The hydraulic casting components made at Rexroth Guss (Lohr am Main, Germany), the subsidiary of Bosch Rexroth AG (Lohr am Main, Germany), are characterized by 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. The transition from 2-D to 3-D design some years ago also prompted the quality department to look for an appropriate measuring solution.
“The requirements for quality assurance have increased with 3-D technologies,” says Frank Mill, manager of product quality management at Rexroth Guss. “In the past, all major dimensions could be taken from a 2-D drawing for tactile measurement, which is no longer the case today. 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.”
Laser Scanning SolutionThe Rexroth Guss quality team discovered that metrology provider Wenzel (Wiesthal, Germany) was offering a low-cost noncontact scanner called the Shapetracer. This high-precision line scanner was developed together with Pointmaster evaluation software from Wenzel Knotenpunkt (Swabian Balingen, Germany), a C-technology specialist.
The Shapetracer is a compact scanner that can be connected to the Renishaw (Wotton-under-Edge, United Kingdom) PH6M or PH10M probe head via the Renishaw Autojoint connection and, consequently, adapted to many coordinate measuring machines (CMMs). In conjunction with the PH10M rotary swivel head, five-axis measurement is possible.
The quality team agreed from the outset that the new laser scanner should be used on a CMM with a stable support table, rather than a mobile measuring arm. “Our future intention is to fully automate the measurement of several workpieces or die plates with multiple molding stations,” says Mill. “That is only possible on a large stationary measuring machine.” Rexroth Guss purchased a Wenzel RS 1012 horizontal arm machine, and the quality department now has a complete system from a single source.
A further step is the installation of a sensor changer to fully 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 is not a problem.
High-Performance SoftwareThe Rexroth Guss quality team has not yet used all of the functions offered by the Pointmaster software. “Target data to actual data comparisons are at the top of our wish list for getting started in the new measurement technology,” explains Achim Werthmann, measurement technician at Rexroth Guss, who is well versed in operating the scanner solution. “One of our next priorities will be surface reconstruction.”
Roughly summarized, the workflow in target-to-actual comparisons is as follows: The workpiece is scanned and the Pointmaster kinematics module supports the operator, and the software takes over full command of the measuring process. The measuring machine can be completely and virtually mapped so that simulations of the measuring process also are possible. A point cloud is generated from the scan data. From that a polygon model with a homogenous surface is computed. Finally, the polygon model is converted into a computer-aided design (CAD) model in initial graphics exchange specification (IGES) format. The quality technicians receive the CAD data, also in IGES format, from the design department. Both data sets now are aligned and compared with the software.
Pointmaster highlights the differences between the two models in pseudocolors; the variances in dimension can be discretionally selected and flagged. All dimensions are listed and contrasted in the evaluation protocol.
“As this involves complex workflows, the measurement technician must fully master the individual process steps,” says Mill. “For this reason it was important to us to build up the relevant expertise inside our company.” In the transition phase to 3-D, an external measurement service provider was appointed. “Any outsourcing of such work involves a lot of uncertainties as well as time loss.”
This applies particularly to components with complex inner workings. These parts must be dissected in order to simulate both the external and internal contours. “This software means it is no problem to construct a model from the component scanned in different directions,” says Werthmann.
The system provides a series of functions, both for aligning the components and for manufacturing homogeneous contour transitions in order 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. After the CAD model has been generated, the finest surface structures become visible. Ingrained lettering, for example, appears distinctly and clearly legible on the display screen.
Wenzel Group GmbH & Co. KG
+49 6020 201-0
BenefitsThe Shapetracer laser scanner enables the Rexroth Guss quality assurance team to measure the curves of free-form surfaces designed in 3-D.
With Pointmaster software, the team can perform target-to-actual comparisons, identifying how well manufactured parts reflect their CAD models.
Because it can build a model from scans taken from different directions, the software allows the team to model parts with complex designs.