FANUC G68.2 - 5-Axis Tilted Work Planes - LinkedIn

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While true 5-Axis simultaneous machining has definitely become more popular, the truth is that 5-Axis simultaneous is a very small part of machining when viewed in the context of all machining operations.

The overwhelming majority of 5-Axis machining operations are actually what can be more correctly classified as 3 + 2 operations. Meaning, the 5-Axis machine will position to a specific orientation with its two rotary axes then perform standard 2-1/2 Axis & 3-Axis operations.

With smaller work-pieces, 5-Axis machines with Table/Table or Dual-Rotary Table configurations accomplish this by simply rotating to align the Work Plane to be parallel to the XY-Plane (G17) and perpendicular to the Spindle/Z-Axis. With the larger work-pieces that are typical of Aerospace, Energy and Automotive industries, this type of machine is impractical.

Large work-pieces that require machining from various orientations are typically done with Head/Head or Articulating Head 5-Axis machines. A Head/Head machine achieves the required orientations by rotating and aligning the Spindle/Tool Axis to be perpendicular to the Work Plane that will contain the features to be machined.

In the past, this presented additional challenges as the CNC Control Systems were not powerful enough to help manage the various Work Plane orientations. Because of this limitation, many CNC Programming functions, that are taken for granted for 3-axis, could not be used. Circular Interpolation, Cutter Radius Compensation and Drilling Cycles are all standard programming tools that couldn't be used. This would be reflected in the NC code by large NC program files that were largely point-to-point movements.

As CNC Control Systems have become much more powerful, this problem is largely a thing of past. Almost every CNC control used on 5-Axis machines today has some version of a function for handling Tilted Work Planes. Among these are FANUC and SIEMENS, two of the most popular CNC Control Systems today.

While Tilted Work Plane functionality has pronounced benefits with Head/Head type 5-Axis Machines, It may also be used with Table/Table and Head/Table Hybrid 5-Axis machines. The reason for using it with such machines is decidedly different. We'll address those reasons in a future article.

Let's first take a look at how FANUC handles Tilted Work Planes for a Head/Head type 5-Axis Machine.

The FANUC command for Tilted Work Planes is G68.2 . G68.2 is the Absolute Mode (G90) command and is the most common. (NOTE: There is a variation of this command defined by G68.3). G68.4 is the Incremental Mode (G91) command.

The G68.2 Tilted Work Plane function allows user to define the Work Plane by Euler Angles, Roll-Pitch-Yaw, 3 Points, 2 Vectors, Projections Angles. The method of defining the Work Plane is designated by the P address.

• G68.2 P0 (Euler Angles)
• G68.2 P1 (Roll-Pitch-Yaw Angles)
• G68.2 P2 (3 Points)
• G68.2 P3 (2 Vectors)
• G68.2 P4 (Projection Angles)

NOTE: When the P is not specified, a P0 is assumed for using Euler Angles.

Since Roll, Pitch and Yaw Angles are the most common used in Aerospace, let's construct a G68.2 command using Roll, Pitch and Yaw Angles.

G68.2 Roll Pitch Yaw Syntax

G68.2 P1 Q123 X_ Y_ Z_ I_ J_ K_

The P1 indicates a Tilted Work Planes definition via Roll, Pitch and Yaw Angles. X,Y,Z define the location of the Origin of the Tilted Work Plane using the base WCS (Work Coordinate System) of the part as the reference point. I,J,K define the Roll (about X), Pitch (about Y) and Yaw (about Z) Angles. The Q123 indicates the order in which the rotary axes are rotated. The order used will depend entirely upon the kinematic definition of the rotary axes for a given machine.

Q123 is the default for the I, J and K values. If the Q is not specified, Q123 is assumed.

In the above example, we have the following Tilted Work Plane properties.

• Local Coordinate System Origin : (200.0, 0.0, 50.0)
• Order of Rotary Axis Rotations : I, J, K (X, Y, Z)
• Rotation about the X-Axis (Roll) : 30 Degrees
• Rotation about the Y-Axis (Pitch): 0 Degrees
• Rotation about the Z-Axis (Yaw) : 90 Degrees

This Tilted Work Plane is defined with the following G68.2 Statement.

