Improve Your On-Machine Inspection Accuracy

May 19, 2003
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On-machine inspection using probing technology is gaining popularity, because it delivers shorter cycle Arial, and increases quality and productivity. Driving the growth of on-machine inspection is the shift to flexible machining and automated processing, which helps manufacturers meet the shorter lead times and tighter accuracy specifications the market expects. Facilitating this shift are advances in probe technology, improved machine repeatability, and sophisticated computer-aided design and manufacturing software.

Traditionally, part verification has been performed offline with a coordinate measuring machine (CMM). Moving the part offline requires additional setups and part handling, making it a time-consuming process. Inspecting the part with a probe while it is still on the machine tool reduces these nonvalue-added activities.

However, this type of inspection must take into account positioning errors that occur during machining, and are likely to be repeated during the inspection process, because the part is measured on the machine on which it was made. Laser vector measurement can help resolve these volumetric positioning errors and improve the accuracy of on-machine inspection. Integrated equipment and software can make on-machine inspection a viable process.

Because gage accuracy requires a 4-to-1 ratio, machine-measuring accuracy must be four times more accurate than the specified part accuracy, according to a National Institute of Standards and Technology standard that replaces the previous 10-to-1 ratio requirement. This new technique for volumetric calibration and compensation helps ensure machine tool accuracy. Computer software uses the measured volumetric positioning errors to generate a lookup correction table to list what errors need to be compensated for, and allows the on-machine measurement software to volumetrically compensate for the machine positioning errors. Additionally, a machine tool operator with minimal or no training can use the technique. In 2 to 4 hours, a machine operator can measure and compensate volumetric errors for a working volume of about 1-cubic meter. Without the technique, this process can take as long as 16 hours.

Movement increases error potential
Machine movement is complex. For example, for each axis of motion there are six possible errors including three linear errors, as well as pitch, yaw and roll angular errors. On a three-axis machine, that equates to 18 errors. There are three errors for squareness, for a total of 21 possible errors, on a single, three-axis machine. These linear displacement, straightness, squareness, angular and nonrigid body errors determine the performance accuracy of a computer-numerically controlled (CNC) machine tool or a CMM.

For on-machine inspection, the major inherent machine positioning errors are linear displacement, squareness, straightness and thermal distortion. For most CNC machine tools, linear displacement errors or leadscrew pitch errors are measured by a laser system, and the controller compensates for those errors. Squareness errors, the out-of-square condition between the two axes, grows as the machine moves away from the line of travel, which has already undergone compensation for pitch errors.

Straightness errors, caused when the guide way is not perfectly straight, usually because of weight shifting or overhanging dur-ing axis travel, may lead to positioning errors. Thermal expansion and distortion errors, where temperature change causes the leadscrew to grow or the temperature gradient to distort machine geometry, also affect the positioning errors.

Volumetric measurement and compensation
By determining volumetric positioning errors, a lookup correction table can be generated for the on-machine measurement software to compensate for the machine positioning errors. In a typical application, conventional laser measurement identifies linear displacement errors for three axes and generates the leadscrew, pitch-error compensation tables for each axis. However, when compensating for the pitch error of three, three-linear axes is not enough because of large squareness and straightness errors, which can cause positioning errors over the working volume.

Unlike conventional laser interferometer measurement that only measures the displacement error, the laser vector method measures vertical straightness and horizontal straightness errors in addition to displacement errors. With four setups, all three displacement errors, six straightness errors and three squareness errors are determined. Then a volumetric positioning error compensation table is generated. The cost of the equipment is low, and its setup and operation is simple.

Vector measurement technique
The basic concept of the laser vector measurement technique is that the laser beam direction, which is the measurement direction, is not parallel to the motion of the linear axis. Therefore, the measured displacement errors are sensitive to the errors both parallel and perpendicular to the direction of the linear axis. For example, the measured linear errors are the vector sum of errors projected to the direction of the laser beam, such as the displacement errors that are parallel to the linear axis, the vertical straightness errors that are perpendicular to the linear axis, the horizontal straightness errors that are perpendicular to the linear axis and the vertical straightness error direction. By collecting data with the laser beam pointing in three different diagonal directions, nine different types of errors including linear, pitch, yaw, roll and squareness are identified. Because the errors of each axis of motion are the vector sum of the three perpendicular error components, this measurement is called a vector measurement technique.

In conventional body diagonal measurement, the displacement is a straight line along the body diagonal, enabling a laser interferometer to calculate the measurement. For the vector measurement described here, the displacements are along the X axis, then along the Y axis and then along the Z axis. The trajectory of the target or the retroreflector is not parallel to the diagonal direction. The deviations from the body diagonal are proportional to the size of the increment. A conventional laser interferometer is unable to make these measurements without being out of alignment, even with an increment of a few millimeters.

A laser Doppler displacement meter, using a single aperture laser head and a flat-mirror as the target, can tolerate large lateral deviation because any lateral movement or movement perpendicular to the normal direction of the flat-mirror will not displace the laser beam, therefore maintaining the alignment. After three movements, the flat-mirror target moves back to the center of the diagonal again, so the size of the flat-mirror only has to be larger than the largest increment. The flat-mirror target is mounted on the machine spindle and it is perpendicular to the laser beam direction.

In a conventional body diagonal measurement, all three axes move simultaneously along a body diagonal and data is collected at each preset increment. With the vector measurement, all three axes move in sequence along a body diagonal and data is collected after each axis is moved. Therefore, data is collected three times, and errors caused by axis movement are separated.

Probing software and lookup table
Most CNC machine tools have software available for on-machine inspection, enabling a probe to replace the cutting tool in the spindle, so the machine can be used as a CMM to collect part dimension data. With a volumetrically calibrated and compensated CNC machine tool, the volumetric positioning errors can be tabulated as lookup tables or compensation tables and stored in the control memory to correct measured probe positions. By using volumetric error correction to eliminate inherent errors in the machine tool geometry and positioning, accurate dimensional measurement is achieved. With volumetric error compensation, a CNC machine tool provides the same high accuracy as a CMM and satisfies the 4-to-1 gage accuracy ratio.

As a result, on-machine inspection allows a CNC machine to verify part accuracy. Instead of checking parts after machining, on-machine probing makes inspection a part of the process. On-machine probing is a growing application for process improvement, providing benefits such as lower process cycle times, faster setup, verification of tooling and tool-wear compensation. In the age of digital manufacturing and absolute quality assurance, on-machine probing with volumetric compensation is making a major contribution to improvement in quality and throughput, as well as reducing costs.

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