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During continuous 5-axes operations, whenever a rotary axis approaches an orientation where the tool becomes parallel or nearly parallel to the axis of rotation, the machine may unexpectedly start to describe large rotations with that rotary axis. These rotations, although large, have very little effect on the tool-to-part relative orientation. This situation is commonly referred to as a “singularity” orientation. A rotary axis is said to be “singular” when the tool axis vector is parallel to the axis of rotation of that axis.
When handling singular orientations, CAM systems may generate Cutter Location motions where the tool axis vector changes slightly from one point to the next. This is due to internal manufacturing tolerances used by the CAM system to compute the motions.
The example below illustrates a series of CAM-generated motions with very small variations in the tool axis vector.
- GOTO / -625.43042,-1366.35596, 495.17419,-0.000183,-0.031493, 0.999504
- GOTO / -625.43988,-1371.03906, 495.02615,
-0.000019,-0.014431, 0.999896
- GOTO / -625.44177,-1375.00012, 494.99750,
0.000012, 0.000000, 1.000000
- GOTO / -625.43588,-1379.99731, 495.08881,
-0.000089, 0.018208, 0.999834
- GOTO / -625.42999,-1384.99451, 495.18011,-0.000190, 0.036410, 0.999337
In the case of a machine with a “CA” or “CB” 5-axes Gantry head, this will result in very large C-axis rotations while cutting, which are very likely to damage the part. Here is the regular post-processed output for the motions above:
- N2012 X-625.43 Y-1366.356 Z495.174 A1.805
C-.333
- N2013 X-625.44 Y-1371.039 Z495.026 A.827
C-.075
- N2014 X-625.442 Y-1375. Z494.998 A-.001
C-90.
- N2015 X-625.436 Y-1379.997 Z495.089 A1.043
C-179.72
- N2016 X-625.43 Y-1384.995 Z495.18 A2.087
C-179.701
ICAM post-processors provide a very efficient way of handling singularity situations, by defining a singularity cone around the axis of revolution. When the tool axis falls within this cone, it will be adjusted to minimize the move of the singular rotary axis. To enable this functionality, the following command must be inserted at the beginning of an operation which may result in singularity:
VTLAXS / ANGLE
- , toler
The toler argument represents the value of the half-angle of the singularity cone, measured in degrees.
Here is the output produced with an ICAM post-processor for the same motions, when using the VTLAXS command with an angular tolerance of 0.5 degrees:
- N2012 X-625.43 Y-1366.356 Z495.174 A1.805
C-.333
- N2013 X-625.44 Y-1371.039 Z495.026 A0.
- N2014 X-625.442 Y-1375. Z494.998
- N2015 X-625.436 Y-1379.997 Z495.089 A-1.043
C.28
- N2016 X-625.43 Y-1384.995 Z495.18 A-2.087
C.299
And here is the output produced when specifying an angular tolerance of 1 degree:
- N2012 X-625.43 Y-1366.356 Z495.174 A0.
C0.
- N2013 X-625.44 Y-1371.039 Z495.026
- N2014 X-625.442 Y-1375. Z494.998
- N2015 X-625.436 Y-1379.997 Z495.089
- N2016 X-625.43 Y-1384.995 Z495.18 A-2.087
C.299
Benefit to User
Simply insert one PP command to avoid “Head-dancing” issue.
For more information or comments, please do not hesitate to contact Phil at TechTipTuesday@icam.com
