The benefits of having both additive and subtractive functionality in a single, self-contained system are being realized, and the equipment advancements are unfolding quickly. The flexibility of applying different laser wavelengths and deposition rates is vastly improving in these additive hybrid machines. Every material and every application requires a nuanced process for the most efficiency for these machine tools to be practical in a manufacturing environment. Users are learning how to get the most out of theses systems.
One of the areas pushing the technology surrounds real-time, adaptive CAM software spurred by the current workpiece condition, garnered from metrology device data, such as probes and vision systems.
If an out-of-tolerance condition is trending, the software, via the machine control, can automatically adjust characteristics of the laser and subtractive tools to avoid scrap parts.
New vision device pixel counts are rising with the ability to characterize and analyze preset parameters and the melt pool. Adaptive control ensures that as the process moves between additive and subtractive, the intended surface or feature is maintained in production. It’s important that the machine maintains common centerline integrity between the additive deposition nozzle and the subtractive cutting tool. Aim for a sub-15 micron volumetric accuracy within the work envelope.
A trend with our customers in the aerospace jet engine turbine sector is using the hybrid approach and creating a much simpler casting/forging strategy. The complex features are added—in our case via laser deposition—and then finish-machined conventionally (subtractive). The benefits to these customers are lower raw material costs (and greater material yield for the material supplier), greater cutting tool life and less need for high priced, highly engineered cutting tools for machining.
In a simplified real-world scenario example, the first operation on the casting/forging is scanning the current geometric condition of the workpiece. A variety of metrology devices could be used for this that can be integrated into the machine, or if it’s scanned on a station just outside of the machine’s work envelope, a structured light scanning system or laser scanning system might be used. The point cloud data from that scan is collected offline on a shop-hardened and extra-powerful PC stationed near the machine. That data is compared with the CAD model of the workpiece, functioning almost as a “master” gage, and any geometric variances are noted and passed on to the CAM software. The CAM software makes an adaptive shift of the toolpath to ideally position the additive structure elements on the workpiece based upon the present part condition.
In this scenario, the material is deposited and then a subsequent, programmable scan occurs in the machine to determine the current condition of the workpiece and its geometry. The instrument data and the CAM software do their work, and any variance information is fed back to the machine control and the subtractive machining operations ensue. We are experimenting with a heat treat routine in our laser head, as well as an eddy current device in our system.
There are also new processes for using water soluble within a cycle that includes additive layering. In this technique, an air blow-off operation removes much of the volume of coolant still clinging to the part, followed by the laser applied at a wide focus. The surface is now prepared for a new feature to be added to it via laser cladding. The machine also needs protection from powder particles, and one of the hallmarks of a precision machine tool is its mechanical accuracy. To protect sensitive attributes like precision-ground ballscrews and way components, make sure it has special guarding and other protections similar to those used in graphite milling.
The combination of metrology hardware, adaptive CAM software and connectivity to plant-wide systems is making additive hybrid machine tool applications ever more practical on the shop floor.
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