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 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_ • P1 indicates a Tilted Work Planes definition via Roll, Pitch and Yaw. • 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. • I,J,K define the Roll (about X), Pitch (about Y) and Yaw (about Z). • The Q123 indicates the order in which the rotary axes are rotated.   In the above example, we have the following Tilted Work Plane properties.
This Tilted Work Plane is defined by this 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. 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 along the POSITIVE axis normal towards the origin: A-Axis rotates in the plus direction CCW about (parallel to) the X-axis. B-Axis rotates in the plus direction CCW about (parallel to) the Y-axis. C-Axis rotates in the plus 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. However, our G68.2 definition is actually defined in the
reverse order even though our kinematically correct rotary axis angles
are being used. 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. In my next article, we'll deal with the SIEMENS CYCLE800 function for Tilted Work Planes.  |
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FANUC G68.2 - 5-Axis Tilted Work Planes
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