The Hard Truth

Superabrasive wheels cost more than conventional-abrasive wheels, but users are willing to pay the price in exchange for superior-quality surfaces, increased productivity, and lower overall production costs. To see a return on their investment, however, they must use the correct truing and dressing techniques for these types of wheels.

Truing alters the wheel to ensure that it is running concentric with the spindle axis. When necessary, truing also forms a precise shape on the face of the wheel. Truing some types of bonds results in a smooth, closed face that cannot cut, because there are no superabrasive grains protruding past the level of the bond on the wheel’s outer diameter (OD). In order to expose these abrasive grains, the user must dress the wheel by eroding some of the bond around these grains.

Fortunately, truing and dressing are not the hurdles to the effective use of superabrasive wheels they once were. In the past, users who employed conventional techniques found that the hardness of the diamond or cubic-boron-nitride (CBN) grains quickly eroded their truing and dressing tools. Users today avoid problems by truing and dressing with devices and methods developed especially for superabrasive wheels. Although the superabrasive grains’ hardness makes truing and dressing these wheels a challenge, in another sense it simplifies wheel maintenance. Once users have mastered the proper conditioning techniques, they will find themselves employing these techniques infrequently, because superabrasive wheels maintain their geometry and sharpness much longer than conventional wheels do.

The Basics

Two variables determine what conditioning procedures are needed and what techniques can be used. One variable is the type of superabrasive grain being used in the wheel. A CBN wheel can be trued by single-point or multipoint diamond tools or diamond abrasives. The softer CBN grains do not erode the diamond as quickly as diamond grains do. With diamond grains in the wheel, the options are limited to techniques that employ a conventional-abrasive grinding wheel to grind the superabrasive wheel to its proper truth and form.

The other variable is the type of bond used to hold the superabrasive grains in place. Both resin- and metal-bond superabrasive wheels must be dressed as well as trued to achieve the best possible performance. This is because these bond materials lack porosity, and truing leaves the grains solidly embedded in the wheel’s OD surface. Vitrified-bond wheels, which are porous, do not require dressing, because the pores themselves leave enough space around the grains to allow them to cut. Single-layer, or electroplated, wheels, with a one-grain-deep coating of superabrasive on their OD, rarely require any conditioning at all.

Other variables determine how easily and quickly the wheel can be trued. Generally, users find wheels with high concentrations of superabrasive or large grains harder to true. The variable porosity of vitrified-bond wheels has an impact on the truing process as well. The more porous the wheel, the easier it is to true.

Tools and Techniques

A user preparing to true a superabrasive wheel can choose from a wide variety of methods. These techniques typically fall into one of five categories.

Stationary tool truing. As the name implies, in stationary tool truing the tool does not move. To bring the truing point in contact with the entire wheel face, the rotating superabrasive wheel must move across the tool. The tool is easy to install on the grinding machine, because it can be fixtured to the machine’s bed. Some CNC grinding centers simplify installation even further by providing built-in stationary truing devices.

Despite this ease of installation, however, stationary tool truing is not the ideal truing technique for superabrasive wheels. Its drawbacks include excessive truing forces and low tool-wear resistance. This method will only true CBN wheels, because it uses single-point or nib diamond tools. And the method is limited to vitrified- and resin-bond wheels, because the truing point would smear rather than erode a metal bond.

Typically, single-point stationary tools are only used to true small CBN wheels in small-scale applications. Because the heat and forces of the truing operation are concentrated on one point, the tools’ diamond point wears too rapidly to be used productively for larger wheels or applications. Productivity suffers with the large-scale use of single-point tools, because the operator frequently has to stop the process, either to turn the diamond point to present a sharp edge to the wheel or to change the truing tool.

Users who want an easy-to-install truing tool for larger applications can use nib truing tools, which feature a number of small diamond points in a metal bond. When nib truing tools are used, the heat and forces of the truing operation are distributed over several points and, hence, take longer to reach a level high enough to accelerate tool wear. However, when nib tools are used on superabrasive wheels, their rate of wear is still too great for high-production applications.

Rotary powered truing. Many users have found modern rotary powered truing to be a faster and more accurate alternative to stationary truing. This method also is limited to vitrified- or resin-bond CBN wheels. Rotary powered truing uses cutters or cups, which are round wheels with diamond cutting points on their periphery. The wheels are mounted on a motorized unit that rotates them against the superabrasive wheel. This unit can be mounted on the machine table. As an alternative to table mounting, the motorized unit may be incorporated into the machine design by the machine’s manufacturer, or it can be easily retrofitted to a sufficiently rigid machine in good condition.

Rotary powered units can be used to true superabrasive wheels with straight or profiled faces. Straight-faced wheels are trued with rolls or cups. To true a profiled wheel, the user may employ a diamond roll mounted on a rotary powered unit to perform plunge truing, which trues the entire width of the wheel’s face at one time, or a rotary cutter mounted on a powered unit may be used to trace a profile into the wheel. A third method for truing a profiled wheel using rotary powered equipment involves crush truing, in which the rotating truing wheel actually crushes the bond into the desired shape.

Rotary powered truing offers significant advantages over stationary truing. The truing wheel’s rotation distributes heat and forces over a wide area, significantly reducing the amount of wear and damage generated by the truing process. The forces in stationary truing typically increase as the superabrasive wheel rotates against the truing tool; these forces do not increase as rapidly or as much with rotary powered truing. The rotating truing wheel also allows the user to control the amount of material removed during the truing process.

Form truing. There are two basic methods for truing a form onto a vitrified- or resin-bond CBN wheel off the grinding machine. Both methods are typically performed on an optical grinder, in which the CBN wheel and the diamond cutting wheel are brought together as they rotate at two different speeds. The optical grinder’s built-in comparator allows the operator to precisely true the wheel to the desired form.

