Introduction to the Actualities of Dressing and Truing
A grinding wheel is composed of bonding media and abrasive (conventional wheels) or super abrasive grits. Bonding media in either conventional or super abrasive grinding wheels may be generally classified as resinoid, metal, or vitrified, where resinoid constitutes the softest bond and vitrified, the hardest. Typical grit material in conventional grinding wheels includes aluminum oxide and silicon carbide. In super abrasive grinding wheels, typical grit material includes diamond and cubic boron nitride (CBN).
Dressing is the operation that involves removing bonding material in order to expose the sharp grits in the grinding wheels. It is a sharpening process that is periodically required to maintain consistency and quality grinding wheel surface finish. The truing process involves the manipulation of the grinding wheel geometry. This process is responsible for creating the grinding wheel profile and assuring that the profile is concentric. Oftentimes, “dressing” and “truing” are used interchangeably, even though they are separate operations. In most processes, however, both operations are accomplished simultaneously. As a specific profile in a grinding wheel is being produced (truing), the correct grit exposure (dressing) is also being generated.
Comparison between the Single Point versus the Rotary Dressing
Single Point (CNC Dresser)
The traditional method of wheel dressing and truing with single point dressers usually equipped with diamond(s). Sometimes, a cluster of single points or a solid block is used. By advancing a quill from the dressing unit towards a rotating grinding wheel, forms are generated and surface finishes are obtained. (In some cases, the single point may have both infeed and cross feed motions). The surface finish of the grinding wheel will be affected by varying the amount of material removed from the grinding wheel per pass as well as the rate at which the single point traverses across the grinding wheel.
Single point dressers generate more friction between the dresser and the grinding wheel than do rotary dressers. Because single point dressers are stationary, they inherently tend to “rub” the bonding agent away from the grinding wheel during the dressing/truing process. The increased friction generated produces heat, which is transferred to the grinding wheel. The heat causes the grinding wheel to expand, affecting the precision of the intended grinding wheel form. In extreme cases, the additional heat generated may also change the characteristics of the bonding agent in the grinding wheel. This can result in an inconsistent or “gummy” grinding wheel surface finish.
Rotary dressing and truing features a spindle that will drive a dressing and truing roll, creating the desired profile into the grinding wheel. The spindle is generally powered pneumatically, hydraulically, or electrically. During the dressing/truing process, the dresser unit advances the dressing/truing roll on the spindle towards a rotating grinding wheel. (Again, the dressing/truing mechanism may have both infeed and cross feed motions). As with single point devices, the surface finish of the grinding wheel will be affected by changing the amount of material removed from the grinding wheel per pass as well as the rate at which the rotary dressing and truing spindle traverses across the grinding wheel. Moreover, with a rotary dressing and truing spindle, a grinding wheel’s surface finish may be manipulated by changing the speed and direction of the dressing/truing roll.
Rotary dressers actually “cut” the bonding agent away from the grinding wheel. This cutting action produces chips as material is being removed. Less friction results with this process because most heat generated is transferred to the chips, as opposed to being transferred to the grinding wheel. Hence, less thermal expansion is observed and consistent bonding material properties are maintained.
Super abrasive grinding wheels are harder than conventional grinding wheels. More heat is generated as the wheel hardness increases. Therefore, because rotary dressing/truing generates less heat, it is essential for dealing with super abrasive grinding wheels.