In-process measurement supports a feedback precision control system to reduce workpiece surface form error. Without it, workpiece surface must be measured offline causing significant error in workpiece positioning and reduced productivity. In order to offer better performance, a new in-process optical measurement method based on use of dual coolant displacing media is proposed and studied, which uses air and liquid phases together to resist coolant and achieve in-process measurement of workpiece form profiles. In the proposed new design, coolant is used to replace the previously used clean water to avoid the problem of coolant dilution. Compared with the previous methods, the distance between the applicator and the workpiece surface can be much relaxed to 1 mm. The result is 4 t...[ Read more ]

In-process measurement supports a feedback precision control system to reduce workpiece surface form error. Without it, workpiece surface must be measured offline causing significant error in workpiece positioning and reduced productivity. In order to offer better performance, a new in-process optical measurement method based on use of dual coolant displacing media is proposed and studied, which uses air and liquid phases together to resist coolant and achieve in-process measurement of workpiece form profiles. In the proposed new design, coolant is used to replace the previously used clean water to avoid the problem of coolant dilution. Compared with the previous methods, the distance between the applicator and the workpiece surface can be much relaxed to 1 mm. The result is 4 times larger than before, thus permitting measurement of curved surfaces. The use of air is up to 1.5 times less than the best method previously available.

The experimental results showed that the proposed new method can also resist coolant from multiple directions and deal with coolant layer with thickness up to 10 mm. For a sample workpiece with curved surfaces, the relative error of profile measurement under coolant condition can be as small as 0.1% compared with the one under no coolant condition. Problems in comparing measured 3D surfaces are discussed. A comparative study between a Bruker Npflex optical profiler and the developed in-process optical profiler system was conducted. For a surface area of 5.5 mm x 5.5 mm, the average measurement error under coolant condition is only 0.693 μm. In addition, the error due to the new method is only 0.10 μm when compared between coolant and no coolant conditions. Thus, the use of dual coolant displacing media can accurately measure the workpiece surface which is fully submerged in the opaque coolant.

Characteristics studies of the sensor and the in-process optical profiler system were conducted. Firstly, the experimental result showed that the measurement error was increased with the height of curved surface. This should be related to the position on detector of triangulation sensor, and the reasons were discussed. Besides, to investigate the effects of measurement parameters on the in-process optical profiler system and find out key parameter, a fractional factorial test was designed and conducted. The table velocity was found as the most important measurement parameter, and the error became significant with the increase of table velocity.

The proposed new method is advantageous when compared with the previous in-process form profile measurement methods. It should be very useful for in-process optical form profile measurement in precision machining.

Keywords: In-process form profile optical measurement, Coolant, Dual coolant displacing media, Error assessment, Surface alignment.