The first form-truing method uses a rotary diamond cutting wheel that is typically only 0.060" wide. This wheel is guided along the form of the CBN wheel by CNC. To accurately trace the profile into the wheel and avoid rapid or accidental movements, which can chip the CBN wheel, the machine must be able to move in extremely small increments.

The second method uses a rotary diamond cutting wheel that is the same width as the CBN wheel. A mirror image of the CBN wheel’s desired form is molded into the cutting wheel’s face. When the cutting wheel is plunged into the CBN wheel, the form is transferred. The recommended tool for this operation is a small-grit diamond rotary cutter. Truing a wheel with this method requires precision infeeds of only 0.000002 to 0.000004 ipr.

Brake-controlled truing. This method can be used to true either diamond or CBN wheels, and it can be used with metal-bond wheels. Its use of a conventional-abrasive wheel rather than a diamond wheel makes it highly versatile. Users typically select an aluminum-oxide (Al2O3) or silicon-carbide (SiC) M-grade wheel with an 80 grit for this application. The truing wheel is mounted on a braking mechanism, which is, in turn, mounted on the grinding machine’s table. This mechanism controls the truing wheel’s speed, preventing it from spinning as fast as the superabrasive wheel when the two come in contact. The difference in speed causes the conventional-abrasive wheel to scrub the face of the superabrasive wheel, thereby eroding the bond.

This scrubbing action quickly cuts the desired geometry into the wheel face and trues the wheel to its mounting arbor or the machine spindle. Although the conventional-abrasive wheel wears more quickly than the superabrasive wheel it’s truing, it will typically condition several wheels before it must be replaced.

Grinding to truth and form. A method that can true either diamond or CBN wheels and put the desired shape on profiled wheels entails mounting the superabrasive wheel and its adapter or mounting flange in the work position on a grinding machine and grinding the face with a conventional-abrasive wheel. Typically, this operation is performed on a tool-and-cutter grinder, an OD grinding machine, or a commercially available machine designed specifically for truing superabrasive wheels. Because this procedure is highly effective and fast, many manufacturers use it when producing superabrasive wheels.

To true a superabrasive wheel, the user selects a conventional-abrasive grinding wheel that has a diameter equal to or larger than that of the superabrasive wheel. A 100-grit Al2O3 or SiC wheel in the J or K grade generally is used. He must then set up a proper speed ratio between this wheel and the superabrasive wheel by adjusting the speed at which the superabrasive wheel turns. The superabrasive wheel’s speed generally is set at about 10% of the conventional wheel’s speed. So, with the conventional-abrasive wheel running at spindle speed (approximately 5,500 sfm), the superabrasive wheel speed would be set to 550 sfm. Both wheels should be traveling in the same direction at the point of contact.

The Sharp-Dressed Wheel

After any of the truing operations described, a nonporous wheel’s face will be closed and dull. Dressing will be necessary to erode some of the bond on the surface and expose the sharp edges of the superabrasive grains. Abrasive-stick dressing is probably the most common way for users to condition superabrasive wheels that must be dressed after truing. A stick of bonded conventional abrasive is pressed against the face of the wheel as it turns. The user should select a stick with an abrasive grit one or two sizes finer than that of the superabrasive wheel. The sticks are available in the same abrasive grades as conventional-abrasive wheels. For most applications, J-grade conditioning sticks are suitable. For more aggressive dressing action, H-grade sticks can be used.

These sticks can be manually pressed against the wheel, but an automated device designed for production-grinding applications will deliver a more reliable and precise level of dressing. Automated devices are suitable for a variety of wheel faces. They can be mounted on the grinding machine, and they can be driven by air, electric, or hydraulic power. When an automated device is equipped with a spring-loaded magazine that automatically loads sticks into position, the productivity of the dressing process is increased. Magazines are available that hold up to 20 dressing sticks.

With manual dressing, the user can use the amount of stick consumed to judge when the wheel has been properly prepared. A general rule of thumb is to use half a stick per wheel. Using stick consumption as a gage only provides a general indication of wheel sharpness, however. With an automated device, the user can judge wheel sharpness by the dressing unit’s infeed rate. This is a constant value that provides a more accurate and reliable gage.

Gaging Results

In general, heavy stick consumption paired with unsatisfactory grinding can be remedied by using either a slightly harder stick grade or a lower infeed rate. Wheel manufacturers suggest starting at a low infeed rate using fairly soft H- or J-grade sticks. Then, over the course of several dressings, the user can optimize the operation by gradually increasing the hardness of the stick grade or the infeed rate until the desired wheel-face roughness is achieved.

Adjustments to the dressing procedure may be called for if the user observes burn on the workpiece or poor stock-removal rates after grinding with a dressed wheel. Burn may be caused by a wheel face that has not been opened enough, but it also may be caused by improper coolant application. The user should first ensure that ample coolant is being directed at the grinding zone. If the problem persists, then he should modify his dressing technique. With a manual dressing operation, this would mean increasing the amount of stick consumed. An automated operation might require an increase in infeed rate or dressing pressure.

Severe or excessive dressing causes low stock removal and high wheel wear. To eliminate this problem, use a softer or finer-grit stick, lower the dressing forces, or decrease the abrasive volume.

If a user has selected a wheel with an extremely aggressive cutting action, he may need to restore its action with periodic stick dressing. This removes swarf and exposes the grain. If the wheel has been properly trued and dressed prior to its first use, it needs only this minor conditioning to perform well for most operations.

About the Author

Leonard Pukaite is senior product engineer at Norton Co., Worcester, MA.