G68.2 P1 Q123 X200.0 Y0 Z50.0 I30.0 J0.0 K90.0

There is another code associated with the use of G68.2 that creates a great deal of confusion in what it actually does. That code is G53.1. FANUC defines G53.1 as Tool Axis Direction Control. A much simpler and clear explanation is that G53.1 will cause the automatic positioning of the rotary axes required by the Tilted Work Plane and align the Tool/Spindle Axis to be perpendicular to the Tilted Work Plane. This results in the Tool/Spindle Axis being the Z-Axis of the LCS (Local Coordinate System). G53.1 must be output immediately after the G68.2 statement.

Caution must be exercised when using G53.1 as it will NOT adjust for the current tool location and it is possible to cause a serious collision if a proper approach position is not defined prior to the G68.2 Tilted Work Plane definition.

Another code that is even more misunderstood than G53.1. That code is G53.6. This essentially applies RTCP for the tool orientation positioning. Like G53.1, it must be immediately after the G68.2 call. However, G53.6 is not supported if G54.4 Work Setting Error Correction (Part Skew/Roto-Translation) will be used.

With the larger 5-Axis machines used in Aerospace, it's quite common to find C-Primary/B-Secondary or C-Primary/A-Secondary Head/Head machines. These configurations lend themselves, quite nicely, to the use of the Roll, Pitch, Yaw method of Tilted Work Plane definition as these relate directly to the specific rotary axes of a given machine.

Looking down along the POSITIVE axis normal towards the origin:

A-Axis rotates in the + direction CCW about (parallel to) the X-axis. B-Axis rotates in the + direction CCW about (parallel to) the Y-axis. C-Axis rotates in the + direction CCW about (parallel to) the Z-axis.

Therefore:

• Roll Axis = Rotary Axis 'A'
• Pitch Axis = Rotary Axis 'B'
• Yaw Axis = Rotary Axis 'C'

For our example, let's assume we have a C/A Head/Head 5-Axis machine. The secondary axis is also sometimes referred to as the 'Slave' axis in that its position is dependent upon where the C-Axis is currently located. For this configuration, we must define the order of axis rotation as Q312. The C-Axis is the primary axis and will rotate first, The A-Axis is secondary and will rotate second.

Once we have defined a Tilted Work Plane, we can then program standard toolpath operations as if they are in the XY-Plane (G17). Any operation defined within a G68.2 statement and G69 cancellation of the Tilted Work Plane is done using the LOCAL XY-Plane and LOCAL coordinates of that Tilted Work Plane.

Below is a graphic that displays the practical application of Tilted Work Planes for 3+2 machining along with 5-Axis simultaneous machining in the context of an actual part. You may download the actual NC Code file as well as the original Mastercam file that was used to generate it.

You can also download a second NC Code File that uses G68.3 for creating the Tilted Work Plane. The graphic below displays the use of G68.3 for the same Tilted Work Plane orientation in our original G68.2 example above. However, like G53.6, G68.3 is not supported if G54.4 Work Setting Error Correction (Part Skew/Roto-Translation) will be used.

In my next article, we'll deal with the SIEMENS CYCLE800 function for Tilted Work Planes.

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Share 123 24 Comments Chase Hettinger 6y

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Could you use a “G52 X.001Y.001” after a g68.2 & g53.1 to comp that face? If your machine isn’t accurate or you just need to comp a certain face a little bit to get those feature locations dialed in?

Like Reply 1 Reaction Basavraj Indi 6y

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Well explained, Thanks for this useful information.

Like Reply 1 Reaction kaleem khan 7y

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Very informative article.nice

Like Reply 1 Reaction Yunus Çoban 7y

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Hello Tim, Thanks for your post, it is very clear.  My question is about alternative angle solution. For some cases, (generally based on the machine head geometry) the machine  rotational axes must  use other angle solution to avoid clashes between  the machine head and the part.  For  example, Heidenhain SEQ -/+ command is used for the alternative angle soluion.   For Fanuc, what is the way of this? Sample NC: ...... N23 (HEAD ROTATION: B90. C0.)            (Other angle solution : B-90 C:180)  N24 G68.2 X0. Y0. Z0. I90. J90. K0. N25 G53.1 ....... Thanks, Yunus

Like Reply 1 Reaction Cristinel Pricop 7y

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thanks

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