New Measuring System Performs Continuous Radial Scanning

The typical coordinate measuring machine (CMM) resembles a multi-axis traveling bridge-type vertical machining center. Indeed, the movements of the measuring probe look very much like a cutting tool in motion as it follows a tool path. Of course, the CMM doesn't make any chips. For prismatic workpieces, the conventional configuration of the CMM and the machine tool is logical and effective.

But not all workpieces are prismatic. Many are round, cylindrical or cone-shaped. Very often, it is advantageous to machine such non-prismatic workpieces on machine tools that are not configured as a VMC is. A turning center, vertical turning lathe (VTL), or other type of machine tool may be more appropriate. The same can be said for the CMM that that might measure these parts. A very different configuration might have its advantages. Such a measuring system that seems to prove this point has now been introduced by TSK America(Pine Brook, New Jersey). The new CMM, called Radiance Radial Measuring System (RMS), represents several years of development effort. It is being offered as the solution to complex rotary measuring challenges.

The patented machine design permits unrestricted rotary scanning of parts such as gears, turbine blades, cones, cylinders and other shapes with reduced cycle times and improved accuracy for complex measurements, the developers say. The measuring range is 1 m diameter and 0.6 m vertically, thus the designation Radiance 1006. The machine occupies less than 2.5 square meters of floor space.

"The main advantage to the Radiance is that for the first time, users can complete a full radial scan of a non-rotating part without probe changes or reorientation," says Jack Epstein, TSK product manager and developer of the Radiance concept. "It permits new levels of freedom for measurement of non-prismatic work which would normally have required multiple probes and a rotary table," he points out. The workpiece is stationary, and the measurement system travels in the surrounding cylindrical coordinates.

This configuration permits economical application of complex polar metrology by eliminating the need for a rotary table. The infinitely bi-directional rotation of the Radiance bridge provides the fourth-axis of measurement, while only three axes are in motion. The synchronized motion of two axes provides an interpolated straight line of motion for prismatic work, according to Mr. Epstein. This reduces inspection time and avoids discontinuity between measuring segments typical of conventional coordinate measuring machines. The inaccuracies inherent to rotary table measurement are also avoided.

Developers state that the Radiance Measuring System features linear accuracy in the X and Z axes of 2 microns; in the U2 (X and Z axes) of 2.0 + L/500 microns. Angular accuracy is 3 arc-seconds. Acceleration is 1,000 mm/s2 along the linear axes (X and Z), and 250 mm/s2 along the Y (round) axis. Measuring speeds are said to be up to 250 mm per second.

Because the workpiece is stationary, there is no possibility of dynamic error related to varying workpiece loads on a table, such as an off-balance condition. Because there are no accelerating forces on the workpiece, workholding requirements are reduced, or in some cases eliminated.

The new system can be used with laser, video, and other types of conventional probes, for contact or noncontact measurement. Its radial motion interprets the measurement of complex, round parts in a more suitable and reliable way than is possible with a rotary-table system, Mr. Epstein says.

The granite surface table is the reference base for the rotational axis of the system. The system's measuring accuracy is substantially supported by this surface plate, which is precisely squared and finished to a Class 1 flatness. The precision-ground circumference of the surface plate is designed to provide stability and high accuracy over long-term use.

The alloy columns support a ceramic bridge (X axis) across which travels the ceramic Z-axis ram fitted with the user's choice of probe. A rotating frame joining the two columns ensures the strength and rigidity of bridge structure. A linear encoder system with a dual-head reader is mounted on the circumference of the surface plate. Bi-directional closed-loop, continuous reading of scale is thus achieved without interruption of measurement and subsequent loss of data.

An electromechanical slip ring at the top of the bridge permits 360-degree bi-directional rotation while maintaining power and control to the system. The use of this component provides a continuous pneumatic and electrical connection between rotating and stationary sections of the machine.

A built-in controller package includes a single-board computer, motion controller and the probe system. A dedicated 32-bit controller software module acts as a gateway to the host while controlling motion and data acquisition. The operator interacts with the host application software, which communicates with the controller module. The host application is a 3D software package for basic geometry. Proprietary gear inspection software and airfoil software packages are available.