by Andrew Brooks
The term “3D scanning” is typically used to refer to the scanning of free form surfaces, such as elements of a car body, exterior aircraft surfaces, sheet metal stampings and other curved surfaces. For scanning these surfaces, optical metrology is typically the best alternative, because of the sheer volume of data points the technology can collect quickly, up to and sometimes more than half a million a second. That volume of data, even though composed of individual points, reproduces–as closely as any digital technology can–the organic curves and angles of a free-form surface.
In the search to bring contact metrology’s level of accuracy and optical data volumes together, metrology vendors have been working on developing the capabilities of sweep scanning, where a contact probe is dragged across a machined surface rather than repeatedly applied at discrete points.
“With lasers, yes you can collect millions of points very quickly in a short period of time,” says Dr. David Chang, technical sales manager, measurement & automation products for Renishaw (Canada) Ltd. “But with the noncontact technology will have to deal with noises– ambient light and other factors when you’re trying to measure the part.”
Renishaw is one metrology vendor that is working on enabling touch probes to gather a much higher volume of data points and compete with laser and optical scanners in coverage of free form surfaces. At Renishaw, the approach is called sweep scan. Initially developed in the company’s Revo platform, the technology employs a pivoting probe head that moves back and forth as the CMM drive system itself moves in a single direction.
“Revo can scan the surface at 500 millimetres per second,” Chang says. “It covers every point and reports up to 2000 points per second. So you can collect a lot of high quality accurate data from a 3D free-form surface.” The system maintains tight control over tip deflection, which enables the Revo to tackle curved surfaces as well as flat planes, and maintain accuracy at high speed.
Renishaw is developing Revo as a multi-sensor platform that will include non-contact sensors such as a surface finish probe. “Before, for surface finish tests, you had to have another gauging station,” Chang says. “They were usually done manually, which adds labour cost. Surface finish measurement depends a lot on the alignment of the sensor, the tip, and if the operator doesn’t align it properly they get a different reading. But we have the means to automate the whole process.” SMT