Computer Numerically Controlled (CNC) Machines:
Introductory Concepts of Machining
Machining is basically the removal of material, most often metal, from the workpiece, using one or more cutting tools to achieve the desired dimensions. There are different machining processes, such as turning, milling, boring, etc. In all these cases metal is removed by a shearing process, which occurs due to the relative motion between the workpiece and the tool. Generally, one of the two rotates at a designated and generally high speed, causing the shearing of material (known as chips), from the workpiece. The other moves relatively slowly to effect the removal of metal throughout the workpiece. For example, as seen above in a turning operation of lathes, the “job”
or the workpiece rotates in a chuck, while the tool moves in two dimensions translationally. On the other hand, in milling, it is the cutter that rotates on a spindle, while the workpiece, which is fastened to a table, moves in X-Y dimensions. While a precise and high-speed rotational motion is needed for a good finish of the machined surface, dimensional accuracy, precise position and velocity control of the table drive are essential.
For all metal-cutting processes, the cutting speed, feed, and depth of cut are important parameters. The figure below shows the important geometry for the turning process. The cutting speed, which is a measure of the part cut surface speed relative to the tool. Speed is a velocity unit for the translational motion, which is maybe stated in or meters/min. The depth of cut, DOC is the depth that the tool is plunged into the surface. Feed defines the relative lateral movement between the cutting tool and the workpiece. Thus, together with the depth of cut, feed decides the
cross-section of the material removed for every rotation of the job or the tool, as the case may be. Feed is the amount of material removed for each revolution or per pass of the tool over the workpiece and is measured in units of length/revolution, length/pass, or another appropriate unit for the particular process.
What is Computer Numerical Control?
Modern precision manufacturing demands extreme dimensional accuracy and surface finish. Such performance is very difficult to achieve manually, if not impossible, even with expert operators. In cases where it is possible, it takes much higher time due to the need for frequent dimensional measurement to prevent overcutting. It is thus obvious that automated motion control would replace manual “handwheel” control in modern manufacturing. The development of computer numerically controlled (CNC) machines has also made possible the automation of the machining processes with the flexibility to handle the production of small to medium batches of parts.
In the 1940s when the U.S. Air Force perceived the need to manufacture complex parts for highspeed aircraft. This led to the development of computer-based automatic machine tool controls also known as the Numerical Control (NC) systems. Commercial production of NC machine tools started around the fifties and sixties around the world. Note that at this time the microprocessor has not yet been invented. Initially, the CNC technology was applied on lathes, milling machines, etc. which could perform a single type of metal cutting operation. Later, an attempt was made to handle a variety of workpieces that may require several different types of machining operations and to finish them in a single set-up. Thus CNC machining Centres capable of performing multiple operations were
developed. To start with, CNC machining centers were developed for machining prismatic components combining operations like milling, drilling, boring, and tapping. Gradually machines for manufacturing cylindrical components, called turning centers were developed.
Advantages of a CNC Machine:
CNC machines offer the following advantages in manufacturing.
• Higher flexibility: This is essential because of programmability, programmed control, and facilities for multiple operations in one machining center,
• Increased productivity: Due to low cycle time achieved through higher material removal rates and low set up times achieved by faster tool positioning, changing, automated material handling, etc.
• Improved quality: Due to accurate part dimensions and excellent surface finish that can be achieved due to precision motion control and improved thermal control by automatic control of coolant flow.
• Reduced scrap rate: Use of Part programs that are developed using optimization procedures
• Reliable and Safe operation: Advanced engineering practices for design and manufacturing, automated monitoring, improved maintenance, and low human interaction.
• Smaller footprint: Due to the fact that several machines are fused into one. On the other hand, the main disadvantages of NC systems are
• Relatively higher cost compared to manual versions
• More complicated maintenance due to the complex nature of the technologies
• Need for skilled part programmers.
The above disadvantages indicate that CNC machines can be gainfully deployed only when the
required product quality and the average volume of production demand it.
Classification of NC Systems
CNC machine tool systems can be classified in various ways such as :
1. Point-to-point or contouring: depending on whether the machine cuts metal while the workpiece moves relative to the tool
2. Incremental or absolute: depending on the type of coordinate system adopted to parameterize the motion commands
3. Open-loop or closed-loop: depending on the control system adopted for axis motion
Point-to-point (PTP) systems are the ones where, either the workpiece or the cutting tool is moved with respect to the other as stationary until it arrives at the desired position and then the cutting tool performs the required task with the motion axes stationary. Such systems are used, typically, to perform hole operations such as drilling, boring, reaming, tapping, and punching. In a PTP system, the path of the cutting tool and its feed rate while traveling from one point to the next are not significant, since, the tool is not cutting while there is motion. Therefore, such systems require only control of only the final position of the tool. The path from the starting point to the final position need not be controlled.
In contouring systems, the tool is cutting while the axes of motion are moving, such as in a milling machine. All axes of motion might move simultaneously, each at a different velocity. When a nonlinear path is required, the axial velocity changes, even within the segment. For example, cutting a circular contour requires sinusoidal rates of change in both axes. The motion controller is therefore required to synchronize the axes of motion to generate a predetermined path, generally a line or a circular arc. A contouring system needs the capability of controlling its drive motors independently at various speeds as the tool moves towards the specified position. This involves simultaneous motion control of two or more axes, which requires separate position and velocity loops. It also requires an interpolator program that generates the position and velocity setpoints for the two drive axes, continuously along the contour. In modern machines there is the capability for programming machine axes, either as point-to-point or as continuous (that is contouring) Before the next type of classification is introduced, it is necessary to present the basic coordinate system conventions in a machine tool.
