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Multi-Axis Calibration is Transforming Machine Tool Accuracy

Multi-Axis Calibration is Transforming Machine Tool Accuracy

I have written recently about rapid machine-tool verification and Etalon Multiline which enables self-calibrating machine tools. While both of these technologies have an important role to play in improving quality, the Renishaw XM-60 multi-axis calibrator may be a more significant development. This laser interferometer system is used to perform a fairly conventional laser calibration process. What sets it apart is that all six degrees of freedom for a given axis can be calibrated simultaneously in a single setup. This greatly reduces the time required to calibrate the machine saving both the cost of skilled labor and machine downtime.

Under ideal circumstances, an Etalon Multiline system will require far less downtime and no skilled labor machine calibration. However, this requires permanently installed instrumentation with significant capital outlay. It may also be impractical for all but the largest machines. The Renishaw XM-60 needs to be set up for each axis, but this means that a single instrument can be moved between many machines, greatly reducing capital expenditure and enabling flexible implementation.

Measuring multiple degrees of freedom saves time

For typical machine-tools, which have their axes arranged in series, calibration involves measuring and correcting for the kinematic errors in each axis. Any given location on an axis can have six positional errors, corresponding to the six degrees of freedom for a rigid body in motion. The six degrees of freedom are made up of three translations and three rotations. For example, there is a translational error along the x-axis due to issues with the axis encoder and control, as well as two translational errors in y and z due to the straightness of the x-axis. The x-axis has rotational errors due to alignment and straightness, with rotation about the axis often described as roll while the two rotations about the perpendicular axes are described as pitch and yaw. The value of these six error sources depends on the location along of the axis. The number of individual measurements required depends on the size of the machine. Typically, at least 10 measurements would be made.

Using a conventional laser calibrator, each degree of freedom requires an independent setup. Optics must be attached to the machine, the laser positioned on a tripod, and careful alignment carried out. This is a skilled operation and the time to perform it for the 18 error sources present in three axes adds up significantly. Furthermore, since each axis is measured independently, the errors, measured with respect to best fit straight-line motion, are not the nominal axis path. This means that three additional measurements must be carried out to determine the squareness between the axes. This results in 21 error sources requiring separate instrument setup and alignment processes. The measurements of straightness between the axes are normally performed using another instrument such as a granite square, a ballbar or a special purpose laser instrument such as the Renishaw XM-60.

This approach also assumes that the errors at a given location along an axis are repeatable and do not depend on the location of other axes. This approach is usually accurate enough, although in reality there is some interaction between the axes due, for example, to gravity induced sag. A more accurate approach, therefore, is volumetric compensation. This means that instead of simply measuring errors at a number of discrete locations along each axis, the errors are measured at the corresponding locations in a 3D grid. For example, for a three axis xyz machine with errors measured at 10 locations in each axis the conventional approach would involve 18 errors multiplied by 10 giving 180 error measurements. In this example, the volumetric approach would involve the same 18 errors being measured in each location in a 103 grid giving 18,000 error measurements. More significantly, instead of each error source requiring a single setup it would need to be measured along 102 lines. This would require 1,800 setup and alignment procedures, something that could require weeks or months of work on a machine. For this reason volumetric compensation has, in the past, rarely been carried out on production machines.

Using the Renishaw XM-60 multi-axis calibrator, the number of setup and alignment processes required for conventional calibration is reduced from 21 to just 6. Perhaps more significantly, for a volumetric compensation with 10 locations along each axis, number of setups required is reduced from 1,800 to 300. In reality, a smaller number of lines along each axis may be used and volumetric compensation starts to look feasible.

How the XM-60 Works

Conventional laser calibrators use a single laser interferometer. A laser interferometer works by splitting the laser beam into two beams which travel along different paths before being recombined. A detector observes the recombined beam. Differences in the distance travelled by the two beams can be observed as changes in the interference fringe pattern. Different setups can be used to measure different dimensions. For example, a fixed length reference path may be compared with a measurement path to measure displacement along the axis of the measurement path. Alternatively, the two paths may be directed at angles either side of the direction of motion so that pitch or yaw causes a relative change in length between the two paths. Different arrangements of beam splitters, mirrors and prisms can be used to measure each degree of freedom as the machine is moved along a given axis.

The XM-60 uses the same interferometric principles as a conventional laser calibrator. However, by splitting the beam into four separate measurement beams it is able to simultaneously perform all of these measurements. This is basically just permanently fixing a cluster of conventional optics meaning that alignments can be manufactured into the instrument and multiple measurements can be carried out in time. However, the way roll (rotation about the x-axis) is measured is more unique. For this, Renishaw uses a new patented approach which uses a polarized laser beam. Roll can then be measured by observing the angle of the polarization plane.

This technology could enable cost-effective volumetric compensation techniques for production equipment, resulting in more accurate machines.