Machine Design

Gantry approach brings accurate scanning on a budget

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Laser scanners that digitize features of 3D objects generally have fallen into two categories. At the high end have been $200,000+ coordinate-measuring machines (CMMs) built on granite bases. Manually operated scanning heads served the entry-level market.

But, "CMMs are beyond the budget of many companies," says Martin Schuster, founder and chairman of Laser Design Inc., Minneapolis (www.laserdesign.com), a maker of 3D laser scanners. "And manual models can scan only small objects and with limited accuracy."

In response, LDI recently assembled a scanner line around gantries from Techno-Isel, New Hyde Park, N.Y. (www.techno-isel.com). The gantries provide a stable, rigid, and turnkey scanning platform that helps LDI build complete machines for about 50% less than existing models of comparable size.

Techno's use of heavy cast aluminum side plates support the X axis for increased stiffness. Bases contain THK, Schaumberg, Ill. (www.thk.com), ball-screw linear-motion slides. Antibacklash ball screws efficiently transmit power through rolling-ball contact between the nut and screw. The arrangement provides high rigidity and longer life than Acme screws and nuts that rely on sliding-friction contact for operation. The result: 0.004 in./ft positioning accuracy, 0.0004 in. resolution, and +/-0.0004 in. repeatability, all on a per-axis basis. That's about one-fifth as good as most CMMs when compared over the same metrics though more than accurate enough for jobs with less-stringent tolerances.

Mirroring complex shapes

LDI scanners use an active stereoscopic technique called laser triangulation to compute distance to an object's surface. Here, a line-range scanner illuminates the part with a line of laser light while a digital camera collects up to 15,000 coordinates/sec. That's about 100 times faster than point-range sensors, says the company.

Special inspection software compares deviation of scan data to CAD files. Other software produces Nurbs surfaces from scan data to create CAD models. Scan data can also be turned into STL and CNC toolpaths for part manufacturing.

One major theme park uses the scanners to generate 3D CAD files from concept models. Engineers may build a CAD model and NC toolpaths for a 60-ft-high cartoon character by scanning a 3-ft-high model, for example. The scanning approach takes about one-twentieth the time needed for CAD models prepared by hand, Schuster says.

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