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General Superabrasives Knowledge

By
    Steven J. Kendjelic



Diamond is the hardest known abrasive (7000-9000 Knoop hardness) followed by Cubic Boron Nitride (C.B.N.)  (4700 Knoop hardness) typical hardened tool steels are 60 Rockwell measured on the C scale.  Due to the process of manufacturing plated and vitrified grinding wheels, increased wheel speed is possible.  

The diamonds used in today's superabrasive grinding operations conduct heat faster than any metal.  Careful coolant application is necessary to reduce and control the heat buildup during the cutting operation in the deep arc of contact. The high heat promotes the grain to pullout of the bond. While the metal bonded and resin bonded grinding applications with C. B. N. and diamond wheels conduct heat faster to the cutting area. The vitrified bonded grinding wheel provides a cooler cutting action. The addition of pore forming agents in the manufacturing process permits greater amounts of porosity in the vitrified grinding wheel.

This illustration is using a diamond disk to generate the required wheel geometry. This type of wheel forming utilizes the machine tools Y and Z-axes with the Cartesian coordinates directed from the computer numerical control (CNC). The advantage of this system is that multiple wheel forms can be generated using a multifunction-dressing device in place of a dedicated form roll.

Vitrified C.B.N. permits the use of rotary dresser or diamond disks to generate forms.  Preforming the abrasive wheel is usually the preferred method.  Diamond dressers are only used to true the super abrasive grinding wheel.  To condition the wheel an aluminum oxide abrasive stick is used to relieve the bond material.  Grit size for the abrasive stick is usually two times greater than the C. B. N. wheel being dressed.  The typical types of Borazon (GE product name) CBN are the following.  Type 1 is a medium uncoated product suited for vitreous and electroplated bond systems, Type 2 are specifically designated to promote crystal retention in resin bonded systems, this product is nickel coated. CBN 500 is a high strength monocrystalline product used in electroplated as well as vitreous and metal bond systems.  This is probably one of the most common products used in precision grinding.  CBN 550 is a microcrystalline abrasive used in vitreous bond systems. This product is used mostly for high metal removal rates in materials like alloy steels and nickel-based superalloys used in aerospace applications.  

In addition to cooling the cutting area during any grinding operation the additional requirement of any cutting fluid is good lubricity and sufficient pressure to remove swarf from the wheel.  The higher lubricity requirement leads the application toward operating using straight oil as a coolant. The need for better cooling during grinding leads toward a water soluble type coolant.  Some new types of coolants allow higher percentages of coolant to be used at pressures in excess of 200 pounds per square inch without foaming.  These new formulations in coolant technology satisfy both requirements for lubricity and cooling.

The high heat generated during grinding attacks the carbon in the diamond-grinding wheel, which creates additional heat to the grinding wheel. Typical concentrations used in C.B.N. grinding wheel applications are 40%.

The advantages of high performance grinding are that parts can be processed from solid in the hardened state, with better form holding capabilities and minimal burrs to the part sometimes eliminating secondary deburring operations.

Creep feed grinding is used to remove a large amount of material (vertical Infeed) with a slow grinding feedrate.  This process allows high metal removal rates (MRR) to be accomplished in a very short time as opposed to surface grinding.  Tangential forces are much greater in creep feed grinding due to the increase in the arc of contact by the deep cut.  Creep feed grinding does require high horsepower machines that are extremely rigid with a closed loop system to monitor the process. The high heat in creep feed operations using diamond or C.B.N. promotes wear flats in the wheel face, which creates more heat during the grinding operation. Usually the grain is pulled from the wheel face during grinding when it becomes dull.

In creep feed grinding higher wheel speeds are required due to the long arc of contact.  When wheel speed is increased the duration of the grain contact is decreased and so is the chip load. Higher surface speeds of grinding wheels alone have provided a substantial increase in productivity.  The higher wheel speed allows a higher volume of material to be removed from the workpiece, decreasing the cost per part.   In any superabrasive operation vibration at the point of contact is a situation, which must be avoided.  

Due to the extreme hardness of the superabrasive grain any vibration can lead to increased wheel usage and chatter to the work piece.  The vibrations in the contact zone cause the grain to fracture.  This can present a problem when polycrystalline is used.  The applications engineer has applied this abrasive for it's ability to fracture and expose new sharp points and when vibration is introduced wheel life decreases.   

In the use of Resin grinding wheels with smaller depths of cut the workpiece will rub the bond providing a sharpened wheel.  In large depths of cut where chip clearance is required, is where resin bonded grinding wheels reach their limit.  Using the maximum diameter grinding wheel possible also allows the heat to be dispersed.

Multi-axis CNC grinding systems provides the end user with continuous improvement abilities to reduce manufacturing costs.

The ability to accomplish multiple grinding operations in one clamping reduces the setup time required to manufacture today complex components.  The use of multiple fixtures on multiple machines increases the possibility of misfixturing.  The stack-up errors as the part is located and move to another station often result in increased scrap parts, further driving up manufacturing costs.

Conventional abrasives traditionally contribute to disposal costs as the abrasive, coolant residue and swarf removed from the grinding operation pose environmental problems.  Filter paper costs are increased due to the amount of grinding wheel abrasive deposited on the paper.

When using superabrasives grinding wheels the majority of the swarf consists of workpiece material.  This alone will reduce filtration costs since there is a minimal amount of grinding wheel grain being lost to the process.  It is also a known fact that coolant life is extended when using superabrasive grinding wheels due to the reduced amount of contaminates in the coolant.