Increasing 5-axis accuracy by probing, auto-tuning

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Geometric tuning to compensate for errors

When a machine is installed, it’s typically leveled on the shop floor. After 30 days or so, the weight of the machine forces it to settle, however, and this can cause the axes to go out of square with each other, meaning the angles are out of 90-degree alignment and no longer perpendicular, which leads to geometric error (see Figure 1).

For angularity, the operator must then correct the “pitch-and-yaw” twist that develops in the machine. Yaw refers to a rotation in the Z axis, pitch is rotation in the Y-axis, and roll is rotation in the X-axis. When performed manually, tuning for rotary axis misalignment is a painstaking process involving many parameter measurements using a tooling ball, complex mathematical calculations and manual data entry. These tasks can take as long as five hours or more to complete, and require someone with leveling and alignment training. This process is also prone to some degree of human error.

When tuning for rotary axis misalignment, precision is an absolute necessity. Consider the following scenario: 5-axis is a three-dimensional world. On a 5-axis machine, you must compensate for plus and minus error, up and down error, and side-to-side error. When you consider all the linear axes involved, there are a total of 16 different parameters (just through the X, Y and Z-axes) that need to be set. Once they are set, the control software will reference each 5-axis move, taking the error into account, to ensure that you are moving to the right place.

Because this is a three-dimensional environment, if any tuning adjustment is incorrect, it not only affects one axis, but all five axes. The pivot points on the rotary axis will be wrong, and the X-axis pitch and yaw are both tied to these parameters. You might proceed thinking you’re at 0/0 on all five axes, but perhaps the centre line is off by 0.001 inch at zero on the table. If you’re making a 10-inch part, multiply this error by 10. You could literally miss the part with the drill.

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