The OG™ System has gone through a study to compare with an interferometry-based flatness device. There exist evidences that the OG™ System correlates with the interferometry-based device very well on flat parts. However, the interferometry-based device failed to detect an artificial recess on a part with surface discontinuity while the OG™ System demonstrated its ability of measuring the entire surface.

INTRODUCTION

Flatness is one of the important geometric tolerances often specified in many engineering designs that require flat surfaces. Instruments used to measure flatness have evolved for decades. One of the early instruments utilizes a precise potentiometer, driven by gravity to measure "slopes" along the grid webs. The other approach utilizes a dial indicator on a flat granite table in which the table works as a reference. Other instruments like surface profilers are commonly used. These devices have some drawbacks. They are typically time-consuming in data collection. Therefore, either the number of data points is limited or the area measured is restricted.

The arrival of interferometric devices brought flatness measurement into a new era. A surface can be digitized into thousands of points in seconds. The interferometry-based devices provide great accuracy and have been prevailing in industry, automotive, semiconductor, hard disks, etc. The industry started to have a more comprehensive view of flatness given the quantity of data points acquired in a reasonable period of time. However, interferometric devices are NOT the final solution. They have some detrimental failure modes. For instance, a combination of large area and surface discontinuity poses a mission impossible for this kind of devices. Harsh factory floor environment is another killer to the delicacy of an interferometric device.

OG™ 2000, as a system to answer the call for flatness measurement of automotive valve bodies (a combination of large area and complex surfaces, often with surface discontinuity), has evolved to challenge all the flatness measurement devices, including interferometric devices, in resolution, accuracy, and throughput. Furthermore, the OG™ System provides the ruggedness that supercedes all other technologies with the same degree of accuracy.

This technical note is intended to explore the performance similarity and fundamental difference between the OG™ System and the interferometry-based devices. The information provided in this note is based on a study conducted by OG Technologies, with the help from a major US manufacturer. Parts used in the study were measured on both the OG™ 2000 system and a flatness measurement device that is based on grazing-incidence interferometry.