holders use the 7:24 steep taper, the same nominal taper used by CAT and the older NMTB tooling. The “7:24” means the diameter grows 7 units for every 24 units of length, which is a shallow enough wedge that the taper releases cleanly instead of locking in the spindle. That self-releasing behavior is what allows fast automatic tool changes, and it is the opposite of a self-holding Morse taper that has to be knocked out.

Because the taper is steep, a BT holder seats on the taper surface and registers against the spindle, then the drawbar pulls on the pull stud to hold it. The taper sets the location and concentricity, so a clean, undamaged taper is what keeps runout low. BT40 and CAT40 share this 7:24 taper and the same gauge-line diameter, which is why people confuse them, but they are not interchangeable.

BT30, BT40 and BT50 Dimensions

The key BT dimensions are the gauge-line diameter (the large end of the taper), the flange diameter, and the pull-stud thread. BT30 has a 31.75 mm gauge diameter and a 46 mm flange. BT40 has a 44.45 mm gauge diameter and a 63 mm flange. BT50 has a 69.85 mm gauge diameter and a 100 mm flange. All three use the 7:24 taper and the JIS B 6339 standard.

Quick reference:

Full dimension grid (inch, with mm in parentheses):

These are the JIS B 6339 (MAS 403) reference dimensions. Individual holder bodies add length and gauge length depending on whether they are collet chucks, shrink-fit holders, end mill holders, or face mill arbors, but the spindle-side numbers above stay fixed for a given BT size.

Pull Studs and Retention Knobs for BT Holders

BT pull studs, also called retention knobs, are metric and sized by holder: M12 for BT30, M16 for BT40, and M24 for BT50. They are governed by JIS B 6339, and the drawbar grabs the pull stud to clamp the holder into the spindle. The pull stud is a wear and safety item, so it gets checked for thread condition and seating surface as part of normal tool-holder care.

BT pull studs are not CAT pull studs. CAT holders use inch threads (5/8″-11 on CAT40, 1″-8 on CAT50), while BT uses the metric M-threads above. The shapes and angles also differ to match each spindle’s drawbar. Mixing a CAT pull stud into a BT holder, or the reverse, can prevent proper clamping or damage the drawbar, so the rule on the shop floor is to keep BT and CAT pull studs separated and matched to the holder.

BT30 vs BT40 vs BT50: Which Size?

The right BT size is set by your machine spindle, not by choice, so match the holder to the spindle taper your machining center already has. BT30 is the smallest of the three, used on compact and higher-speed mills where mass and tool-change speed matter. BT40 is the general-purpose workhorse for most vertical machining centers. BT50 is the largest, used on big, rigid machines for heavy roughing and large cutters.

The trade-off across the sizes is rigidity and capacity versus speed and weight. BT50, with its 69.85 mm gauge diameter and 100 mm flange, carries the most cutting load and the largest tools, but the heavier holder slows tool changes and suits lower spindle speeds. BT30, at a 31.75 mm gauge diameter, changes fast and runs well at high rpm but carries lighter cuts. BT40 sits in the middle and is why it is the most common size in the used market.

BT vs CAT Tool Holders

BT and CAT use the same 7:24 taper and the same gauge-line diameter for a given number, but they are not interchangeable. A BT40 and a CAT40 both seat a #40 steep taper at 44.45 mm, yet they differ in flange thickness and style, in pull-stud thread (BT40 uses M16 metric, CAT40 uses 5/8″-11 inch), and in the drive-key and orientation-slot arrangement. A BT40 holder will not run correctly in a CAT40 spindle.

BT is the Japanese standard and is built symmetric about the spindle axis, which helps balance at higher spindle speeds. CAT is the common American standard. If you are sorting a mixed tool crib, the fastest way to tell them apart is the pull stud (metric vs inch) and the flange shape. For a full breakdown, see CAT40 and CAT50 holders, and if you are weighing tapers for a high-speed job, how HSK compares to CAT and the difference between BT and HSK cover the other interfaces.

Tool Holder Care: Keep the Taper and Flange Clean

BT holder performance depends on a clean taper, flange, and pull stud. Any dirt, chips, oil, or coolant film left on the taper, the spindle bore, the collet, or the collet pocket raises total indicator runout (TIR), and high runout wears the cutting tool, the holder, and the spindle nose faster. Wipe the taper and flange before every load, and inspect the taper for nicks or fretting.

BT holders run in automatic tool change (ATC) cycles, so the gripper groove and pull stud also need to stay clean and undamaged for reliable handling. A holder with a dinged taper or a worn pull stud should be pulled from service. For the full routine, see the importance of tool holder care and maintenance.

Buy Used BT Tool Holders at 40-60% Off New

You do not need to pay new prices for BT holders. Tool Holder Exchange stocks inspected used BT30, BT40, and BT50 holders, typically 40-60% below the cost of new, and each holder is checked for taper and pull-stud condition before it ships. For a shop adding capacity or rebuilding a tool crib, used holders in good condition do the same job as new at a fraction of the spend.

See what we have in BT40 tool holders we have in stock, or call 517-514-5196 to ask about a specific size, type, or quantity. We also carry collet chucks, end mill holders, and shrink-fit bodies in BT tapers. You can also shop Superior Machine and Tool’s selection of BT tool holders. Beyond holders, you can buy used and surplus carbide tooling from us as well.

Sell Your Used BT Tool Holders to THE

If you have surplus BT holders sitting idle, Tool Holder Exchange buys them. We purchase used BT30, BT40, and BT50 holders in any quantity, from a single box to a full tool crib, and we handle shop closures, retirements, and machine changeovers where a shop switches taper systems and no longer needs its old holders. You get paid for tooling that would otherwise gather dust.

Getting a quote is simple: tell us the sizes, types, brands, and rough quantities, and we evaluate and make an offer. Start a quote on our sell your used tool holders page or call 517-514-5196. The faster we see what you have, the faster you get an offer. We also sell your carbide tooling for you, and if your holders are name brands such as Kennametal tool holders, those carry strong resale value.

Frequently Asked Questions

Ready to buy or sell? Visit our BT40 tool holders page for current inventory and quotes.

For how BT compares to every other type, see our Tool Holders 101 overview.