Programmable Automation Technologies. Daniel Kandray
accomplish. Cutting complex surfaces may require movement in the direction of all three axes. However, such an operation is not possible on a traditional manual mill; numerical control technology was developed to specifically address this limitation.
During the 1940s a contractor to the U.S. Air Force by the name of John Parsons began experimenting with methods to produce more accurate inspection templates for helicopter blades. The inspection templates were a complex airfoil shape. Machining these shapes accurately was a challenge. Parsons’ method involved calculating points along the airfoil’s shape and then, using two operators (one for each axis), manually moving the machine tool to each of these points. Because the calculations were so complex, Parsons used a punch card tabulating machine to perform the calculations. The punch cards would be fed into a card reader at the machine, which would read the data, then pass the information on to a machine controller, which in turn directed the motion of each of the machine axes.
Figure 3-2 Exploded view of manual vertical milling machine
Figure 3-3 View of dovetail slides of manual vertical milling machine
Figure 3-4 Manual machine with power-driven x-axis
This method, although much more accurate than manual machining, was still very time-consuming. However, the Air Force was much impressed and awarded Parsons a contract to develop a machine to provide automated control of the axes.
Parsons’ machine concept is, fundamentally, the system that all-modern CNC equipment uses today. However, today’s systems are substantially upgraded due to the rapid development of computer technology. Punch cards were replaced by magnetic tapes, which in turn were replaced by electronic files. These electronic files, or “part programs,” are now either created directly at the machine or developed offline at a separate computer. Figure 3-5 shows an older numerical control—or “NC”—machine, which used programs stored on magnetic tape. Note the size of the tape reader/machine controller. Drive motors for the axes are also visible. Prior to the advent of computer technology most machines were referred to as “NC machines.” They were hardwired using vacuum tubes, transistors, and relay technology. In the 1970s and 1980s, microcomputers replaced the aging hardwired technology. These machines were called computer numerical control, or CNC, machines. However, NC is still often used interchangeably with CNC. Figure 3-6 shows a modern CNC machine center.
Figure 3-5 Vintage numerical control (NC) machine
Figure 3-6 Modern computer numerical control (CNC) milling machine
CNC technology has been applied to numerous machine tools, including lathes, mills, electric discharge machines (EDM), and flame, laser, and plasma cutting machines. Non-machine tool examples include coordinate measuring machines (CMM), component insertion machines (assembly machines), wire-bending machines, and polymer composite filament winding machines. Figure 3-7 shows a numerical controlled wire-bending machine. Essentially, any type of processing equipment that needs to move a tool relative to a workpiece is a prime candidate for CNC technology. Thus, a CNC system can be broken into four major components:
1. Processing equipment/machine tool
2. Drive mechanism/positioning system
3. CNC controller
4. Program of instructions.
Figure 3-7 CNC wire-bending machine
3.2.1 Processing Equipment/Machine Tool
For processing equipment in general, the workpiece-tool relative motion is executed in two or three directions; however, as many as five different directions can be controlled in modern equipment. Each direction corresponds to an axis of the machine. A standard CNC mill has three axes: two horizontal axes corresponding to the x,y-plane and a vertical axis for movement of the spindle corresponding to the z-axis. The axis with the longest travel is generally labeled “x-axis.” Another standard in the machine tool industry is the correspondence of the axis of the machine’s spindle with the z-axis. Hence, for a lathe, the tool motion is specified in the direction of the x- and z-axes. The tool moves in the x direction, in and out of the workpiece orthogonal (at right angles) to its rotational axis. This is shown in Figure 3-8.
Figure 3-8 CNC lathe with axes labeled
Additional axes are often added to mills in the form of multiaxis rotational tables, or fixturing, resulting in four or five axis machines. This is discussed in greater detail in later chapters. As mentioned previously, any type of processing equipment where controlling the location of the workpiece relative to the location of a tool is important is a prime candidate for CNC control.
3.2.2 Drive Mechanism/Positioning System
Movement along the various axes of the machine is accomplished with mechanically guided high precision linear bearings, called slides or ways, and a lead screw. A carriage or table is moved along the slide (or axis) by the lead screw. The lead screw transforms rotational motion from an electric motor into linear movement along an axis. Early machines used hydraulic motors controlled with servo valves, an approach still used for very large machines.
The majority of modern CNC tools have electric servomotors to drive the lead screw. Figure 3-9 shows a cutaway view of a typical servomotor closed loop system, for controlling one axis. In order to perform complex contouring in the x,y-plane, two axes are required, one for each direction. This is accomplished by mounting one axis control system on top of another. This is evident in Figure 3-9. One axis control system moves the table across the machine, the other moves the saddle, and hence the table, in and out of the machine. By adding vertical axis control to the spindle, complicated three-dimensional shapes can be machined. Orchestrating and synchronizing the motion in all three directions is the task of the CNC controller.
Figure 3-9 CNC mill servodrive components
3.2.3 CNC Controller
The CNC controller directs all machine functions while executing the program of instructions. It interfaces with the machine operator during machine and tool setup. It selects the desired tooling and positions the workpiece in the correct location relative to the tool. It turns on spindle and coolant flow and moves the workpiece and/or tool along the correct tool path at the specified feed rate, often directing the motion along three axes simultaneously. It can also make decisions and instruct other machines to perform a specified task. For example,