CO-ORDINATE SYSTEM

CO-ORDINATE SYSTEM

Before going further into programming it is necessary to know some fundamental details
regarding directions of tool movement, sign convention followed for directions of tool travel and
axes co-ordinate system, different methods of dimensioning, units of dimensioning system etc.,

1. CO-ORDINATE AXES :
The directions of movement in a machine tool are based on a co-ordinate system allocated
to the axes of motion of the slides.
As in the conventional lathe there is movement of slides in two directions along two axes,
one axis being parallel to the spindle axis ( Z-AXIS) and the other axis perpendicular to the spindle
axis (X- AXIS).
The axis parallel to the spindle axis is called Z-axis. The axis perpendicular to the spindle
axis is called X-axis.
Since the machine co-ordinate system has two axes, X and Z, it is referred to as the X-Z
co-ordinate system

2. DIRECTIONS OF MOVEMENT :
Consider that a tool is positioned in I quadrant of X-Z co-ordinate system.
Conventionally speaking,
Along Z-axis any movement towards the spindle is considered as movement in the
negative direction and any movement away from the spindle is considered as movement in
the positive direction.
Along X-axis any movement towards the operator is considered as movement in the
negative direction and any movement away from the operator is considered as movement
in the positive direction.

TYPES OF CO-ORDINATE SYSTEMS:-

1. Absolute Co-Ordinate system
2. Incremental  Co-Ordinate system

Absolute Co-Ordinate system:-

In the figure, X and Z axes are divided into equal increments on either side of the origin.

To represent a point “P1” which is located at 3 divisions to the right of the origin along Zaxis

and 4 dimensioning above the origin along X-axis, it is written as P1(X4,Z3). The

positive value for X and Z is given because the point is located in the positive direction

with respect to origin in both the axes.

Similarly to represent P2, P2 is written as P2(X5,Z-3) as it is in the positive direction along

X-axis and in the negative direction along Z-axis. Thus all the points are referred to the

fixed origin. The origin is also called as ABSOLUTE ZERO or FIXED ZERO. Since all

the co-ordinate are defined with respect to fixed zero, this system is also called as FIXED

ZERO SYSTEM OF DIMENSIONING

INCREMENTAL SYSTEM 

With incremental system of dimensioning, target point co-ordinates do not refer to the

fixed zero. Target point co-ordinates will be defined with respect to present position.

Assuming that tool is positioned at point P0, if a reference is made for point P1, coordinates

of P1 are defined with respect to point P0.

Co-ordinates of P1 with respect to P0 are X4Z3.

Similarly if a reference is made for point P2, co-ordinates of P2 with respect to P1 are

X4Z5.

Again if a reference is made for P0, co-ordinates of P0 with respect to P2 are X-8Z-8.

Thus incremental system doesn’t have a fixed origin. The origin changes from point to

point. In other words the origin is floating and hence FLOATING ZERO SYSTEM OF

DIMENSIONING.

Co-ordinates of P1 with respect to P0 are X4 Z3

Co-ordinates of P2 with respect to P1 are X4 Z5

Co-ordinates of P0 with respect to P2 are X8 Z8