Most CNC coordinates are read and written as cartesian coordinates.

This sounds complicated but are more familiar to most people as rectangular coordinates; the type used for basic grids such as map reading - read along the X - Axis (Across) first followed by the Y - Axis (Up). These are the coordinates used in plan view for CNC Milling. For CNC milling we have three axes; the milling machine also uses the Z - Axis (true up and down).

This sounds complicated but are more familiar to most people as rectangular coordinates; the type used for basic grids such as map reading - read along the X - Axis (Across) first followed by the Y - Axis (Up). These are the coordinates used in plan view for CNC Milling. For CNC milling we have three axes; the milling machine also uses the Z - Axis (true up and down).

The point where the axes cross is known as the origin, X=0 Y=0 and Z=0; for a machinist this is known as the datum. The datum will be defined by the machinist and will be used to define machining coordinate positions.

Cartesian (or any other coordinate systems) can be expressed as absolute or relative (some call incremental). We will discuss absolute in this blog; all coordinates are expressed from one fixed point. with relative coordinates the datum moves with the coordinate; hence the expression is incremental from the current position.

The position of the datum relative to the geometry of the part determines the direction of the coordinates. If the part geometry is to the left of the datum the X axis coordinates must be negative (-); if the part geometry is below the datum the Y axis coordinates must be negative. The same is true for the Z axis coordinates.

For this reason many machinists will try to place the part geometry in the 1st quadrant. This is so the coordinates and program will have no negative X and Y figures.

For Z it is common for the datum to be place at the top of the part, this is so all cutting moves have a Z- figure and all safe Z's are Z+ figures.

It is more common for the machinist to determine the datum; this is the first task before trying to determine any coordinate positions. It some cases manufacturing engineers will specify the datum, this will be shown on the engineer drawing with the symbol as shown above.

For this reason many machinists will try to place the part geometry in the 1st quadrant. This is so the coordinates and program will have no negative X and Y figures.

For Z it is common for the datum to be place at the top of the part, this is so all cutting moves have a Z- figure and all safe Z's are Z+ figures.

It is more common for the machinist to determine the datum; this is the first task before trying to determine any coordinate positions. It some cases manufacturing engineers will specify the datum, this will be shown on the engineer drawing with the symbol as shown above.

To recap, consider the datum position, remember the rectangular or cartesian system, and remember the direction or quadrant rules.

If you were to program the 4 points shown with a centre datum the coordinates would be as shown

If you were to program the 4 points shown with a centre datum the coordinates would be as shown

Check the exercise download page for worksheets