The slot on right with visible burr was ground with a plated CBN wheel; All other slots ground with Vitrium³ bonded wheel.


TG2 grain is manufactured by a sol-gel extrusion process


resulting in a hard, tough microcrystalline grit having an aspect ratio of about 8:1. Unlike standard aluminum oxide grains, that can fracture under low to moderate forces, the TG2 grain is tougher and less friable, and, when subjected to high forces, will tend to micro-fracture thereby creating a new sharp cutting edge. Because of a natural low loose pack density of high aspect ratio grains, wheels made with TG2 grains have an open struc- ture providing increased space for coolant and chip removal. Te testing at the Higgins Grinding Technology Center


started at a specific material removal rate of 1 in.³ per minute per inch of wheel width, which is comparable with removal rates currently used with plated cBN wheels. As the removal rate increased, power increased; however, the specific grind- ing energy or the energy to remove a cubic inch of material decreased significantly. It is important to understand how high removal rates


influence wheel life and cost since the wheel will wear or break down when we grind the material. Wheel life is most often characterized by the ratio of the volume of material removed to the volume of wheel loss, referred to as the G-ratio. Unlike other grinding processes and wheels, the exceptionally high removal rates did not cause these Nor- ton Vitrium³ wheels to break down at significantly higher rates. Form loss is another area of concern, as with any slot grinding process the wheel corner radius needs to be


restored during the truing/dressing process. Wheel life is calculated by adding wear occurring during the grind and the dress amount necessary to restore form.


Flight-Critical Components Since all rotating engine components are considered


flight-critical, these components must meet stringent surface integrity criteria for thermal damage and abusive machining. Engineers at Norton contracted an independent laboratory with experience in the inspection of machined and ground aerospace components to inspect the ground slots, resulting in no evidence of thermal damage or white layer. In addi- tion, surface distortion and strain depth did not exceed 0.001" (0.025-mm) depth on any of the slots. By designing a wheel with the latest Norton Vitrium³


bond and the high aspect ratio TG2 grain, customers have a new solution available for transforming high-nickel cast- ing, forgings or wrought blanks to near final shape. Since grinding wheels can be dressed on the machine and are not susceptible to catastrophic failure due to edge break- down or depth-of-cut notching, grinding brings a new level of reliability and consistency to heavy stock removal processes. The Norton standard bonded or superabrasive products can then finish the components quickly and easily while preserving the strict surface integrity require- ments of our aerospace customers. ✈


Aerospace & Defense Manufacturing 2014 157


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