4 August 2022

Turrets are used for holding and automatic changeover of cutting tools. The turrets can be of various types (VDI, BOT, BMT) and differ in the maximal numbers of tools they can accommodate. There are 8-, 12- and 24-station turrets, though some turrets are designed for different numbers of tools. In the most cases, the modern CNC turret lathes are equipped with 12-station turrets.

Common turret types

Common turret types


VDI (Verein Deutscher Ingenieure) turrets have dedicated holes on the outside circumference to mount the necessary toolholders. Main sizes: VDI16, VDI20, VDI30, VDI40, VDI50, VDI60. The larger is the lathe, the larger turret is installed. The most popular tool holders are VDI30 (Haas SL-10, DMG MORI 310 ecoline) and VDI40 (Haas ST and DS series, DMG MORI 510 ecoline).

VDI toolholders are offered in quite a wide scope. Axial collet holders, quick change holders, chucks for drills and internal boring bits, left-hand and right-hand, as well as long and short radial cutter holders.

VDI turret

VDI toolholders have a serrated shank to fix them inside the turret holes. Inside each turret hole, there is a mating serrated shaft. This shaft is tightened with an Allen wrench to clamp the toolholder. The changeover of a tool holder is very quick as it requires loosening of only one screw.

VDI ID toolholders

There are multiple types of radial tool holders for rectangular cutting tools (B1–B8). The selection of such holder shall be based on the type of the cutting tool (right- or left-hand) and the location of the clamping screw inside the turret hole (top or bottom).

VDI OD Toolholders

For instance, Haas lathes have bottom position of the serrated shaft, and the toolholder shall therefore be inserted with the serrated side down. At the same time, if we are going to use a right-hand cutting tool, we shall select a left-hand tool holder B2 (short) or B6 (long). In that case, the cutting tool will be oriented the tip down, and the lathe spindle will rotate clockwise (M03).

If we are going to use a left-hand cutting tool, we shall use an inverted left-hand tool holder B4 (short) or B8 (long). The cutting tool will be oriented the tip up, and the lathe spindle will rotate counter clockwise (M04).

VDI turret can be combined on the lathe with live tooling for milling operations. Live tool holders can be of axial and radial type. Axial tool holder is oriented along Z-axis (spindle rotation), while radial live tool holder is oriented along X-axis (diameter). There are adjustable angle live tool holders allowing the service persons to set the angle required for current operations. In the most cases, the live tool holders for ER32 collets are used (clamped shank up to 20 mm), but some live tool holders can accommodate ER25 or ER40 collets.

VDI Live tooling (driven tools)

Some models of live tool holders can be equipped with internal cutting fluid supply, while some models have no internal cutting fluid supply. Anyway, in the first turn we shall consider the total shank length and the type on engagement. Different lathe manufacturers use different types of engagement of live tool holders with the drive shaft, such as keyed, splined or geared. Haas lathes have the following dimensions of splined connection.

VDI Driven tool specifications

The shank length (X) can have 3 values.

Driven tool specifications

VDI Driven tool specifications

Lathe modelsShank length, mm
SL-20 and SL-30, VDI40 turret104,80
All ST lathes except ST-40/45/L. All DS lathes. SL-20 and SL-30 with hybrid VB turret. SL-40 with VDI turret. TL-15 and TL-25117,55
SL-40 with hybrid VB turret. ST-40/40L, ST-45/45L130,25


The next turret type is BOT (Bolt-On Tool turret). BOT turret can not be combined with live tooling. On the other hand, it allows using a sub-spindle. Axial BOT tool holders are attached to the turret with four bolts (radial mounting). Rectangular external turning tools are installed directly into the turret slots and fastened with dedicated clamps. They do not require any additional tooling.

Hybrid VB (VDI/BOT) turret

The tools are rigidly fastened, which suits well for heavy-duty batch-turning, but the holder replacement requires more time. Unlike VDI, 4 bolts shall be loosened instead of 1. The tooling range is not very wide, and it is more expensive. Cutting fluid supply through a tool is possible. This is provided via internal channels inside the holder and its rear wall.

BOT toolholders

Exchange of BOT tool holders between lathes is not always possible. The distance between mounting bolts can be different. Even the same manufacturer can produce different tool holders for different lathes. For instance, Haas ST-20/25/30/35, DS-30 have centre-to centre distance 80х45 mm, while ST-10/15 – 70х45 mm. The 80х45 mm tool holders also suit for Okuma LB15/25/LB15II/LB300, and 70х45 mm tool holders – for Okuma LLC-15-2S. Many tooling manufacturers specify lathe models for which a certain BOT tool holder is suitable in their catalogues.


Hybrid VB (VDI/BOT) turrets shall be also mentioned. They have both VDI mounting holes and BOT slots arranged in alternating pattern (6 VDI + 6 BOT).

Hybrid VB (VDI/BOT) turret

Such turrets allow working both with live tooling and with a sub-spindle. Here, the maximal number of live tool holes is equal to VDI mounting holes.


The next widely spread turret type is BMT (Base Mount Tooling turret). The following sizes are provided: BMT45, BMT55, BMT60, BMT65, BMT75.

BMT turret

In the most cases, such turrets are installed on Y-axis turning centres with live tooling. The drive of a live tool holder is inside the turret. It neither covers any part of the lathe’s working sit, nor collides with the tailstock. The tooling is mounted radially, fastened with four bolts and is secured in position with four locating keys.

BMT OD and ID toolholders

The tool holder shank diameternbsp;(D) corresponds to the number in the size designation. For BMT65, the shank diameter is 65 mm. This size suits for Haas lathes, many medium-size Doosan lathes, Hyndai Wia, Smec.

BMT driven tools

The collet of BMT live tool holder can be tightened only in operating position and only with the use of two wrenches to avoid damage of the disk inside the turret. This can be also performed when the tool holder is removed from the lathe. The video blow shows the preparation of BMT live tool holder to the work.

As a rule, BMT turrets are 12-station, but 24-station versions are also offered. The maximal number of live tool holders and mounting holes does not change. To accommodate additional turning, dedicated multi-seat tool holders and 15-degree turret indexing instead of 30 are used. The multi-seat tool holders are also offered for lathes with a sub-spindle.


Each turret type has its own advantages and disadvantages.

BMT turret is often installed on turning centre with live tooling. The drive is hidden inside the turret and does not hinder machining, while the radial and axial live tool holders allow the use of both main spindle and sub-spindle.

BOT turret is a good choice for a common 2-axis lathe. Radial external turning tools are installed directly into the turret slots without the use of tool holders. This turret also allows operations with a sub-spindle.

VDI turret can not be used with a sub-spindle, but allows application of live tools. There is a wide range of VDI tool holders. They are cheap and can be easily replaced. This is convenient for retooling..