CNC lathes are widely used in making components with asymmetric geometry. These are generally machined with 2-axis control. (Z-axis parallel to the spindle and X-axis perpendicular to spindle axis). A CNC lathe is shown in Fig. 12.15. Some CNC lathes have an additional Y-axis. CNC lathes are usually designed with a slant bed for easy removal of chips, at the same time maintaining the rigidity of a closed box structure. A chip conveyor (either magnetic or mechanical) is usually provided for easy chip disposal. The control console is located on the front on top side. The spindle is driven usually by an infinitely variable speed AC drive through a set of V-belts or poly belts. The high power series motors employed provide almost constant power for a large portion of the speed range. High-speed machines usually have integral rotor spindles. AC servomotors drive the X and Z slides usually. Sliding doors (sometimes automatic) with bulletproof glass windows are provided for safety and visibility of the machining zone. Hydraulic chucks are provided for clamping the workpieces. CNC lathes, being high-speed machines require special highly accurate and well-balanced chucks. Tools are mounted in indexable turrets which can hold 8, 12 or 16 tools.
A tool is called in the part program by the number of the pocket in which it is held. For example T06 means the tool in the 6th pocket. Tool stations are indexed using a Geneva mechanism powered electrically or hydraulically. An encoder or a similar device is used to index the turret. A curvic coupling is used to ensure high accuracy in indexing usually 2 to 3 seconds of an arc. Both axial tools (boring tools, Internal grooving tools, internal threading tools, drills, face grooving tools, taps, dies etc) and radial tools (external turning, facing, threading, grooving, cut off etc) are mounted on the turret. It is a good practice not to mount axial tools in adjacent pockets. Tools, which are used more often, are provided in more than one pocket so that if the tool life of one tool is over the, other tool can take over the machining work without interruption to facilitate tool change or indexing of inserts. Short tool holders are recommended for better rigidity. A variety of adaptors are provided to mount different tools. Tool length offset must be measured and input in the memory of the machine. Some machines will have an automatic tool length offset measuring device, usually mounted on the headstock.
CNC lathes and turning centers are used for chuck work and shaft work. These are
(i) Horizontal machines and
(ii) Vertical Machines
The horizontal machines are further classified as:
(i) Chucking machines
(ii) Universal machines
In chucking machines short workpieces held only in the chuck are machined. Examples of such components are gear blanks, flanges, pump impellers, bearing races etc. Universal machines have a tailstock, which supports long workpieces like shafts at their ends.
i. Chucking Machines
Chucking machines usually have shorter beds and a single saddle with hexagonal, octagonal, disc or drum type turret, which accommodates both ID and OD tools and two independent saddles with separate turrets. Many of the present day chuckers offer optional swing-in-tail-stock to facilitate shaft work. During normal working as a chucker, the tailstock is swung away from the work area. Chucking machines are fitted with additional sub-spindle to carry out both first and second operation in the same set up. These spindles will have synchronized drives to enable transfer of work from one spindle to another. In some machines the subspindle is fixed and work transfer is achieved with the aid of two turrets. Chucking machines are often fitted with bar feeding mechanisms. Twin spindle chuckers ensure high productivity. These are used for workpieces, which are to be finished in two end operations. The first operation is performed on the first spindle and the second operation on the second spindle. These will have an automatic work transfer system, which transfers the workpiece from one spindle to another. The advantage of these machines is that they ensure a continuous supply of parts for assembly operations. A typical example of a workpiece suited for such machines is the impeller of a centrifugal pump.
ii. Universal lathes
Universal lathes are suitable for both chucking and for bar work. They are often fitted with steady rest and follower rest to accurately machine slender components. 4-axis machines have two turrets; each mounted on an independent slide and facilitates simultaneous machining with two tools.
Some lathes have rotating tools in the turrets to facilitate off-axis machining such as drilling, milling, reaming, tapping, boring etc. These machines have in addition to the conventional X and Z-axes, CNC control of the spindle rotation i.e. C-axis. Such machines are known as turning centers. Turning centers eliminate the need for separate settings for milling, drilling and allied operations. Turning centers enable the components to be finished in one set up thereby reducing lead time and improving the accuracy of machined parts. An automatic tool changer (ATC) can be used with CNC lathes also using the Block tooling system of Sandvik or Multiflex tooling system of Widia. CNC turning centers have rotating tools on the turret. With these tools operations like off-center drilling, milling (keyway or slot milling), tapping etc. can be done on workpieces after turning. A servomotor is provided on the tool turret to drive the live tools.
This requires an additional axis called C-axis (spindle orientation control). Some machines will have special main drive, which will also serve the function of C-axis drive. Flexible turning centers are fitted with gantry loading and unloading devices, software and hardware for in-process and post-process metrology, automatic tool changers, automatic tool offset measuring devices, automatic offset correction, tool magazines, automatic chuck changers, chuck jaw changers etc., This type of machines use cartridge type tools which facilitate automatic tool change. Flexible turning centers easily get integrated in a CIM set up.
Productivity of CNC turning centers could be increased by attaching a gantry device for unloading and loading of components. Another approach is to use a robot for loading and unloading the component.
iii. Tooling and Turret slide
The present trend is to have a single heavy disc type turret capable of accommodating both external and internal working tools. Some machines have two independent slides with one turret on each slide, one for external working tools and the other for internal tools. This arrangement will need 4-axis CNC. External and Internal machining can be carried out simultaneously. Alternatively both slides can be fitted with turrets carrying external turning tools to facilitate simultaneously machining of steps of the shafts from either direction.
Vertical CNC Lathes (Turn Mill Centers)
Vertical CNC lathes are widely used for machining heavy components. Figure 12.17 shows a typical vertical CNC lathe. Some of these machines can also be used for milling operations. Such machines are sometimes known as turn mill centers. This type of machines is very popular in aerospace industries where components of large diameters need to be turned, drilled and milled. The machine shown in Fig.12.17 has five axes four longitudinal axes and one rotary axis.
Multitasking is a recent concept in CNC machines. Multitasking machines incorporate several processes in a single work center. For example, if a component requires milling, turning and grinding, a multitasking machine can be designed to carry out all these operations. This type of machine reduces the number of set ups and therefore reduces cycle times. Accuracy is improved as there are fewer set ups. Very often designers break down complex components into a number of parts to facilitate machining.
Multitasking machines eliminate this need thereby increasing the integrity of the part. Another advantage of multitasking is the possibility of cost reduction because of reduction in set ups, eliminating the need for several fixtures.