The basic components of an CNC control system are the program of instructions (also called the part program or G-code), the controller, servo drives for each axis of motion, and feedback devices for each axis of motion. The controller commands the servo drives to move the machine axes to drive the tool along the tool path specified in the part program. Both linear and rotary motions can be precisely controlled simultaneously.
NC systems can be classified by the method used to control machine slides, the number of axes, positional information, the feedback mode, the interpolation method, or the data format.
Machining centers with three, five, seven, or more axes, which can generate very complex surfaces that cannot be produced with conventional machines, are available.
The methods available for controlling the relative motion of axes are the following:
(1) point-to-point (usually for two-axis machines such as drilling machines with single- or dual-axis control);
(2) straight cut (control along a path parallel to a linear or circular machine way); and
(3) continuous path or contouring (continuous control along a path in two or more axes).
An increasing level of control sophistication is required as we move from 1 to 3..
The positional information is either absolute (predetermined or fixed datum) or incremental (referenced from the current position).
Absolute control systems are closed-loop systems, which rely on angular encoders or linear displacement encoders to determine absolute axis positions; feedback from the encoders is continuously compared to a reference value by a microprocessor, which adjusts the slide speed to eliminate deviation between the absolute position and the reference value. Incremental control systems can be either closed or open-loop systems. An open-loop incremental control system operates without feedback; a stepping motor is used as an actuator that receives the number of pulses corresponding to a specified displacement directly from the controller.
An open-loop control system has better dynamic characteristics than a closed-loop system but does not provide positional verification. Closed-loop systems with dynamic error compensation are required for high-speed contour milling. The communication rate for each individual axis processor for the servo interface can be the determining factor for the maximum cutting rate.
All controllers offered by various manufacturers were proprietary. But, there has been a recent trend toward open-architecture controls. Often, open control means using PC front ends and interfaces to proprietary machine tool controls that have full connectivity via standard networking and communications protocols. Open controls are enabling machine tools to take advantage of the latest software, networking, and operating system technologies, and to communicate with open-architecture factory automation software, resulting in more flexibility. The controller now communicates with ancillary equipment controls or an on-board programmable logic controller (PLC).
Types of CNC Machines
There are two general classes of CNC machine tools: CNC lathes or turning centers, which perform
turning, boring, facing, threading, profiling, and cutoff operations, and CNC machining centers,
which are used primarily for milling, boring, drilling, and tapping.
CNC Lathes
In most CNC lathes, tools are held in a turret, which rotates to bring a specific tool to bear. All CNC lathes have a headstock with spindle, and most have a tailstock. Turning centers usually used quick-change modular tooling.
In addition to standard CNC turning centers, there are a number of advanced types with additional capabilities. CNC automatics are similar to turning centers but include more axes, rotating tooling (live tooling), and multiple slides and spindles. These machines are also called multifunctional machines or mill-turn machines when they are equipped with live spindles in the turret. On CNC automatics, a job can be divided into segments so that many tools can work on different areas of a workpiece simultaneously. Cycle times can thus be shorter than on CNC lathes, and idle time for part setup and handling is often reduced. They are especially useful for finishing smaller parts with limited machined content in a single setup.
CNC lathes and automatics are modern equivalents of cam-driven screw and bar machines,
which they have largely displaced.
Machining Centers
Machining centers are usually classified by spindle orientation (vertical or horizontal) and the number of axes controlled.
On a vertical spindle machine, the workpiece is mounted on a horizontal bed; on a horizontal spindle machine, the workpiece is usually mounted on a vertical fixture or table, which is more compliant.
Vertical machines are preferred for large workpieces, flat parts, and especially for contoured surfaces in dies so that the thrust force is absorbed directly by the bed of the machine. Vertical gantry or bridge-type milling machines are used for very large workpieces because their two-column design gives greater stability to the cutting spindle(s).
Horizontally configured machines are more versatile because four sides of the workpiece can be machined without re-fixturing if a rotary indexing worktable is available. Horizontals are finding increasing use in surface machining, since they provide increased access on larger complex parts and have less restriction on vertical height of the workpiece. Horizontal machines are preferred for untended use since they allow for easy chip and coolant evacuation. In North America, horizontal machines are often preferred in high-volume applications for increased ease and safety of maintenance. Horizontal machines are also often preferred for dry or minimum quantity lubrication (MQL) applications since they can be adapted to eliminate chip accumulations in the work zone more easily.
Universal machines have heads that rotate to act as a horizontal or a vertical machine. The combination of tilts and swivels available in the spindles and tables allows the workpieces to be addressed at various compound angles.
The common nomenclature for axes and rotations.
The primary axis directions on the machine are designated by the Cartesian coordinates X, Y, and Z. The corresponding rotary axes are A, B, and C. Secondary linear axes aligned to X, Y, and Z, often on a table, are called U, V, and W or X′, Y′, and Z′. The Z-axis is commonly aligned with the spindle, whether the spindle is horizontal or vertical. On a horizontal machine, the Z-axis may correspond to motion of a spindle carried in a column, and the W-axis may refer to motion of a table toward a stationary spindle or the extension of a quill from a stationary spindle.
