Conventional engine lathe or a turret lathe is a common machine in just about every machine shop. A lathe is used for machining cylindrical or conical work, such as shafts, rings, wheels, bores, threads, etc. The most common lathe operation is removal of material from a round stock, using a turning tool for external cutting. A lathe can also be used for internal operations such as boring, as well as for facing, grooving, threading, etc., if a proper cutting tool is used. Turret lathes are usually weaker in machining power than engine lathes, but they do have a special holder that stores several mounted cutting tools. An engine lathe has often only one or two cutting tools mounted at a time, but has more machining power.
Typical lathe work controlled by a CNC system uses machines known in industry as the CNC turning Centers – or more commonly – CNC lathes.
The term ‘turning center’ is rather unpopular, but an accurate overall description of a computerized lathe (a CNC lathe) that can be used for a great number of machining operations during single setup. For example, in addition to the standard lathe operations such as turning and boring, CNC lathe can be used for drilling, grooving, threading, knurling and even burnishing. It can also be used in different modes, such as chuck and collet work, bar feeder, or between centers. Many other combinations also exist. CNC lathes are designed to hold several tools in special turrets, they can have a milling attachment (live tooling), indexable chuck, a sub-spindle, tailstock, steady rest and many other features not always associated with a conventional lathe design. Lathes with more than four axes are also common. With constant advances in machine tool technologies, more CNC lathes appear on the market that are designed to do a number of operations in a single setup,many of them traditionally reserved for a mill or machining center.
Types of CNC Lathes
Basically, CNC lathes can be categorized by the type of design and by the number of axes. The two basic types are a vertical CNC lathe and a horizontal CNC lathe. Of the two, the horizontal type is by far the most common in manufacturing and machine shops. A vertical CNC lathe (incorrectly called a vertical boring mill) is somewhat less common but is irreplaceable for a large diameter work. For CNC programmer, there are no significant differences in programming approach between the two lathe types.
Number of Axes
The most common distinction of different CNC lathes is by the number of programmable axes. Vertical CNC lathes have two axes in most designs available. The much more common CNC horizontal lathes, commonly designed with two programmable axes, are also available with three, four, six, and more axes, thus adding extra flexibility to manufacturing of more complex parts.
A typical horizontal CNC lathe can further be described by the type of engineering design:
- FRONT lathe ( an engine lathe type )
- REAR lathe ( a unique slant bed type )
Slant bed type is very popular for general work, because its design allows cutting chips to fall away from the CNC operator and, in case of an accident, forces the part to fall down into a safe area, towards chip conveyer.
Between the categories of flat bed and slant type lathes,front and rear lathes, horizontal and vertical lathe designs, there is another variety of a lathe. This category describes CNC lathes by the number of axis, which is probably the simplest and most common method of lathe identification.
A typical CNC lathe is designed with two standard axes – one axis is the X-axis, the other axis is the Z-axis. Both axes are perpendicular to each other and represent the typical two-axis lathe motions. X-axis also represents cross travel of the cutting tool, Z-axis represents its longitudinal motion.All varieties of cutting tools are mounted in a turret (a special tool magazine) and can be external or internal. Because of this design, a turret loaded with all cutting tools moves along both X and Z axes, which means all tools are in the work area at all times.
Following the established standards of milling machines and machining centers, the only machine axis capable of making a hole by methods of drilling, boring, piercing or punching, is the Z-axis.
In CNC lathe work, the traditional axis orientation for a horizontal type of lathe is upwards and downwards motion for the X-axis, and left and right motion for the Z-axis, when looking from the machinist’s position. This view is shown in the following three illustrations Figure 3-1, Figure 3-2, and Figure 3-3.
This is true for both front and rear lathes, and for lathes with three or more axes. Chuck face is oriented vertically to the horizontal spindle center line for all horizontal lathes. Vertical lathes, due to their design, are rotated by 90 degree,where the chuck face is oriented horizontally to the vertical spindle center line.
In addition to the X and Z primary axes, the multi-axis lathes have individual descriptions of each additional axis, for example, the C-axis or Y-axis, used for milling operations, using the so called live tooling.
This is the most common type of CNC lathes. The work holding device, usually a chuck, is mounted on the left side of the machine (as viewed by the operator). Rear type, with slant bed, is the most popular design for general work. For some special work, for example in the petroleum industry (where turning tube ends is a common work), a flat bed is usually more suitable. Cutting tools are held in a specially designed indexing turret that can hold four, six, eight, ten, twelve and more tools. Many such lathes also have two turrets, one on each side of the spindle center line.
Advanced machine tool designs incorporate tool storage away from the work area, similar to design of machining centers. Tens and even hundreds of cutting tools may be stored and used for a single CNC program. Many lathes also incorporate a quick changing tooling system.
Three-axis lathe is essentially a two-axis lathe with an additional axis. This axis has its own designation, usually as a C-axis in absolute mode (H-axis in incremental mode), and is fully programmable. Normally, the third axis is used for cross-milling operations, slot cutting, bolt circle holes drilling, hex faces, side faces, helical slots, etc. This axis can replace some simple operations on a milling machine, reducing the setup time for the job. Some limitations do apply to many models, for example, the milling or drilling operations can take place only at positions projecting from the tool center line to the spindle center line (within a machining plane), although others offer off-center adjustments.
The third axis has its own power source but the power rating is relatively lower when compared with the majority of machining centers. Another limitation may be the smallest increment of the third axis, particularly on the early three axis lathes. Smallest increment of one degree is certainly more useful than an increment of two or five degrees. Even better is an increment of 0.1 degree, 0.01 degree, and commonly 0.001 degree on the latest models. Usually, lathes with three axes offer a very fine radial increment that allows a simultaneous rotary motion. Those with low increment values are usually designed with an oriented spindle stop only.
From the perspective of CNC part programming, the additional knowledge required is a subject not difficult to learn. General principles of milling apply and many programming features are also available, for example, fixed cycles and other shortcuts.
By design, a four-axis CNC lathe is a totally different concept than a three-axis lathe. As a matter of fact, to program a four-axis lathe is nothing more than programming two two-axis lathes at the same time. That may sound strange at first, until the principle of a four-axis CNC lathe becomes clearer.
There are actually two controls and two sets of XZ axes, one for each pair (set) of axes. Only one program may be used to do the external – or outside – diameter roughing (OD) and another program to do the inside – or internal – roughing (ID). Since a four-axis lathe can work with each pair of axes independently, the OD and ID can be machined at the same time, doing two different operations simultaneously. The main keys to a successful 4-axis lathe programming is coordination of the tools and their operations, timing of the tool motions and a generous sense of healthy compromise.
For several reasons, both pairs of axes cannot work all the time. Because of this restriction, special programming features such as synchronized waiting codes (typically Miscellaneous Function), the ability to estimate how much time each tool requires to complete each operation, etc., are required. There is a level of compromise here, because only one spindle speed can be used for both active cutting tools, although feedrate is independent for both pairs of axes. This means that some machining operations simply cannot be done simultaneously.
Not every lathe job benefits from 4-axis machining. There are cases when it is more costly to run a job on a 4-axis lathe inefficiently and it may be very efficient to run the same job on a 2-axis CNC lathe.
Six-axis CNC lathes are specially designed lathes with a twin turret and a set of three axes per turret. This design incorporates many tool stations,many of them power driven, as well as back-machining capabilities. Programming these lathes is similar to programming a three-axis lathe twice. Control system automatically provides synchronization, when necessary.
A small to medium size six-axis CNC lathe is popular choice of screw machine shops and industries with similar small parts and large volume applications.