June 2012 / Volume 64 / Issue 6|
By John R. Besse, Saint-Gobain Abrasives Co.
Understanding and controlling wheel truing and dressing forces when rotary plunge dressing.
Rotary plunge truing and dressing is the use of a full-form diamond roll plunged into the grinding wheel face in parallel axis to each other. This is the most accurate and productive way to true and dress, or form, a grinding wheel.
It is important to understand the relationship of dresser speed ratio, or relative speed, of the diamond dressing roll and the grinding wheel and its effect when grinding. The equation for speed ratio is Vr (sfm) ÷ Vs (sfm), where Vr is roll velocity and Vs is wheel velocity. Often, the industry standard of 80 percent (+0.8) unidirectional, where the dressing roll and grinding wheel travel in the same direction at the point of interaction, is used. This is often achieved with a perfectly stiff dressing system.
Reverse-plated diamond rolls used in dressing operations. All images courtesy Saint-Gobain Abrasives.
However, most dressing systems are the least-stiff area of the grinding operation in terms of dynamic and static stiffness because of space, structural and economic limitations. Also, this ratio may have to be
The dresser speed ratio impacts dressing and grinding performance—mechanically and geometrically—when applying vitrified-bond wheels. For this article, truing and dressing vitrified wheels are the same process because—unlike resin- or metal-bond wheels—they generally don’t need subsequent dressing after truing.
These tendencies also apply to rotary profile truing. However, because rotary profile truing uses much narrower rolls, the radial forces are an order of magnitude lower.
Wheel Sharpness, Roundness
Typically, dressing achieves the desired grinding wheel sharpness to impart the specified surface finish. Sharpness, however, is secondary to wheel roundness. If a dressing system is not stiff or dressing parameters overpower it, it becomes difficult to achieve wheel roundness. Often, “lobeing,” an out-of-roundness condition, occurs as a result of high radial truing or dressing forces.
Lobeing occurs when the grinding wheel is not perfectly round and transposes this imperfection to the workpiece. This also causes excessive wheel wear because a pulsing action occurs, mottling the lobeing. Surface finish becomes rougher than what the abrasive grit size would normally generate.
To obtain the optimal dresser speed ratio and successfully true and dress the grinding wheel, the relative speed of the diamond dresser to the wheel must be adjusted to balance the desired sharpness with the allowable radial forces.
Historic data shows the effect of the relative speed of the dressing roll to the grinding wheel. Unlike a positive speed ratio where there’s a unidirectional condition, a negative speed ratio implies a counterdirectional point of contact between the grinding wheel and dressing roll (Figure 1).
Figure 1. Unidirectional (top) vs. counterdirectional dressing.
Figure 2. Speed ratio vs. surface roughness.
As the relative speed increases, the wheel’s surface roughness decreases, which improves workpiece surface finish and decreases the radial dressing forces (Figure 2). However, the improvement in surface finish is the result of a duller wheel face and higher grinding forces, or a higher specific grinding power.
As the relative speed decreases, it leads to a coarser surface finish on the workpiece, but a sharper grinding action, or lower grinding forces. This causes higher dressing forces. However, the dressing system may not be stiff enough to endure those parameters, and the grinding wheel will not be properly trued.
If lobeing occurs as a result of high dressing forces, the diamond dressing roll may not wear via attrition, the desired mode of wear, and catastrophic wear occurs instead.
These limitations in dressing system stiffness can also depend on the type of grinding operation because high dressing forces can’t be applied to a small wheel on an extended arbor or quill. Deflection is eminent and the dressing forces must be lowered. There are times when the direction of the roll and wheel must be reversed to obtain a high-enough relative dressing-roll-to-grinding-wheel speed to lower the radial forces enough to get roundness and diamond dressing roll wear through attrition.
When selecting a dresser speed ratio, be sure it does not put the system into a resonant frequency because the resulting excessive vibration will create problems, such as a poor surface finish as a result of chatter (see Figure 5). If this occurs, increase the relative speed 10 percent.
Diamond roll wear can also dictate the speed ratio, or relative roll-to-wheel speed. This is often seen when using hard abrasives like silicon carbide and superabrasives. A +0.8 speed ratio may be effective for a 60-grit, vitrified-bond, aluminum-oxide wheel but not for a 120-grit, vitrified-bond CBN wheel (see Figure 4). A +0.8 speed ratio may be overly aggressive, though not necessarily due to the finer CBN grit—120 has more cutting edges per unit area than 60-grit Al2O3 and therefore does not have to be as roughly dressed. Also, the rougher dress may accelerate wear in the CBN wheel and diamond dressing roll.
