CNC | Axes Coordinate 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…,

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 and the other axis perpendicular to the spindle
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.

Machine Origin

It is considered that the spindle nose front face is lying on X-axis and spindle axis as Zaxis
thus the intersecting point of the two axes is the machine origin.

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.

Methods of Dimensioning

There are two methods of dimensioning;
a. Absolute system of dimensioning or fixed zero system of dimensioning
b. Incremental system of dimensioning or floating zero system of dimensioning

Absolute system of dimensioning or fixed zero system of dimensioning

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 Z axis 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 called as FIXED ZERO SYSTEM OF DIMENSIONING.

Incremental system of dimensioning or floating zero system of dimensioning

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.

Units of Dimensioning

Irrespective of whether absolute or incremental system is followed for dimensioning a
component, it is possible to use both inch system and metric system of units.
With inch system all dimensions will be considered as in inches.
With metric system all dimensions will be considered as in mms.
NOTE :
1. Generally all the programs are written in absolute system.
2. All the programmed X values will be in diameters.
Different coordinate systems and reference points used on a CNC machine are explained below.

Machine Origin Coordinate System

Machine origin co-ordinate system is the axes co-ordinate system having the origin at
machine zero.

Machine Zero – M

The points of intersection of the spindle axis and the plane of spindle front face is taken as
machine zero.
Similar to machine co-ordinate system, we have work piece co-ordinate system having the origin at work zero and the reference point co-ordinate system having the origin at machine reference point.

Work Piece Coordinate System

This is the co-ordinate system having the origin at work piece zero. This co-ordinate system is used for the purpose of programming co-ordinates of different points on the work
piece.

Work Piece Zero – W

Work piece zero is the zero defined for programming the work piece dimensions. It can be
freely selected.
The coinciding point of the work piece front face and the work axis is selected as work
zero.

Machine Reference Point Coordinate System

This is the machine co-ordinate system having the origin at machine reference point.

Machine Reference Point – R

There is a difference between machine origin and machine reference point. A programmer
has to know this part clearly
The reference point R is a point defined by the machine manufacturer which is to be approached when the control is switched on and which synchronizes the system. The reference dimension is defined in the machine data.
With the machine co-ordinate system having the origin at reference point, whenever the
slides are taken to reference point the display values of the actual position (on the CRT
screen) will be zero.
With machine co-ordinate system having the origin at machine zero, reference point coordinates are to be defined with respect to machine zero. For all the actual requirements keeping the slides at the machine reference point, distance between the machine zero and the turret zero (turret front face and axes intersecting point) is established in both the axes and defined as the reference point co-ordinates.

Parking Position – T

This is an intermediate or an arbitrary point on the machine which is defined by the operator. The position is selected such that, it would be convenient for tool indexing, inspection of component or tool, loading and unloading of component etc.