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Chapter 2 Numerical Control. The Beginnings of CAM

Contents

2.1 Numerical control machines

2.1.1 Basic components of NC

2.1.2 Coordinate system in NC

2.1.3 Programming methods

2.1.4 Machine tool applications

2.1.5 Machine tool technology

2.1.6 Economics of NC

2.2 NC Part programming

2.2.1 The NC tape

2.2.2 Methods of NC part programming

2.2.3 NC part programming using CAD/CAM

2.2.4 Computer-automated Part programming

2.1 Numerical control machines Numerical control machines or NC machines can be defined as a form of programmable automation in which the process is controlled by numbers, letters and symbols. These numbers, letters and symbols are coded in an appropriate format to define a program of instructions for a particular workpart or job. This code can be changed or altered when a different job is to be performed.
   The applications of numerical control range over a wide variety processes. We divide the applications into two main categories:
1. machine tool applications such as drilling, milling, turning and other metal working
2. non-machine tool applications such as assembly, drafting and inspection.
2.1.1 Basic components of NC There are three basic components constituting an NC system:
1. Program of instructions
2. Machine control unit
3. Processing equipment
   The program of instructions is the detailed step by step commands that direct the processing. The code is consisted of commands referring to instructions giving the position of the machine spindle or cutting tool with respect to the part. The most common medium for the code to be stored was an 1-in wide punched tape, however, today in computer numerical control machines the program is stored on tapes, floppy discs and being transferred through communications cables.
    The machine control units consists or electronic and control hardware responsible for reading and interpreting the code and convert it to mechanical motion
    The processing equipment is the component that performs the useful work and performs the machining operations. It consists of the worktable and spindle as well as the motors and controls needed to drive them.

2.1.2 Coordinate system in NC In NC machines in order to make things easier for the part programmer, the viewpoint is adopted that the workpiece is stationary while the tool is moved relative to it.
   Two axes x and y, are defined in the plane of the table as shown in figure 2.1. The z axis is perpendicular to this plane and movement in the z-axis is controlled by the vertical motion of the spindle. The positive and negative directions are also shown on figure 2.1. NC drilling presses are classified as two or three axis machines depending on there capability to move the spindle or not. However, machines such as milling, presses etc. are classified as three axis machines.
   In additions machines such as milling or boring drill posses the capability of rotating of one or more axis as these are specified in figure 2.1 as a, b, and c rotational axes.
   For turning operations, two axes are normally needed to command the movement of the tool relative to the rotating workpiece. The z axis is the axis of rotation of the workpart and the x axis defines the radial location of the cutting tool. This arrangement is illustrated in figure 2.2.

FIGURE 2.1 Machine tool coordinates system for NC

FIGURE 2.2 x- and z- axes for NC turning

2.1.3 Programming methods There are basically two methods of programming or positioning:
1. Absolute programming
2. Incremental programming
   The absolute programming means that the tool locations are always defined in relation to the zero point
   The incremental programming means that the next tool location must be defined with reference to the previous tool location.
   It is must be noted though that in cases of manual part programming the absolute method should be preferred because of the fact that when incremental programming is used and an error occurs in the coding of the part program the error will be transferred throughout the program. As a result the program must be rewritten in contrast with absolute programming where only the line with the error should be corrected.
   The difference between the two methods is being illustrated in figure 2.3.

FIGURE 2.3 Absolute versus incremental programming

2.1.4 Machine tool applications The most common application of numerical control is for machine tool control. This was the first application of NC machines and today is the most important commercially. Metal machining is the most common application of NC machines. By machining we mean the process by which the geometry of the work is changed by removing excess metal. The material is removed by means of the relative motion between the cutting tool and the workpiece. By controlling the action of the tool against the work, the desired geometry is created.
   This kind of metal work is considered to be the most versatile production process because it can be used to create a wide range of shapes and surface finishes and also can produce at high production rates and at low-cost finished products.
   There are five basic types of machining processes which are the following:
1. Turning
2. Drilling
3. Shaping
4. Planing
5. Milling
6. Grinding
   For each of this machining methods there are certain parameters that have to be specified prior to work and are called cutting conditions. Although for each process there are certain differences the most common parameters encountered in almost all of them are the speed, feed and depth of cut. These cutting conditions are illustrated in figure 2.4 for a turning operation.

