NC MACHINE PART PROGRAMMING METHODS
The processing tape used for programming can be prepared using any of the several different methods of NC part programming. NC part programming represents one of the elements in the broader procedure called process planning.
Process planning is a function of manufacturing engineering in which the sequence of individual production operations for making a part is planned. The part programming methods include:
- Manual part programming
- Computer-assisted part programming
- Manual data input
- NC programming using CAD/CAM
- Computer-Automated part programming
- Voice NC part programming
MANUAL PART PROGRAMMING
In the more familiar manual part programming, the part processing instructions are written in the form of coded machine language and are documented on a form called the part program manuscript. These instructions describe the positions of the tool relative to the workpiece that the machine must follow in order to perform the processing of the part. On the manuscript, they appear in the form of a tabular listing of data which includes other commands such as speeds, feeds, tooling, and others. A punched tape, magnetic tape or as the case may be, is then prepared from the manuscript directly.
MANUAL DATA INPUT (MDI)
Manual and computer assisted part programming are methods that require a relatively high degree of formalistic documentation and procedure. Also, substantial time is required to;
Write the part program (either using the manuscript or the computer language code)
Punch the tape and validate the program.
To achieve that, two or more persons will be involved i.e. the programmer and the machine operator.
A potential method of simplifying the procedure is to have the machine operator perform the task of programming at the site of the machine tool. This is called manual data input (MDI), because the operator manually enters the program data and commands directly into the machine control unit at the site of the processing machine, without the need of the punched tape. This consequently simplifies the programming procedure, since it permits machine operators rather than part programmers to do the programming. ACRT (Cathode ray tube) and an alphanumeric keyboard further simplify the communication between the operator, programmer, and the machine.
Entering the programming commands into the controller is typically done using a menu-driven procedure in which the operator responds to questions posed by the NC system about the job to be processed. In this interactive mode used facilitate the programming process, the system queries the operator about the details of the machining job so that the operator types in the program, responding to the sequence of questions. The sequence of questions is designed so that the operator inputs the part geometry and tool motion in a logical and consistent manner. A computer graphics capability is often included in the MDI programming system to permit the operator to visualize the machining operations on the work part. A minimum of:
Training in NC part programming
The ability to read the engineering drawing of a part and
A sound knowledge in machining processes is all that is required of the machine operator.
MDI systems are perceived as a way for small machine shops to introduce NC into its operations without the need to acquire the special NC part programming equipment like the tape punch, a separate computer, tape reader, etc. and to hire a part programmer.
The limitations of manual data input are as follows:
There is the risk of programming errors as the job becomes more complicated. For this reason, MDI is usually being applied for relatively simple parts.
Since there is no punched tape to document and save the program, the most economical MDI applications are those in which the batch is made only once. This limitation can be overcome by attaching a storage device like tape puncher and reader, disc drive, magnetic tape cassette drive, etc. to the controller for saving and reading repeatedly, the desired program.
Again, the MDI system can be wrongly used if the machine is always allowed to stand idle while the operator is inputting the programming instructions. Efficient use of the system, however, dictates that the programming of the next part be performed while the current part is being machined. That reduces the changeover tune from one job to the next.
NC PART PROGRAMMING USING CAD/CAM
The term CAD/CAM typically refers to a computer interactive graphics system equipped with the software to accomplish certain functions in a geometrical description in design and manufacturing. The term CAD/CAM actually means computer-aided design and computer-aided-manufacturing.
In the part programming using CAD/CAM, the system is again equipped with NC programming software used to facilitate the part programming tasks. In this method of part programming, the programmer works on a CAD/CAM workstation to enter the machining commands. The actions indicted by these commands are displayed on the graphics monitor, which provides visual feedback to the programmer. Also, certain positions of the programming cycle usually done by the part programmer are automated by the NC programming software, to reduce the total programming time required.
Recall that the two tasks of the programmer in computer-assisted programming are:
Defining the part geometry and specifying the tool path. Advanced CAD/CAM systems have the capability to automate portions of these two tasks.
COMPUTER AUTOMATED PART PROGRAMMING
In the CAD/CAM approach to NC part programming, several aspects of the procedure had been automated. In the future, it should be possible to automate the complete NC part programming procedure, when this is done, it can be referred to the fully automated procedure as computer-automated part programming. Given the geometric model of a part that has been defined during product design, the computer automated system would possess sufficient logic and decision-making capabilities to accomplish NC part programming with no human assistance.
This can most readily be done for certain NC processes that involve well-defined, relatively simple part geometries. Examples are in point to point operation such as drilling, wire-wrap machines, and electronic component assembly machines. In these processes, the design data can be processed to generate NC program for the particular system.
VOICE NC PROGRAMMING
Voice programming of NC machines, abbreviated VNC involves vocal communication of the machining procedure to a voice input NC tape preparation system. NC allows the programmer to avoid steps such as writing the program by hand, key-punching or typing and manual verification.
To perform the part program process with VNC, the speaker speaks into the headband microphone designed to reduce background acoustical noise. Communication of the programming instructions is done in shop language with such terms as “turn”, “thread”, “mill line”, etc., together with numbers to provide dimensional and coordinate data.
Before the voice input system can be used, it must be trained to recognize and accept the individual’s voice pattern. This is accomplished by repeating each word of the vocabulary about five times, to provide a reference set, which can subsequently be compared to the voice commands given during actual programming.
In talking to the system, the programmer must isolate each word by pausing before and after the word. The duration of the pause must be at least one-tenth of a second. This allows the speech recognition system to identify boundaries for the uttered command so that its wave characteristics can be compared with the words in the reference set. Typical word input rates under this restriction are 70 words per minute.
As the words are spoken, a CRT terminal in front of the operator verifies each command and prompts the operator for the next command.
One of the principal companies specialized in voice input systems is Threshold Technology INC., of Delran, New Jersey. The entire vocabulary for the Threshold system contains about 100 words. Most NC programming job can be completed by using about 30 of these vocabulary words. When all the programming instructions have been entered and verified, the system prepares the punched tape for the job.
