Flat surfaces and straight lines are rare among the parts produced at Triumph Fabrications (Shelbyville, IN), which is mainly why the company upgraded its machine tool probe on one of its high-accuracy portal mills to a strain-gage-based Renishaw RMP600.
As an aerospace stretch-forming specialist, there is hardly a part or a die in Triumph’s shop that is not sporting a parabolic, compound curved or cone shape. Rapid, precise measurement of machining work on these parts-while the parts are still fixtured on the machine tool-is a core skill the company’s computer aided design (CAD) and computer aided manufacturing (CAM) department developed and refined to facilitate lean manufacturing. But it took an RMP600 strain-gage probe to bring the refined touch needed for consistent, high-accuracy measurement of small hole diameters, contours and geometric tolerances.
With no lobing-or true 3-D triggering characteristic-and a repeatability of 0.00001 inch, the strain-gage probe delivers high accuracy when triggered from any direction after a one-time calibration routine. The RMP600, combined with Renishaw’s OMV Pro software, gives the company’s Zimmerman CNC 5-axis portal mill coordinate measuring machine (CMM)-like capabilities, allowing measurement results that routinely come within 0.001 inch when double-checked with other devices. As a result, offline checks of work on a true CMM have been eliminated, saving the company many hours of production delays, potential damage and accuracy problems that arise when moving and re-fixturing high-value parts.
According to Triumph, it is hard to quantify the time savings, but it can be in the range of five times the cycle time for the part. For example, if a part can be machined in one hour, the extra setups caused by offline measurement can be five hours. The key is that the part is processed in one setup, improving overall cycle time and delivery.
Keys to SuccessTriumph Fabrications’ core capability is the stretch-forming of sheet, extruded, rolled shape and light plate metals for the aerospace industry. The company’s press capacity ranges from 10 to 750 tons, with the ability to stretch-form reverse forms using bulldozer attachments of 150 and 400 tons on its Cyril Bath presses. The company also has unique stretch forming capabilities to support complex single-plane and multi-plane forming of a variety of metals.
Various types of computer numerical controlled (CNC) machine tools are used to produce features on these formed parts, as well as to shape the forming dies themselves. Supporting this work upstream and downstream is a CAD/CAM department equipped with Catia V4 and V5, Vericut 7.0, Renishaw OMV Pro, AutoCAD, Inventor and other desktop engineering tools.
The other key to Triumph’s success is programming the model and geometry. The probing program runs on the CNC to collect the measurement data, which is then evaluated offline in OMV software with CMM-type algorithms. The software compares the measurement data with the map/model file and produces graphical and numerical reports. The software reports on the deviations from the nominal and will plot a graph to show the ranges.
Unlike a conventional machine tool probe, the strain-gage-based RMP600 does not have to be calibrated for each vector, which saves significant cycle time in a 5-axis environment.
The RMP600 is fundamentally different from conventional machine tool probes in that its patented Rengage sensing mechanism eliminates lobing, an error inherent to all conventional machine tool probes. If the lobing is excessive and high enough to impact measurement accuracy, operators must compensate by calibrating the probe along each measurement vector. This requires a complex calibration cycle and management of numerous probe offsets.
The RMP600 eliminates this. Strain gages measure the contact force applied to the stylus and generate a trigger after the strain threshold is exceeded. This results in low trigger forces, less stylus bending, submicron repeatability, no lobing and a true 3-D triggering characteristic. The RMP600 delivers significantly higher metrology performance, particularly on 3-D surfaces where many sensing directions are used, or in setup, when approach vectors to the workpiece may not be known. Logic inside the probe eliminates unexpected triggers resulting from shock and vibration by determining whether the strains seen at the gages are caused by contact with the part surface or a random event.
High ConfidenceWith its knowledge of the Zimmerman CNC 5-axis portal mill machine’s capabilities, condition and calibration history, Triumph Fabrications believes an inspection on its machine is equal to a CMM inspection. The machine is tested and calibrated to essentially the same requirements as its CMMs, and easily capable of producing part profile accuracies of 0.015 inch.
Triumph can verify its metrology performance by probing a known artifact, which has all the features it would encounter in its parts. Because of the high accuracy of this machine and its 5-axis capability, the strain-gage probe has proven to be a better match for Triumph’s measurement requirements. Periodic double checks against other measurement devices rarely show deviations greater than a thousandth of an inch, giving Triumph a high level of confidence and operational efficiency.