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Grinding Over Machining

David Graham
By David Graham Applications Engineer, Norton | Saint-Gobain Abrasives

The aerospace industry is known for using materials that defy machining with conventional tools and processes. The properties of these materials, high strength at high temperature that allow the components to survive in the hostile environment of an aerospace engine are the same attributes that make them difficult to machine.

David Graham

Whether the parts are cast, forged or made from sintered powdered metal, most all have 50 percent or more of their original volume removed before final form is reached. This is most commonly accomplished by turning, milling and broaching.

Because of the material properties and the high value of the parts, these operations are usually run at conservative feeds and speeds to ensure that the tools don’t fail or damage the part.

Regardless of the parameters and part tolerances used in machining, the surface quality degrades as the tool wears, which can reduce the part’s life in the engine.

In contrast, a grinding wheel is easily dressed—keeping the cutting edges of the abrasive sharp and the wheel shape constant, resulting in consistent finishes within close tolerances.

A good example is an aerospace customer that needed to cut slots into a disk of IN718 material and wanted to compare a milling process with grinding.

The engineers at Norton | Saint-Gobain Higgins Grinding Technology Center conducted an evaluation with two different abrasives: one with Norton Targa ceramic aluminia TG2 grain and the other with Norton Quantum ceramic alumina 5NQX grain. Two plates of IN-718 were stacked and four 1/2" wide slots were ground 1/2" deep for each condition without dressing the wheel. A depth of cut of 0.100" was arbitrarily selected and feed was increased in increments of 20"/min. until wheel wear was deemed too high. In the case of the 5NQX wheel, a feed rate of 70 ipm was reached before wheel wear was deemed too high. In the case of the TG2 wheel, the feed was increased to 180 ipm. Material removal rates of the TG280 and 5NQX46 wheels were compared with rates stated by different end mill manufacturers. Grinding proved to be the most effective, productive material removal process.

Aerospace Turning

Newer nickel alloys used in aerospace can be even more difficult to machine than legacy materials, such as IN 718.

A customer was having difficulty with turning one of these new nickel alloy materials. Inclusions in the material were causing unpredictable tool failure, and tool life was very low due to the high strength of the material.

Using the new Norton Vitrium3 bond along with the extruded TG2 grain, Norton engineers were able to grind the part at a feed of 0.025" per part revolution and a work speed of 160 ipm, reaching a removal rate of 4.0in3/min/in.

Under these conditions, eight cubic inches per minute were removed using a two inch wide wheel. With the tool change capabilities in today’s machines, separate wheels can be used to reach and finish complex part geometries in much the same way as a traditional CNC lathe.

If the surfaces require grinding after turning, then the turning operation can be eliminated.

Cost savings on aerospace manufacturing processes can come from many places, whether in reduced capital expenditure, consumable tooling, logistics, or cycle time reduction.

Today engineers can identify opportunities where an abrasives solution will outperform traditional machining processes.

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