Lathe turret types and tooling

Lathe Turret Types and Tooling

During the lathe machining of cylindrical parts, the cutting tool must remain stationary at its attachment point, while processing is achieved through the rotation of the workpiece in the lathe's spindle and the movement of the support with a tool along the X and Z axes. 

Given the technology and geometry of the turning tool, unlike milling machines (where the tool rotates in the spindle and the tool changer is relatively simple to move out of the work zone), we are forced to place the maximum number of processing tools in the CNC machine's working zone. 

Lathe manufacturers have always sought to maximize the number of turning tools available and the speed at which they can be changed. Universal lathes typically utilize quick-change toolholders that can accommodate only 2-3-4 tools. However, in modern CNC turning centers, most manufacturers have switched to turrets.

A turret is the basic device that allows for quick and efficient tool changes in CNC turning centers. It consists of a disk that is attached to a base mounted on an X-axis support. The turret has slots for mounting cutting tools and/or toolholders, including live blocks. Tool change in the turret is accomplished by rotation of the disk itself, while the tool or fixture always remains in its position in the turret.

This arrangement (rotary disc) is recognized by all manufacturers of CNC lathers as most popular way to ensure quick change, maximum capacity of cutting tools, and maintenance of concentricity during position changes (disc rotation).

Common types of turret lathe

At the beginning of this technology, each equipment manufacturer chose the geometry of the turret disc and the method of attaching the cutting tool independently. However, with globalization, today we have arrived at three main types of turrets in CNC lathes: (1) VDI ("Verein Deutscher Ingenieure" = Association of German Engineers), (2) BOT (Bolt-On Tool turret), and (3) BMT (Base Mount Tooling turret), as well as combined VDI/BOT turrets. Below we will consider the 3 main types:

VDI

In VDI turrets, the tool is always mounted through tooling blocks. The turret disc has cylindrical holes (tool slots) with an internal clamping spline/rack that moves thanks to a screw brought to the surface of the turret. VDI turning blocks accordingly always have a cylindrical shank with a spline and rack corresponding to the slot in the turret. VDI head sizes are determined by the diameter of the tool's cylindrical shank: VDI16, VDI20, VDI30, VDI40, VDI50, VDI60, where the numbers indicate the diameter in millimeters. This type of turret provides the fastest retooling (complete block change), as to replace a holder, it is enough to just loosen the clamping screw. Most VDI tooling blocks are standardized in forms, and to select equipment, you only need to know a few parameters.

Main shapes and types of VDI holders are the following:

• B1 - B8 Radial holders

• C1 - C4 Axial holders

• D1 - D2 Universal holders

• E2 For boring bars

• E1 For drills with replaceable inserts

• E4 Collet chucks

• F Morse taper

VDI turrets can optionally be equipped with a driven tool option. 

The disadvantages of VDI include only a relatively low clamping force and high cost (comparing to BOT).

BOT

In BOT turrets standard cutting tools for O.D. machining are mounted directly in the turret without holders in slots. The slots are milled on the front of the turret, and the tool is clamped with a plank. Tools for I.D. machining are mounted through tooling blocks on the verge of the disc in most cases with four bolts. 

The advantages of BOT include simplicity of construction, no need for additional toolholders for O.D. cutting tools, and structural rigidity and affordability. 

The disadvantages of BOT turrets include the lack of the possibility of being equipped with driven tools and the lack of standardization (the grid of mounting holes is often unique to each equipment manufacturer).

BMT

Turrets of this type have incorporated the best from VDI and BOT turrets:

- Rigid construction (as in BOT)

- Standardization of sizes and equipment (as in VDI)

- The possibility of being equipped with driven tools (as in VDI), but with a motor built into the turret, which provides better engagement and torque. 

The cutting tool in BMT turrets is always mounted through tool holders. The slots for tooling blocks are located only on the verge of the turret, have 4 threaded holes for mounting screws, as well as 4 seating slots and a hole, which can be used both for additional positioning and (main function) for accommodating the shank of driven blocks. 

The diameter of this hole determines the size system of BMT: BMT40, BMT45, BMT55, BMT60, BMT65, etc., where the number indicates the diameter in millimeters. 

The only drawbacks to the BMT system are the high cost of the turret and tool holders.