Understanding Polar Coordinate Interpolation

Polar coordinate interpolation is a function that exercises contour control in converting a command programmed in a Cartesian coordinate system to the movement of a linear axis and the movement of a rotary axis is the definition given by the FANUC operator’s manual. These manuals are always hard to reads and understand. This may stem from the fact that they were originally written in Japanese and translated to English but in recent years the manuals have become much easier to interpret. I always wondered why they do not have an index. It has always been necessary to scan through the Table of Contents and find the appropriate topic for a page number.

CVTL’s incorporate this function and is what makes them so versatile and in demand by reducing set up time and material handling to another machine for milling operations.

There are some important factors that need to be considered during part set up and in programming when this function used to drill or profile in milling mode on a lathe.

1.     Proper setting of C0.0 in relationship to features on the part and as outlined by the dimensional drawing.

2.     The machine tool must be in the Cartesian coordinate system at the beginning of the program and remain in this coordinate system up until the rotary axis (Chuck) has been oriented to C0.0. Then G12.1 can be executed.

3.     G12.1 must be turned off and the machine returned to turning mode after milling is complete by executing the command G13.1.

4.     When the machine is referenced between tool changes the “C” axis should also be referenced.

5.     Rapid axis movement (GOO) cannot be used anywhere in the program while polar interpolation is in effect.

6.     Axis movement is calculated in the controls by the use of the law of cosines and position-to-position movement is much slower than lathe mode.

7.     Position movement varies as the diameter of the cut increases or decreases.  Try milling a straight groove 180 degrees across a part.

Keep all these point in mind and the conversion to Polar coordinates will come much easier. A simple way to see and understand what is happening is if you take the part print and hold it in the proper orientation of the part when the machine is at C0.0 and then turn the print as the part turns. You can now visualize what is happening to the coordinate system. It’s as if the Cartesian system is turning in relationship to the “C” axis rotation.

It seems confusing at first but if you write programs by hand, program generation can be much easier when you understand this system. Write your program normally until the machine positions itself at C0.0, invoke G12.1 and then with the print oriented with the part, write the remainder of the program as if it was on a horizontal mill and positioning in the X, Y plane. Don’t forget to turn off polar coordinates and reference all axis before the tool change command.