PCD Grinidng Cup Wheel
In the application of uncoated superabrasive grinding wheels, there are three increasingly prominent problems: insufficient abrasive holding power, insufficient abrasive particle protection, and poor chemical reaction between the abrasive particles and the binder (using metal and The grinding wheel of the ceramic bond has only the last two problems). The use of superabrasive coating technology helps solve these three problems, which improves the grinding performance of the grinding wheel. The technique is to apply a metal-based coating to the superabrasive grains before the abrasive particles are combined with the binder material and form the grinding wheel.
The primary purpose of the abrasive coating is to hold the abrasive particles more firmly in the binder. The actual increase in grip depends on the type of coating and is related to the binder system itself. For example, nickel-based coatings are commonly applied to superabrasive granules of resin bond grinding wheels, which increase the holding force by increasing the surface area between the coated abrasive particles and the binder, thereby increasing the mechanical adhesion between the two. This helps prevent the abrasive particles from peeling off prematurely from the binder, thus extending the life of the wheel.
Abrasive coating of resin bond grinding wheels Super hard abrasive coatings are most commonly used on resin bond grinding wheels, and nickel and copper are the preferred coating materials. Nickel is the most commonly used coating material because it is relatively inexpensive and easy to coat. Copper coatings are generally used in dry grinding because of their excellent heat dissipation in the grinding zone.
The thickness of the coating depends on the coating material. The copper coating typically accounts for 50% of the total weight of the coated abrasive particles; the nickel coating accounts for 30%, 56% or 60% of the total weight of the coated abrasive particles. “When the coating is 30% of the total weight, the holding force on the diamond abrasive particles is certainly not as good as 56%, but it generates less heat,” said Richard Andrews, vice president of technology at General Industrial Diamond (Whippany, NJ). . “Obviously, a 56% nickel coating has a stronger abrasive holding power (longer holding time), but it also produces more heat. Therefore, this is a problem of seeking balance.”
Silver can also be used as an abrasive coating for resin bond grinding wheels, and although it is expensive, it has the lubricating properties required for the grinding wheel. “The silver-coated super-hard grinding wheel is mainly used to grind the spiral groove of cemented carbide drill bits,” said Tim Smith, senior development engineer at Diamond Innovations (Worthington, Ohio).
A newly developed nickel coating has a surface morphology consisting of many spikes. These spikes greatly increase the surface area between the coated abrasive particles and the binder, resulting in a significant increase in abrasive grip and grinding performance.
Abrasive coating of metal and ceramic bond grinding wheels While the resin bond grinding wheel uses a large amount of abrasive grain coating, some metal bond grinding wheels also use abrasive grain coating. “About 80% of the coatings are used for resin bond grinding wheels and the rest are used for metal bond grinding wheels,” said Gabreil Dontu, technical support manager at Diamond Abrasives (New York).
The coating for the metal bond grinding wheel is a metal-carbide based coating - this means that the carbide is formed during the deposition of the coating. The most common coating materials are titanium carbide and chromium carbide. “These coatings perform extremely well in enhancing abrasive grip because chemical bonds are formed at the abrasive/coating interface and metal bonds are formed at the coating/bonding interface,” says Smith.
As noted above, in addition to enhancing abrasive grip, the coating provides other benefits. The metal-carbide-based coating also protects the diamond abrasive particles from the undesirable reaction of abrasive particles and binders that may occur during the manufacture of the wheel. Since the process temperature for processing the metal bond metal powder is much higher than the process temperature for processing the resin bond, at high temperatures, certain elements in the bond etch the diamond abrasive particles, making the abrasive particles fragile and ultimately Reduce the life and grinding performance of the grinding wheel. “For example, iron can etch uncoated diamond and form Fe3C on the surface,” says Smith. “This makes it easy to invert or graphitize the diamond abrasive particles, and the coating helps protect the abrasive particles from this. Ablation."
Another advance in metal bond grinding wheels is the use of a silicon carbide coating (called Si2), whose primary role is to allow the wheel manufacturer to use a high proportion of iron in the bond, and the machined wheel does not heat at high temperatures. Abrasive erosion occurs. “You can use Si2 coatings to protect diamond abrasives in 100% iron-based binders,” says Smith. “Compared to many traditional binder materials – such as the use of expensive and environmentally harmful cobalt – This is a more economical choice."
The superabrasive coating used in ceramic bond grinding wheels exhibits the same advantages as used in metal bond grinding wheels: helping to control potentially harmful chemical reactions between abrasive particles and ceramic binders. Titanium is a typical material for such superabrasive coatings, and the titanium coating helps to mitigate the undesirable chemical reactions that can occur in the binder. "In metal bond grinding wheels, the coating protects the diamond abrasive particles from the binder system; in ceramic bond grinding wheels, the coating protects the binder system from CBN abrasive particles," Smith said.
Also, the coating enhances the abrasive holding power. “The frit used in ceramic binders is not 'wet' or can't be combined well with uncoated abrasive surfaces,” says Smith. Therefore, coating the titanium carbide coating on the abrasive grains can effectively improve the wettability of the abrasive grains and increase the holding power.
Disadvantages of abrasive coatings The only potential disadvantage of using coated abrasive superhard grinding wheels may be an increase in cost. “The current market charge for abrasive wheel coatings is between $0.05 and $0.1,” says Dontu. But because the coating can increase the life of the wheel, this cost is usually worth the money.
In most cases, abrasive-coated superabrasive wheels perform better than uncoated wheels (although there are exceptions). “If the wheel manufacturer does not improve the bonding agent to take full advantage of the performance advantages of the coating, the end user may not see the difference between the coating and the non-coating,” says Smith, “when the bonding agent and coating are used as An overall system can be designed to achieve the best results. For example, iron-based binders can be used with Si2 coatings, but if existing cobalt-based binders are still used, then Si2 coatings will not There is any performance that improves performance."
Choice of abrasive coatings So how do end users know if they should buy a coated or uncoated superabrasive wheel? They need advice from the grinding wheel manufacturer or the superabrasive manufacturer.
“We need end users to share information about the use of the grinding wheel with us, such as what kind of processing the grinding wheel is used and the type of bonding agent used, in order to recommend the right coating concentration or coating material to the user,” says Dontu.
The first thing to consider is the workpiece material, whose properties determine the type of abrasive used. The grinding wheel manufacturer needs to know the hardness of the workpiece material and whether it belongs to the iron family material or the non-ferrous material to determine whether the diamond abrasive grain or the CBN abrasive grain should be selected. Wheel manufacturers also need to understand the surface quality requirements of the workpiece and the material removal rate during processing.
“Designing a grinding wheel is a step-by-step process, and this process always starts with the requirements for use and the final part,” says Dontu. “You must first choose the type of abrasive material, then determine the specific type of material, and then design the size and bonding of the grinding wheel. On this basis, consider whether the coating is needed.”