The coordinate system is defined by the definition of the translational and rotational motion coordinates. Each translational axis of motion defines a direction in which the cutting tool moves relative to the workpiece. The main three axes of motion are referred to as the X, Y., and Z axes. The Z-axis is perpendicular to both X and Y in order to create a right-hand coordinate system, such as shown in Fig. A positive motion in the Z-direction moves the cutting tool away from the workpiece. The location of the origin is generally adjustable. The figure shows the coordinate system for turning as in a lathe while Fig. shows the system for drilling and milling. For a lathe, the infeed/radial axis is the x-axis, the carriage/length axis is the z-axis. There is no need for a y-axis because the tool moves in a plane through the rotational center of the work. Coordinates on the workpiece shown below are relative to the work.
As mention earlier, a part program is a set of instructions often referred to as blocks, each of which refers to a segment of the machining operation performed by the machine tool. Each block may contain several code words in sequence. These provide:
1. Coordinate values (X, Y, Z, etc.) to specify the desired motion of a tool relative to a workpiece. The coordinate values are specified within the motion codeword and related interpolation parameters to indicate the type of motion required (e.g. point-to-point, or continuous straight or continuous circular) between the start and end coordinates. The CNC system computes the instantaneous motion command signals from these code words and applies them to drive units of the machine.
2. Machining parameters such as feed rate, spindle speed, tool number, tool offset compensation parameters, etc.
3. Codes for initiating machine tool functions like starting and stopping of the spindle, on/off control of coolant flow, and optional stop. In addition to these coded functions, spindle speed feeds and the required tool numbers to perform machining in the desired sequence are also given.
4. Program execution control codes, such as block skip or end of block codes, block number, etc.
5. Statements for configuring the subsystems on the machine tool such as programming the axes, configuring the data acquisition system, etc. A typical block of a Part program. Note that the block contains a variety of code words such as G codes, M codes, etc. Each of these code words configures a particular aspect of the machine, to be used during the machining of the particular segment that the block programs.
Interpolation consists of the calculation of the coordinated movement of several axes using the
programmed parameters, in order to obtain a resulting trajectory, which can be of various types,
– Straight line
The interpolation module computes instantly by instant position commands for the servo module, which in turn, drives the motors. There are two types of interpolators, namely:
– Process interpolator (for continuous axes)
– Point-to-point interpolator (for point-to-point axes).
Servo control consists of all the activities which allow several axes to effectively maintain the trajectory calculated by the interpolator. Continuous axes are continuously controlled by the system both for “speed” and for “position” so as to guarantee that the calculated trajectory is maintained. In contrast, for point-to-point axes, there is no guarantee that the trajectory will be maintained. The only guarantee is that the final point will be reached.
Types of servo control for motion axes:
The axes controlled by the axis manager may be divided into various types according to the specific function they perform on the machine tool. Some of these types are described below.
This is a working axis, which may be interpolated along with other axes of the same type. This is necessary for generating specific 2D or 3D contours. The movement of one of the axes can be taken as the master and the other axes slaved to it. The mechanical and electrical features of the slave axis must be identical to those of the master. A coordinated axis can also be rotary and programmed in degrees. Note that for rotary axes, it may or may not be needed to map angular displacements to a (0-2π) interval.
This axis is not required to be interpolated with others, since it is used for only for positioning from one point to another. Such an axis may be viewed as an independent mechanical component fitted with a positioning transducer.
There are two types of spindle axes. For some, only the speed of the axis need to be controlled and not the position by the spindle servo control system. Such an axis essentially realizes a “motorized” tool. For the second type, the speed of this spindle axis, as well as its angular position can be controlled. This has application in controlling threading processes. It is also possible to drive the spindle in coordinated motion, interpolated with the other axes. This uses
the spindle transducer value as the set point for the other axes. A typical example is the C axis in lathes. One can command a controlled acceleration ramp for the spindle rotation command. However, for improved angular positioning, this must be eliminated. It is also possible to have spindle drives without servo control, generally for spindles driven with ac motors. The only control needed in such a case is for reversal of spindle rotation. For control of tool and workpiece motion in the various ways described above, one of two kinds of control systems is employed.
Open Loop Systems:
The term open-loop means that there is no feedback, and in open-loop systems, the motion controller produces outputs depending only on its setpoints, without feedback information about the effect that the output produces on the motion axes. We have already seen that the effects of controller outputs on the plant may not be the same always, since it depends on factors such as loads, parameter variations in the plant, etc. In open-loop systems, the setpoints are computed from the instructions in the Part program and fed to the controller, which may reside in a different microprocessor, through an interface. These motion commands may be in the form of electrical pulses (typical for step motor drives) or analog or digital signals and converted to speed or current set points by the controller. These setpoints, in turn, are sent to the power electronic drive system that applies the necessary voltage/current to the motors. The primary drawback of an open-loop system is that there is no feedback system to check whether the commanded position and velocity have been achieved. If the system performance is affected by load, temperature, or friction then the actual output could deviate from the desired output. For these reasons, the open-loop system is generally used in point-to-point systems where the accuracy requirements are not critical. Contouring systems do not use open-loop control.
Closed-loop control, as described in the module on controllers, continuously senses the actual position and velocity of the axis, using digital sensors such as encoders or analog sensors such as resolvers and tacho generators and compares them with the setpoints. The difference between the actual value of the variable and its setpoint is the error. The control law takes the error as the input and drives the actuator, in this case, the servo motor and its drive system, to achieve motion variables that are close to the setpoints. As we know, closed-loop systems can achieve much closer tracking of setpoints even with disturbances and parameter variations in the system with, say, with temperature. Closed-looped systems, on the other hand, require more complex control as well as feedback devices and circuitry in order for them to implement both position and velocity control. Most modern closed-loop CNC systems are able to provide a very close resolution of 0.0001 of an inch.