program planning

PROGRAM PLANNING

The conversion of engineering blueprint to a part program can manually be performed or with the assistance of a high-level computer language.

In both cases, the part programmers determine the cutting parameters, spindle speed and feed, based upon characteristics of the workpiece, tool material and limitations of the machine tool. Therefore, they must have extensive knowledge of machining process and machine tool.

In a conventional machining system, a machine operator guides cutting spindle on the basis of print information on engineering drawing. The successful operation of a machine tool and quality of the component produced depends mostly on the skill of the operator or machinist developed over many years of practice. But in the CNC machines, the drawings are studied and the information about the workpiece is converted into a series of code numbers by the part programmer. Figure 5.7 illustrates the series of steps followed for developing the part program from the drawing and operating the machine tool. Every movement, path or action of the machine tool to machine the workpiece as required is based on the code numbers. The part program is a complete series of codes required to manufacture a workpiece on machine tool using a CNC machine.

1. Part Program

The part program is a set of instructions proposed to get the machined part starting with the desired blank and NC machine tool. Part programming contains geometric data about the part and motion information to move the cutting tool with respect to the workpiece.

Figure 5.8 shows the layout of the part program procedure. This work is carried out by a part programmer. He prepares the planning sheet and writes the instructions in a coded form which is acceptable to the controller of the machine tool.

2. Methods of Creating Part Programming

The following are various methods of creating part programming.

(i) Manual part programming.

(ii) Computer-assisted part programming (CAD/CAM based programming system).

(iii) Manual data input

(iv) NC programming using CAD/CAM

(v) Computer automated part programming.

In manual part programming, the processing instructions are documented on a form called a part program manuscript. The manuscript is a listing of the positions of the tool relative to the workpiece that the machine must follow in order to perform the processing. The listing may also include other commands such as speeds, feeds, tooling, and so on. A punched tape is then prepared directly from the manuscript.

In computer-assisted part programming, much of the tedious computational work required in manual programming is performed by the computer. For complex, the work part geometries or jobs with many processing steps and use of the computer results in significant savings in the part programmer's time. When computer assisted part programming is used, the programmer prepares the set of processing instructions in a high-level computer language. For complex jobs, this computer language is much easier to use than the lower-level coding required in manual part programming. The high-level language commands are interpreted by the computer, and the required calculations and data processing are accomplished to prepare the NC program for the tape reader (or other input device).

Manual data input (MDI) is a procedure in which the NC program is entered directly into the MCU at the site of the processing machine. Consequently, the use of the punched tape is avoided and the programming procedure is simplified to permit machine operators rather than part programmers to do the programming.

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 term CAD/CAM means computer-aided design and computer-aided manufacturing. In this method, the programmer works on a CAD/CAM workstation to enter the machining commands. The actions indicated by the commands are displayed on the graphics monitor which provides the visual feedback to the programmer. Also, certain portions of the programming cycle are automated by the NC programming software to reduce the total programming time required.

Computer-automated part programming extends the notion of automating certain portions of the NC part programming procedure to its logical conclusion. It automates the complete part programming task using software which is capable of making logical and even quasi-intelligent decisions about how the part should be machined.