Tech Talk
What are "Vitrified Bonded Wheels"?
In the world of vitrified bonds there are really three distinct types of bonds, sintered bonds, high temperature bonds, and low temperature bonds. The development of inert atmosphere low temperature bonds has allowed the use of vitrified CBN and diamond to take off. As of this writing there are many suppliers that use bond temperatures that are too high for diamond and can cause damage to the CBN grain. When you have a good low temperature system, the manufacturer can supply open structure diamond as well as CBN.
This bond system is like using a conventional vitrified wheel and substituting the conventional abrasive with CBN. The open structure and controlled porosity allows for the grinding of even long chip material such as, inconel 718 soft, and soft materials like cast iron. Normally hard materials are of the most applicable for using CBN (materials like CPM10V, M4, T15, 8620, etc).
This bond is very flexible and can be dressed, depending on the wheel size, with either a single point, rotary single radius diamond (prefer 0.020" radius), rotary form dresser or, metal bonded diamond wheel approximately 0.060" (1.5 mm) wide. If a cup dresser is used, best results would have the cup rim address the wheel at 35 to 45 degree angle to the wheel face. Infeed of point contact dressing should be less than 0.0002 per pass (0.005 mm). The traverse rate for point contact dressing will vary but a good starting point would be 0.004" (0.1 mm) per rotation of wheel.
To maximize the life of the wheel by minimizing the amount dressed on each dressing cycle, an acoustic sensor can be mounted on the dresser. This system works by picking up the sound through the coolant as the diamond dresser gets closer to the wheel. Integrated into the machine control system, the acoustic emission created a foot of the wheel face and will stop the dressing process when the wheel is in contact with the dresser across the entire wheel face. This minimizes the amount dressed off the wheel and eliminates operator error.
Although the truing and dressing is accomplished in the same operation utilizing vitrified CBN, studies show that the abrasive stick can be utilized to minimize a break-in period known as reaching "steady state". This is not needed often but is still an option in the arsenal of controlling process.
Truing and dressing are critical for the success of vitrified CBN. The infeed per pass and traverse of the dress are critical for maintaining process control. The operator should never be allowed to make changes without documentation. Also, rigidity of the dresser should not be overlooked, the dresser can pass along inherent weaknesses of its spindle and imbalance to the wheel face.