FIGURE 2.4 Cutting conditions for a turning operation

2.1.5 Machine tool technology Each machining process required, must be carried out on machine tool which it’s task is to perform that certain operation. For this reason NC machine tools have been developed to satisfy nearly all machining processes. The list of NC machines includes:
1. Drill presses
2. Milling machines, vertical and horizontal spindle
3. Turning machines, both horizontal and vertical axis
4. Horizontal and vertical boring mills
5. Profiling and contouring mills
6. Surface grinders and cylindrical grinders
NC machines have also been developed for other metal working processes
7. Punch presses for sheet metal hole punching
8. Presses for sheet metal bending
In more recent years, however, NC machines were to be highly automatic and capable of combining several operations in one setup. This eliminated the need or requiring several machine tools which have been replaced by the one mentioned before. These changes are best exemplified in a new type of machine, the machining center. A machine center is a machine capable of performing several different machining operations on a workpart in one setup, under program control. The machining operations that can be found on a machine center are milling, drilling, reaming, tapping, boring, facing and other similar operations. Of course all these operations where effectively used by the machine center due to its new features which included:
1. Automatic tool-changing capability. Due to the fact that a variety of operations were performed on the same machine, a variety of tooling was needed for this operations. The machine center possesses the feature of automatic tool change. The tools were kept on a magazine or drum and when a specific tool was needed the machine exchanged the tool. This function is automatic and takes place when the program asks for a tool change.
2. Automatic workpart positioning. The machine center has also the ability to rotate the workpart in order to permit the tool to access four surfaces of the part.
3. Pallet shuttle. Another feature of a machine center is that it has two or more separate pallets that can be presented to the cutting tool. This offers the possibility of workpart unloading and loading by a worker while machining takes place on the other pallet.
A machine center is illustrated in figure 2.5.

FIGURE 2.5 NC machining center with five axis control and pallet shuttle showing complex workpart being machined.

2.1.6 Economics of NC A number of reasons come together to justify the extensive use of NC machines in the metal working industry. It has been estimated that 75% of manufacturing is being carried out in lot sizes of 50 or less and such small sizes are the typical applications for NC. The advantages of NC when utilized in this small batches are:
1. Reduced nonproduction time
Although the NC machine cannot change the basic metal-cutting processes it can increase the time that the machine is performing useful work. This is achieved by decreasing the nonproductive time by means of fewer setups, less setup time, reduced workpiece handling time, automatic tool changes on some machines and so on.
2. Reduced fixturing
Simpler fixtures are needed because the positioning is done by the program than the jig or fixture.
3. Greater manufacturing flexibility
NC adapts better to changes in jobs, production schedules and so on.
4. Easier to accommodate engineering design changes on the workpiece
In case of change in the design of the workpiece the program can be altered very easily to satisfy the new need instead of modifying a complex fixture.
5. Improved accuracy and reduced human error
NC is ideal for complicated parts where the chances of human mistakes are high.
2.2 NC Part programming Numerical control machines, as said before, require or need a program to perform work. This programs are called part programs and the part programming is concerned with planning and documentation of the sequence of processing steps to be performed on the NC machine. However, planing of the program requires knowledge of machining, geometry and trigonometry. The sequence of steps which constitutes the part program govern the movements of the processing head with respect to the machine table and workpart.
As it was mentioned before there are many ways for the storage and transfer of the part program to the NC’s MCU (machine control unit). The traditional input medium for over 30 years was the 1-in wide punched tape. However, more feasible ways have been discovered from the evolution of technology such as magnetic tapes or floppy discs. A floppy disc can store the information that exist in several thousand feet of punched tape. Furthermore more direct communication systems are now being used in controlling NC machines. These will be discussed later in the DNC systems.

2.2.1 The NC tape The NC tape was the most common communication medium used in the past for string and transferring information. The punched tape used for NC is 1-in wide. It is standardized as shown in figure 2.6. Despite the low cost of the paper tape, it is not durable thus cannot be used repeatedly. Stronger materials have been developed though for higher production use. The holes were punched either manually with the use of an appropriate typewriter-like machine called the Flexowriter or with the use of the computer.

