The key to maximizing the performance of a production grinding application is having the right truing and dressing tool and using it correctly. In fact, a skillful operator with a quality dressing tool and good dressing technique can often improve the performance of a wheel that may not be the optimum wheel for the application. This skill is particularly important in shops where it isn't practical to have a special wheel for each operation.
Dressing is the process of sharpening the abrasive elements of the wheel. The process breaks down the bond and removes dull abrasive grains to expose new and sharp abrasive particles. Dressing also removes tiny pieces of material from the pores of the wheel face to prevent wheel loading, which can cause vibration and leave burn marks on the workpiece.
Without proper dressing, it's impossible to achieve the best consistency and adherence to specs from even the highest quality abrasive wheel. In fact, when you invest in top-quality grinding wheels, it becomes even more important to dress them properly in order to capture the quality and performance benefits.
Truing is a companion wheel-preparation process performed at the same time as dressing on conventional wheels. With superabrasive wheels, the two processes are accomplished separately, with truing performed first. In superabrasive wheel applications, truing is done with a tool or roll, while dressing often employs a vitrified dressing stick in a secondary operation.
It's important to ensure that the spindle bearings are warm—as warm as normal grinding conditions—prior to truing and dressing. This avoids both the loss of part geometry and abnormal wear of the tool and the wheel. Tools should be handled carefully because diamond is brittle and susceptible to cracking and chipping if the tool is dropped.
Because a diamond dresser is itself a cutting tool, it has to be sharp to do its job. Dull dressing tools glaze the wheel face to produce a dull wheel. To maintain a well-defined and sharp diamond point, rotate the single-point or cone-point tool 1/8 turn at regular intervals. The frequency of these rotations will depend on how often you dress, but a minimum of once per day is a good rule of thumb. Chisel and form tools are typically flipped 180 degrees one time during their life cycles.
Most cylindrical grinders are arranged so that the workpiece and the grinding wheel are on the same horizontal line. The point where the circumference of the work touches the circumference of the wheel is called the work/wheel contact. The diamond tool should dress the wheel as close to this point as possible. On internal grinders, the orientation of the diamond to the work/wheel contact is even more important.
Take Light Cuts
There is always the temptation to take too deep a cut in an effort to reduce dress times. This is false economy. Use the correct amount of infeed instead. Too heavy an infeed overheats the tool and reduces tool life, and valuable grinding abrasive is lost. The net result is a dull tool producing a dull or closed wheel. Dress the minimum amount necessary to restore the wheel's geometry and cutting action, taking off only what you need.
With single-point dressing tools, approach the grinding wheel at a 10- to 15-degree drag angle. This will create a sharpening effect for the tool when the tool is rotated. Multi-point (impregnated) tools do not require a drag angle. Instead, approach the wheel with full-face contact.
The traverse rate, which is the speed at which the tool moves across the wheel, is critical to achieving the desired part finish and metal removal rate. A traverse rate too slow tends to close up the wheel, compromising part finishes and metal removal rates. The slow rate can also cause the wheel to vibrate and burn the workpiece. Faster traverse rates create an open wheel face, removing more metal and accelerating part finishing.
Proper use of coolant speeds dressing and makes it more effective. As a rule of thumb, use a 3/8-inch diameter stream of coolant to remove excessive heat from the tool during dressing, extending tool life. Arrange the coolant nozzle to either flood the entire wheel face or follow the diamond as it moves across the wheel. Never allow the tool to go in and out of the coolant flow while it is in contact with the wheel. Diamonds may crack or cleave during extreme temperature changes.
Filter the coolant to avoid recirculating dirt or chips, which can load the wheel and result in a need for more frequent dressing. Dress dry only when you plan to grind dry (and in such a case, allow frequent intervals for the diamond to cool). After shutting off the coolant flow at shift's end, let the wheel idle for a few minutes. This will help prevent wheel breakage.
Vibration Is The Enemy
It is also crucial to minimize vibration during wheel dressing to avoid diamond marks, gouging and damage to the tool. This means maintaining proper balance, which begins with the grinding wheel structure itself. Density variations and overall wheel geometry affect a wheel's inherent balance, so selection of a well-made product is essential.
Assuming a quality grinding wheel, proper installation will keep the wheel in balance. Follow the manufacturer's directions by, for example, following the mount-up arrows on the wheel to locate the light point. Mounting the wheel with the mount-up arrow pointing upward minimizes imbalance after dressing. Even distribution of coolant also helps to maintain balance.
To further avoid vibration, be sure the tool is tight in its holder and rigidly supported with a minimum amount of overhang. If the diamond tool is not securely held, vibration will cause chatter, diamond marks, gouging and damage to the tool.
The importance of proper truing and dressing cannot be too strongly emphasized. A grinding wheel needs to be trued and dressed before it touches a piece of material. Following these procedures will help ensure that your grinding wheel produces superior results.
About the author: Debbie Simpson is product manager for diamond tools with Saint-Gobain Abrasives of Worcester, Massachusetts.