CNC Milling | G02 and G03 Codes | Helical Interpolation


Helical milling uses an optional control system feature called helical interpolation. In its simplest definition, helical interpolation is a machining operation where a circular interpolation uses three axes simultaneously. This could be a misleading statement because it implies a three dimensional arc or a circle. Such an arc or circle does not exist anywhere in the field of mathematics, it becomes a helix. Both G02 or G03 circular interpolation commands do use all three axes – for example,

G03 X.. Y.. Z.. .. F..

This type of operation is only available for CNC machining centers as an optional feature. Let’s look at the subject of helical milling a little closer.

Helical Milling Interpolation

What exactly is helical milling? Essentially, it is a form of a circular interpolation – it is a programming technique to machine arcs and circles combined with a linear interpolation in the same block, during the same motion.

Other posts on our web site that were related to circular interpolation presented one major feature of that subject. In circular interpolation, there are two primary axes used within the selected plane, with the intent to program an arc motion or a circular motion.

For example, in G17 XY plane (the plane that is most common), a typical format of circular interpolation will be in two forms:

Using arc center vectors IJK for CW/CCW motion :
G02 X.. Y.. I.. J.. F..
G03 X.. Y.. I.. J.. F..
Using radius R for CW/CCW motion :
G02 X.. Y.. R.. F..
G03 X.. Y.. R.. F..

Note that there is no Z-axis programmed. As a matter of fact, if the Z-axis were included in the same block as a circular milling, it will not work – normally. That means it will not work, unless the control system has a special feature called helical interpolation option.

Helical Interpolation

Helical interpolation is usually a special control system option that is designed to be used for cutting a circle or an arc with a third dimension. This third dimension is always determined by the active plane:

  • In G17 XY plane – the third dimension is the Z-axis
  • In G18 ZX plane – the third dimension is the Y-axis
  • In G19 YZ plane – the third dimension is the X-axis

In the active plane G17 (XY), the third dimension is the Z-axis. In the active plane G18 (ZX), the third dimension is the Y-axis and in the active plane G19 (YZ), the third dimension is the X-axis.

In all cases, the third dimension – the third axis motion – will always be a linear motion that is perpendicular to the active plane.

A more formal definition of a helical interpolation can be made, based on the previous statement:

Helical interpolation is a simultaneous two-axis circular motion in the working plane, with the linear motion along the remaining axis.

The resulting three axis motion is always synchronized by the control system and all axes always reach the target location at the same time.

Helical Interpolation Format

General formats for helical interpolation in a program are similar to formats available for a circular interpolation – plane selection is particularly important:

Using arc center vectors IJK for CW/CCW motion :
G02 X.. Y.. Z.. I.. J.. K.. F..
G03 X.. Y.. Z.. I.. J.. K.. F..
Using radius R for CW/CCW motion :
G02 X.. Y.. Z.. R.. F..
G03 X.. Y.. Z.. R.. F..

Plane selection programmed before helical interpolation block determines which axes will be active in the program and what their function will be.

Arc Vectors for Helical Interpolation

Arc vector functions are programmed using the same principles as in circular interpolation but will be different for each plane. Here is a summary in a typical table:

Note that the arc vectors apply to the two axes that form the circular motion – linear motion has no influence whatsoever. If the control system supports the direct radius entry R (instead of the more traditional IJK vectors), the physical center of arc motion is calculated automatically, within the current plane.

Applications and Usage

Although helical interpolation option is not the most frequently used programming method, it may be the only method available for a number of rather special machining applications:

  • Thread milling
  • Helical profiling
  • Helical ramping

From these three groups, thread milling is by far the most common method of helical interpolation applied in industry.