In addition, the high thermal conductivity of CBN can cause a CBN wheel to develop larger wear flats without failure, whereas Al2O3 is an insulator and an Al2O3 wheel fails because of dullness with smaller wear flats. Therefore, a +0.5 dresser speed ratio is usually suitable to effectively dress CBN vitrified wheels. Those wheels are less sensitive to lower speed ratios because they self-sharpen with use. In addition, lower speed ratios extend diamond roll life by exerting lower radial dressing forces.
Figure 3. Effect of crush ratio on normal truing force.
Because of the aggressive shape and hardness of silicon carbide and the extreme hardness of diamond, a high relative dressing-roll-to-wheel speed and counter-directional speed, or negative, ratios are recommended to achieve diamond dressing roll wear via attrition and lower dressing radial forces. Vitrified abrasive products tend to open up, or sharpen, during grinding and will usually be fully sharp after the first roughing cycle.
Vitrified-bond wheels are relatively easier to true and dress than most other bond systems. That’s because they have a glass or refractory bond and at least 50 percent or more inherent porosity, whereas most other bond systems have no porosity and are made from more resilient materials.
Other bond systems for superabrasives often have to be conditioned after truing to make them sharp enough. Therefore, lower speed ratios or higher relative speeds are preferred to achieve low radial truing forces. The conditioning operation using fine, vitrified abrasive sticks plunged into the wheel face determines and controls sharpness.
Too high a relative speed can dull the abrasive. Therefore, only a relative speed necessary to optimize the truing and dressing process should be used.
Figure 4. Different truing parameters are recommended for different abrasives and bonds.
Optimal dressing provides the same level of sharpness anywhere along the wheel face. Optimal dressing, however, is compounded with the “compensation syndrome,” where feeding the wheel into the roll 0.001" radially results in only about 0.0001" of wear along a side angle in the wheel. Sharpness or roughness quickly changes as the speed ratio is lowered or the relative speed is increased. Large changes in geometry will occur when the wheel is too sharp or too dull, depending on where that dullness or sharpness is located along the wheel face.
The speed ratio is monitored by the wheel OD and the roll diameter. Using a +0.8 dresser speed ratio may yield only a +0.5 speed ratio or less along a deep drop in the wheel form or wheel face. The change in wheel sharpness or surface roughness is significant in this deep-drop part of the curve, and the grinding wheel could display a major gradient effect (change in grade hardness across the face or form of the wheel). For example, the wheel could experience as much as a two-grade increase in hardness, caused by the difference in the speed ratio at any given point along the form in the wheel face. Using a +1.2 speed ratio, you may get uniform sharpness along the entire form. The specific grinding energy should be consistent throughout the form.
Dressing Roll Size
Where high radial truing forces are a concern, a small-diameter diamond dressing roll provides a lower equivalent diameter, or contact area, than a large-diameter roll. This also lowers the radial dressing forces. Balancing dressing roll size with the dresser speed ratio can reduce the truing forces enough while achieving adequate or even desired wheel sharpness.
Figure 5. A chattered workpiece produced by a bad dress.
The change in dresser speed ratio as it relates to wheel geometry is less when using smaller diameter dressing rolls vs. larger diameter rolls. This change should be considered when grinding steep angles or radii to get a consistent wheel face. A small-diameter roll will have a shorter life than a larger roll because it has less diamond. However, increasing diamond density can extend roll life.
When dressing with a rotary plunge roll, the dresser speed ratio appears to have the dominant effect on truing and dressing forces because of the high contact area plunge dressing provides. There are other considerations, such as infeed rate and dwell, or spark-out, time. The construction of the roll also affects surface finish and dressing and grinding forces when the diamond particle size, density and bond matrix changes.
Understanding radial forces during the wheel dressing process is as critical as ensuring grinding wheel sharpness, having a robust dressing process and maintaining optimal diamond dresser roll life.
Understanding the effects when changing truing and dressing parameters allows for effective process adjustment, when needed, and assures a consistently robust grinding process. CTE
About the Author: John R. Besse is senior applications engineer at Saint-Gobain Abrasives Co., Worcester, Mass. Contact him at John.R.Besse@saint-gobain.com. The author acknowledges the following people and sources in connection with information in the article’s charts: R.P. Lindsay, Ph.D., Principles of Grinding (1982); R. Schmitt, Ph.D., Truing of Grinding Wheels with Diamond Studded Rollers (1968, University of Braunschweig); Pahlitsh, Ph.D., Dress Roll Speed Ratio and Radial Infeed (1956, University of Braunschweig); and M. Hitchiner, Ph.D., Handbook of Machining with Grinding Wheels (CRC Press).
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