Numerical Control Method and Coordinate System

Numerical Control Method:

To utilize numerical control in manufacturing,the following steps must be accomplished :
1. The engineering drawing of the workout must be interpreted in terms of manufacturing processes to be used. We assume that a portion of this processing is to be performed on as NC machine.
2. The part programmer plans the process for the portion of the job to be done on NC.
3. A punched tape is prepared from the part programmer’s process plan.
4. The tape is checked or verified.
5. The job is produced on an N.C. machine under tape control.

These methods are called as NC procedure. The part programmer is knowledgeable  about the machining process and has also been trained to program for numerical control. It is his resposibility
to plan the sequence of machining steps to be performed by NC and to write these down in some special format. There are two ways to program for numerical control :

  •  Manual Part Programming
  • Computer Assisted part programming

In manual programming, the machining instructions are prepared on a document called a part program manuscript.  Basically, the manuscripts is a listing of the realtive cutting tool/ workpiece positions which must be followed in order to machine the workpiece. A punched tape is then prepared directly from the part programmer manuscript.

The Coordinate system and Machine Motions:

In computer-assisted part programming, much of the tedious computational work is transferred to the computer. For complex workpart geometries or jobs with many machining steps, use of computer results in significant savings in part programming time.  The part programmers work consists of typically two tasks. First he must define the configuration of the workpart in terms of basic geometric elements, like lines, points, planes, circles, and so on. Second he must direct the cutting tool to perform the machining steps along these geometric elements:drill the holes, mill the surfaces flat. the part programmer accomplishes these tasks by using special programming language. The computer then interpretes the program and perform the various calculations neccessary to prepare the punched tape for the punched tape for NC machine tool.
After the punched tape has been prepared a method is usually provided for checking the accuracy of the tape. Sometimes the tape is checking the accuracy of the tape. Sometime the tape is checked by running through a computer program which plots the various tool  movements(or table movements)on paper. in this way major errors in the tape may be discovered.  The acid test of the tape involves involves trying it out on the machine tool to make the part. programminfg errors are not uncommen,and it may requireabout three atempts befor the tape is correct and ready to use.
The final step in the NC procedure is to use the NC tape in the production. This involves ordering the raw workparts,specifying and preparing the tooling and any special fixturing that may be required and setting up the NC machine tool for the job. The machine tool operater function during production is to load the raw workpart in the machine and establish the starting position of the cutting tool with respect to the workpiece. The NC machine then takes over and controls the rest of the operation. When the operation is complete the operater removes that part from the machine and loads new part.
It is customary when manufacturing a batch of parts for the quality control inspector to throughly check the first part for conformance to the engineering drawing. After the accuracy of this initial part has been verified, production of the remaining parts is been completed with relative assurance that the first part will be duplicated in the subsequent cycles.

In order for the part programmer to plan the sequence of position and movements of  the cutting tool relative to the workpiece, it is necessary to establish a standar axis systemby which relative position can be specified. Using an NC drill press as an example, the drill spindle is fixed in vertical position and the table is moved and controlled relative to the spiindle. However to make things easier for the programmer we adopt the viewpoint that the workpiece is stationary while the drill is moved relative to it. According , the coordinate system of axes is established with respect to the machine table. Two axis are defined as shown in figure.

Two axes x and y are defined in the plane of the table . The z axis is defined in the plane perpendicular to the  table and the movement in the z direction is controlled by the vertical motion of the spindle.  The positive and negative directions of motion of the cutting tools are relative to the table along these axes. NC drill presses are classified as eother two axis or the three axis machines,depending on whether or not they have the capability to control the z-axis.
A numerical control machine and similar machine tools use the axis system similar to the drill system. However in addition to the three linear axes, these machines may posses the capacity  to control one or more rotational axes. These axes are used to specify the angle about the x, y and z axes respectively . T o distinguish positive from negative angular motions the right hand rules can be used. Using the right hand with thumb pointing in the direction of positive linear axis direction the fingers of the hand are curled to point the positive rotational direction. For turning operations two axes are required to command the movement of the tool relative to the rotational waorkpieceThe arrangement is illustrated in figure.

 

Other features of the Location system
The purpose of the coordinate system is to provide a means of locating the tool in relation to the workpiece. Depending on the NC machine the part programmer may have several different options available for specifying this location.

FIXED ZERO VERSUS FLOATING POINT ZERO:

The programmer must determine the position of the tool relative to the origin (zero point) of the coordinate system. NC machines have either of two methods for specifying the zero point. The first possiblity is for the machines to have fixed zero. In this case the origin is always located at the same position is for the machine table. Usually that position is the southwest corner of the table and all the tool locations will be defined by positive x and y coordinates.
The second and more common feature on modern NC machines allows the machine operator to set the zero point at any position on the machinetable. This feature is called floating point zero. The part programmer is the one who decides where the zero point should be located.  The decision is based on part programming convienience. For example the workpart may be symmetrical and the zero point should be established at the center of symmetry.

ABSOLUTE VERSUS INCREMENTAL POSITIONING:

Another option sometimes available to the part programmer is to use either an absolute system of tool positioning or an incremental system. Absolute positioning means that the tool locations are always defined in relation to the zero point. If a hole is to drilled at the spot that is 8 inches above the X-axis and 6inches to the right of the y-axis the coordinate location of the hole would be specified as x=+6.000 and y=+8.000. By contrast incremental positioning means that the next tool locations must be defined with reference to the previous tool location.

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