Conventional three-axis machines most commonly have a vertical spindle and three linear axes
(X, Y, Z) but may have two linear and one rotational axis. Horizontal spindle three-axis machines are sometimes used for drilling, milling, and tapping large workpieces.
Four-axis machines typically have three linear axes and a rotational axis on the work table. Horizontal spindle machines often have a B-axis table. Horizontal spindle four-axis machines with A-axis tables or trunnions are used in dry and MQL machining applications since they permit the part to be machined upside down to clear chips by gravity. Vertical spindle four-axis machines commonly have an A-axis trunnion.
Five-axis machining centers are used for contour surface machining on components such as
molds, dies, and airfoils and for positioning on workpieces requiring machining on multiple sides
or at compound angles. Five-axis machining is essential for the first class of applications and often
provides reduced cycle times and increased accuracy for the second. Five-axis machines have three
linear and two rotational axes, with rotations being performed by a table, the spindle, or both. They
are often built up by adding rotary axes to three-axes horizontal or vertical machines. These
configurations are well suited to machining smaller parts, and the choice of a particular configuration
depends on the workpiece dimensions and orientation, the required axis motions, fixturing, and the
available base machine tools. Axes may also be added to the spindle, using a fork and swivel mechanism or a nutating head. This approach is common on large machines, since
it is often not practical to precisely rotate large workpieces.
Hybrid machining centers combine the functions of turning centers and conventional machining
centers, providing the ability to complete all machining operations for many classes of parts in a
single setup. In one example, a tilting milling spindle is used to enable both horizontal
and vertical machining operations as well as boring and milling on multiple faces. This type of
machine can perform turning, milling, drilling, contouring with the C-axis, off-center machining with the Y-axis, milling of angled surfaces with the B-axis, grinding, and other operations.
Such machines may be called multitasking turning centers because in addition to the traditional
X- and Z-axes, they incorporate the Y-axis and rotary C- and B-axis for tilting the turret. As with
the mill-turn machines discussed earlier, such machines can reduce cycle/lead times and work-in process inventory, save up setup and queue time, and potentially improve part quality by eliminating refixturings.
The capabilities of machining centers are characterized by maximum spindle RPM, power, and torque versus speed curves, spindle size, and toolholder adaption, axis drive motor power, rapid feed rate, fastest cutting federate, structural properties (stiffness, damping, etc.), workspace size, and support for networking.
High-speed machining centers (HSMC) operate at spindle speeds of 20,000–40,000 rpm,
have high-acceleration/-deceleration (acc/dec) spindles (i.e., 1.5 s from 0 to 20,000 rpm), high speed (>200 m/min) and high-acc/-dec (>2 g) slides, and high-speed control systems. High-speed
machines usually offer lower torque at high speeds than conventional machines at lower speeds
(i.e., in one case, a 30,000 rpm/20 kW specification yields 6.4 N m torque available at 30,000 rpm
and 29 N m at 2,500 rpm), and therefore, the allowable depth of cut is often reduced at higher
speeds. Some manufacturers offer higher torque, lower-speed spindles for hard metal machining as
an option on base high-speed machines. HSMCs have tighter requirements with respect to machine
tool characteristics, spindle, toolholder, and cutting tool interfaces and balance, vibration charac teristics, and control systems. Spindle, toolholder, and cutting tools have special requirements for coolant application to ensure the coolant is effective at the cutting zone. Dry machining is preferable when feasible in HSMCs to avoid high-pressure and high-volume coolant supply systems possibly generating unbalance at higher speeds. High-speed and high-precision machines require highly rigid feed drive systems and high-speed interpolation control.
In HSMCs, the machining time for one part feature may be significantly lower compared to a conventional machine. The machining time is typically one-third of the total time, with the remainder being used for machine travel, tool changes, spindle acceleration/deceleration, and pallet changes and rotations. As a result, high production rates are best achieved by reducing the noncutting time, and HSMCs permit reduction of noncutting time components due to their fast acceleration capabilities. Continuously raising the cutting speed proves cost-effective in just a few applications, such as aerospace applications in which parts are machined from billets or rough forgings.
In an example time study comparison of a standard CNC machine, a conventional (STD)
HSMC, and an advanced HSMC in machining an aluminum automotive part, the positioning time was found to be the largest contributor to productivity improvement, with the cutting time the second contributor. It shows importance of reduction of time in tool changes automatically in the machine.
Drive dynamics and the dynamic characteristics of the machine structure are also important in
HSMCs. High-speed machining or five-axis machining requires control with look-ahead capability,
acceleration and deceleration control, and collision detection. Look-ahead capability allows the
CNC to read ahead a certain number of blocks in the program, in order to slow down the feed rate
at anticipated sudden tool path direction changes. Nurbs interpolation has been used to interpolate
the tool path so that the control system can change direction along the curve more gradually using
a high average feed rate.
CNC machines allow for operations to be combined using combination tooling since variable
speeds and feeds are available. High-speed interpolation capability also increases tool flexibility
and can lead to reduced tool counts.
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