The N.C. 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 :
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.
The Coordinate system and Machine Motions
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.
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.