The hydraulic casting components made at Rexroth Guss, a subsidiary of Bosch Rexroth, are characterized by their complex, core-intensive construction and many free-form surfaces. These costly parts, made of cast iron or spheroidal graphite iron, are invariably designed in 3-D CAD software. The transition from 2-D to 3-D design prompted the quality department to look for an appropriate measuring solution to inspect their parts.
“ The requirements for quality assurance have increased with 3-D technologies,” says Frank Mill, quality manager at Rexroth Guss. “Earlier, all major dimensions could be taken from a 2-D drawing for tactile measurement, which is no longer the case. Modern 3-D drawings include little in the way of explicit dimensions. The curves of the free-form surfaces are almost impossible to verify using traditional tactile measurement techniques.”
Rexroth Guss chose the Shapetracer from Wenzel Knotenpunkt as its 3-D measurement tool. Shapetracer is a compact high-precision line scanner that can be connected to the Renishaw PH6M or PH10M probe head via the Renishaw Autojoint connection. Consequently, both can be adapted to almost any coordinate measuring machine (CMM). In conjunction with the PH10M rotary swivel head, five axis measurements are also available.
The quality team was united from the outset that the new laser scanner should be used on a CMM with a stable support table, and not just on a mobile measuring arm. In addition, a sensor changer was installed to automate changing between tactile sensors and the laser head. Because the Shapetracer is cabled via the Renishaw Multiwire connection, its storage together with other measurement heads was no problem.
The company has yet to use all of the functions offered by Pointmaster, the software that comes with Shapetracer. “Target data to actual data comparisons are at the top of our wish list for getting started with the new measurement technology,” says measurement technician Achim Werthmann. “One of our next priorities will be surface reconstruction.”
Roughly summarized, the workflow in target-to-actual comparisons is as follows: First, the workpiece is scanned using the Pointmaster kinematics module. The software takes over the full command of the measuring process. The measuring machine can be completely and virtually mapped so that simulations of the measuring process are also possible. Next, a point cloud is generated from the scan data. From that, a polygon model with a homogenous surface is computed using triangulation. Finally, the polygon model is converted into a CAD model in IGES format. The quality technicians receive the CAD data, also in IGES format, from the design department. Both datasets are now aligned and compared with the software. Pointmaster highlights the differences between the two models in pseudocolors; the variances in dimension can be selected and marked by flags. All dimensions are listed and contrasted in the evaluation protocol.
During the transition to 3-D design, an outside measurement service was used for a while. “Outsourcing such work involves a lot of uncertainties as well as some lost time,” says Mill. This is particularly true for components with complex inner workings. These parts must be dissected to simulate both the external and internal contours.
“With this software there is no problem with constructing a model from an existing component that we scan from different directions,” says Werthmann, in praise of the solution.
The system provides a series of functions, both for aligning the components and for manufacturing homogeneous contour transitions to remove any cut surfaces and to add consecutively scanned areas. The assembly of the contours does not rely on mathematical tricks, but always uses real data, so a highly precise scanner is a prerequisite.
“We don’t know of any comparable systems that offer such high precision,” says Werthmann. After the CAD model has been generated, the finest surface structures become visible. Ingrained lettering, for example, appears distinctly on the display screen.