FIGURE 2.6 Numerical control punched tape format as standardized by the Electronics Industries Association (EIA).

2.2.2 Methods of NC Part programming The part program can be prepared for submission to the MCU using any of several different methods of NC Part programming.
   The part programming methods include a variety of procedures ranging from highly manual to highly automated.
1. Manual part programming
2. Computer-assisted part programming
3. Manual data input
4. NC programming using CAD/CAM
5. Computer-automated part programming
   In manual part programming the processing instructions are documented on a form called a part program manuscript. This is a listing containing the positions of the tool relative to the workpiece in order to perform the required task. This listing can also include other information referring to the cutting conditions such as speeds, feeds etc.
   In computer-assisted part programming much of the computational work required in the manual programming is done by the computer. In cases of complex parts this results in a substantial saving of the part programmers time. In computer-assisted part programming the part programmer prepares the set of instructions on a high level computer language. The computer then takes this high level program and performs the required calculations and the data processing in order to prepare the part program.
   Manual data input (MDI) is a procedure entered directly into the MCU at the site of the processing machine. This method eliminates the use of the punch tape since the program is directly entered into the machine. Also the programming is simplified to allow machine operators to do the programming instead of using part programmers.
   NC part programming using CAD/CAM is an advanced form of computer-assisted part programming in which an interactive graphics system equipped with NC programming software is used to facilitate the part programming task. The part programmer works on a CAD/CAM workstation and it provides the machining commands. The programmer has visual feedback on a graphics screen. In addition some programming cycles are automated to reduce the total programming time.
   Computer-automated part programming automates the complete part programming task using software that is capable of making logical and even quasi-intelligent decisions about how the part should be machined.
   Since the scope of this project is not the part programming in any manual method or even semi-automatic the manual programming method, the computer-assisted method and the manual data input method are not going to be discussed any further. All three methods mentioned involve a form of high degree manual task to be performed in order to compose the part program. In the two following sessions of this chapter we will further discus NC part programming using CAD/CAM and computer-automated part programming.
2.2.3 NC part programming using CAD/CAM As discussed in the previous chapter CAD/CAM deals with both design and manufacturing. It is very possible that one of the possible functions of the system is NC part programming. The two tasks required by the part programmer in the computer-assisted part programming was to define the part geometry and specify the tool path. Advanced CAD/CAM systems have the capability to automate portions of both of these tasks.

Geometry definition using CAD/CAM
On a CAD/CAM system the designer performs the design of each part on the computer and as a result the geometry definition of each part is stored in the CAD/CAM database.
The information stored in the database by the design department refers to all geometric, dimensions and material specifications of the part. It is logical then that the part programmer can access the database and recall the geometric definition of the part that is about to be programmed for machining or any other treatment will be subjected to. Therefor the part programmer can save time by skipping the task of defining geometrically the object and proceed directly to the tool path generation.

Tool path generation using CAD/CAM
The second task of the part programmer is the tool path specification. The first part of this though is to select the suitable cutting tool to be used. In CAD/CAM systems tool libraries usually exist with all the tool available currently in the factory. If the tool that is going to be used exists in the tool library, it is selected and the part programmer moves forward to the tool path generation. In the case, however, that the required tool for the operation doesn’t exist, the part programmer can give the specification of a new tool which will be automatically stored in the tooling library and also it will notify the tooling department to construct or purchase the specific tool. Once the cutting tool is selected the tool diameter and other dimensions can be entered automatically for tool offset calculation.
Tool path generation can be done in two basic ways. The first involves the use of interactive graphics system to enter the motion commands one by one, similar to computer-assisted part programming. The second and most advance approach to the issue, is by the use of automatic software routines that might be available on the CAD/CAM system. These are designed as subroutines in the NC programming package that can be called and with the required parameters given to execute the machining cycle.

2.2.4 Computer-automated Part programming Computer automated part programming refers to the most advanced method of part programming. In this highly advanced system the computer will be able to accomplish NC part programming given only the geometric model of the part. It should be pointed out that the software will have the sufficient logic and decision making capability to do so. In addition in this highly advanced system the software will even be able to make suggestions about tooling, speeds and feed rates.
These systems are considered to be the most advanced and further improvements are about to be performed in the future which will enable this system to run unattended without human supervision.

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