Coordinates of Points
Two types of coordinate
systems are used to define and control the position of the tool in relation to
the work piece. Each system has its own applications and the two coordinate
systems may be used independently or may be mixed within a CNC part program
according to the machining requirements of the component. The coordinate
systems used are absolute coordinate system and incremental coordinate
system. In addition, the dimensions can be given in metric system and in inch
system. The two coordinate systems are described in the following paragraphs:
(a) Absolute
Coordinate System. In the absolute
system the coordinates of a point are always referred with reference to the
same datum. The datum positions in the Xaxis, Yaxis and Zaxis are defined by
the user / programmer before starting the operation on the machine. A major
advantage of using absolute system is that it is very easy to check and correct
a program written using this method. If a mistake is made in the value of any
dimension in a particular block, it will affect that dimension only and once
the error is corrected there will be no further problems.
(b) Incremental Coordinate System. In the incremental system
the coordinates of any point are calculated with reference to the previous
point i.e. the point at which the cutting tool is positioned is taken as datum point
for calculating the coordinates of the next point to which movement is to be
made. It is difficult to check a part program written in incremental dimension
mode.
Note. The
difference between absolute system and incremental system can be better
appreciated with the help of component shown below. The co ordinates of points
P1, P2, P3 and P4 in absolute and incremental system are given in the table
below:
Absolute and
Incremental Programming
Point

Absolute system

Incremental System

P1

1,3

1,3

P2

3,2

2,1

P3

4,2

1,0

P4

4,3

0,1

The coordinates of a point
can be given either in inch system or in metric system. In addition, the
position of a point may be defined using the polar co ordinates where the
distance of the point from a specified datum point and the angle from a specified
datum axis give the coordinates of the points.
Most of the CNC/NC machines
are capable to work in the absolute as well as incremental coordinate systems.
While adopting a single system of coordinates will serve to simplify the
programming, the two systems may many times be mixed to save time and avoid
duplication of effort in making the program. It is always convenient to write
the main program in absolute system and subroutines are written in incremental
mode. However, the programmer has to tell the control system whether absolute
or incremental mode and whether system or metric system inch will be used. This
is achieved by giving relevant codes in the program.
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