Length Measurement System for Thin-Walled Parts

In the world of high-precision metrology, all parts are elastic, which means they will deform to some extent under applied force (even if only by a few millionths of an inch). But certain parts, like thin-walled bearing rings, deform much more readily than others, to the point where it is extremely difficult for quality-control technicians to obtain accurate measurements of their undeformed size. In other applications, such as precision balls and spheres, standards call for reporting size in an undeformed state, which frequently calls for a theoretical calculation.

Mahr’s Precimar PLM 600-2 high-precision length measuring system is capable of measuring parts in their undeformed state. This capability is said to be especially important for measuring thin-walled workpieces, such as rolling bearing rings. If a technician checks the inner or outer diameter of a bearing on a length-measuring machine, the bearing can be deformed, even at low applied forces, which distorts the measurement results, the company says.

Deformation correction is fully automated and available as part of the standard application tools set in the Precimar PLM 600-2 system’s controller. The procedure automatically modifies the measuring force in a specified sequence, while reliably recording the corresponding change in the part’s measured size. This enables the system to determine the actual elastic properties of the part, which are used, in turn, to calculate the given length in the unloaded, measured force-free state of the part. The measuring machine initially generates a measuring force of 1.2 N, then, invisible to the user, automatically reduces the measuring force in steps of 0.02 N from 1.2 N to 0.8 N, and determines the corresponding measured values, the company says.

According to the company, the calculated length value is based on a number of individually measured values, thereby yielding an uncertainty level lower than any respective single measured value.

The length-measuring system is designed to test a range of equipment and suitable workpieces quickly and easily, and with the smallest possible uncertainty. Under ideal conditions in a metrology laboratory at 20°C, a measurement uncertainty of MPEE1 (0.085 + L/1500) μm can be achieved (length in mm). The measuring system has a five-axis, finely adjustable, universal object table; a modern, PC-based machine control for several axes; and includes a PC workstation with basic 828 WIN “Free Measuring” software.