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Milling vs. Grinding for Rapid Stock Removal

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

An argument, with test data, for the grinding alternative.

A recent effort by the Norton Advanced Applications Engineering Group demonstrates that for difficult-to-machine materials, grinding can be an economical alternative to other machining processes. The high removal rates achieved with the Norton Vitrium³ wheels provide a robust and reliable process which can be easily automated and is not susceptible to the variability experienced with traditional machining processes due to premature or unpredictable tool failure. The power required to achieve these high material removal rates is significantly lower than traditional grinding processes and only two to three times higher than traditional machining processes.

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Machine set up for test run at the Higgins Grinding Technology Center.

Many aerospace components used in high and low-pressure sections of a turbine are made from high-nickel superalloys which may be cast, forged or sintered using powder metallurgy techniques. These alloys are notorious for being difficult to machine due to their high strength, corrosion and fatigue resistance, and low thermal conductivity. The same attributes which improve their engine performance also result in making the alloys more difficult to machine. Many of these components have as much as two-thirds of their original weight removed to produce the finished component, while turning, milling and broaching processes are traditionally utilized to remove most or all of the material.

Tool Failure

When machining high-nickel alloys, tool failure primarily occurs due to tool edge breakdown through chipping or plastic deformation, or notching at the depth-of-cut line. These materials also have a chemical affinity at high temperature to the tool material, resulting in welding of workpiece material to the cutting edge. The welded material may break free randomly during cutting, taking a small portion of the cutting edge with it, resulting in reduced tool life. The higher temperatures encountered during machining these materials also lead to the formation of a work-hardened layer on the part surface. On subsequent passes, this hardened layer causes increased tool wear, known as depth-of-cut notching. If this notch gets too large, the tool can fail catastrophically. These alloys produce long chips and when the chips are not cleared from the cutting zone, they can lodge between the work and tool thereby breaking the tool. If a component has an interrupted surface, repeated entry and exit of the tool can also cause force and temperature shocking which further reduces tool life, especially in the case of ceramic cutting tools. In combination, these failure modes lead to an unpredictable tool life.

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Open structure of the TG2 wheel (left) in contrast with the 38A wheel.

If a tool should fail on, or close to, final dimension, then the part is removed from production and sent for review to determine if final part performance has been compromised. In addition, operators are asked to not index the insert while taking the final pass to avoid leaving marks on the part surface. Consequently, to minimize production interruptions and to minimize risk of damaging these high-value parts, engine builders typically use conservative values for tool life and operating parameters. They also employ lower cutting speeds than conventional high-speed machining, which is detrimental to productivity.

Another reason to employ lower cutting speeds is to preserve the surface integrity of the components. If machining parameters are too aggressive or the tool is left in the cut too long, the cutting temperature will rise and cause the formation of white layer. White layer occurs due to phase transformation resulting from rapid heating and cooling of the work surface, grain refinement due to severe plastic deformation of the surface, and chemical reaction of the work surface with the environment. White layer affects the fatigue life of the parts significantly, thereby reducing the service life of the part.

Wheel Capacity Testing

Testing was done at the Norton Higgins Grinding Technology Center on IN718 material to demonstrate the capacity of the latest wheel technology combining Norton Vitrium³ bond with the high aspect ratio TG2 grain. One-half inch (12.7-mm) wide slots ¾” (19.1-mm) deep were ground into two stacked 1″ (25.4-mm) thick parts. The removal rate was increased incrementally until either visual burn or excessive wheel corner breakdown occurred.

In the past, grinding processes have been unable to reach high stock removal rates due to a variety of factors, including insufficient space in the wheel face to accommodate large volumes of material, weak grains that dull and fracture prematurely, or weak bonds that take up a large portion of the wheel volume and release the grains prematurely. In wheels without sufficient space, chips can pack into the face of the wheel and these chips rub on the part surface damaging it due to increased frictional heat. Wheels made with the new Norton Vitrium³ bond coupled with their TG2 grain mark a major step in overcoming limitations in wheel construction. The Norton Vitrium³ bond is able to hold onto the TG2 grain under very high forces while at the same time due to the bond’s chemistry taking up less volume in the wheel. This lower bond content, along with the natural porosity of the high aspect ratio TG2 grain, means there is more space to accommodate coolant and a larger volume of chips.

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 structure providing increased space for coolant and chip removal.

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

The 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 grinding 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 Norton 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 addition, 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 casting, 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 breakdown 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 requirements of our aerospace customers.

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