Tips for Choosing CBN Grinding Wheels
When considering the use of CBN (cubic boron nitride) grains in an aerospace part grinding process, the first questions that come to mind may be associated with the grinding conditions and machine capabilities. It’s commonly assumed that CBN grinding wheels need very specific operating conditions that only a re-tooled or new machine can achieve. It’s also typical to think that those conditions are completely different from the ones used by conventional grinding wheels. However, depending on the bond system, some of the conditions may resemble those suitable for conventional wheels.
The first thing to determine when considering a CBN wheel is whether it will bring any advantage to the current process. A cost analysis should be completed to make this determination. In other words, how many parts can be ground using the CBN wheel and what is the cost? This information can be used to make a cost/piece analysis, which can be used for comparison to the current wheel (usually a conventional wheel) to determine if there is value in incorporating the CBN wheel. After cost justifying the use of a CBN wheel, it is helpful to look at the different bond options in order to optimize the grinding process.
The most common bond systems for CBN wheels are resin, vitrified and electroplated matrixes. Depending on the grinding conditions, each system has its own unique requirements which will allow the wheel to achieve optimal performance.
Features: Resin is a bond that gives up easily and it is the most forgiving bond among all. One downside of this bond is that resin does not do well if the grinding conditions generate too much heat. Since the CBN grain is held in place by a mechanical method, a resin bond tends to release the CBN grain before it dulls, so the wheel will keep its cutting ability, but may never use the CBN grain completely.
Dressing Conditions: The wheel can be dressed outside or inside the machine. It is usually done by using another grinding wheel such as silicon carbide, which trues the CBN wheel slowly. A white stick made of Alox could be used to open the structure again after truing the wheel.
Grinding Conditions: Resin bonded wheels can be run wet or dry. The wheel speeds range from 30 to 40 m/s when using coolant, and from 10 to 15 m/s when running in dry conditions without the use of coolant. However, running a dry process can have a negative effect on wheel life.
Cost: The cost of the wheel is based not only on the CBN concentration and wheel size, but also on the thickness of the abrasives layer (usable layer). In many cases it is possible to reuse the core of the wheel, but that can depend on the size of the wheel and if the core material is reusable, such as steel. In some cases the actual technology or manufacturing process doesn’t allow for reuse.
Features: Vitrified bonds are typically used for high-precision processes. They offer the longest life among all of the bond types. This bond will also maintain its cutting ability as required by wheel speed and due to the nature of the vitrified matrix which adheres to the CBN by a chemical method (chemical bond). This helps in retaining the grain, allows for longer life and also permits it to be dressed to re-expose new sharp edges of the CBN grain. The CBN won’t be released until the bond bridge breaks or until there is very little grain retained by the bond and it eventually fails. Vitrified bonds are also good for high temperature applications.
Dressing Conditions: Dressing of a vitrified bond wheel is done with a diamond rotary dresser and requires online dressing inside the machine.
Grinding Conditions: These bonds required higher speeds above 40 or 50 m/s - ideally 60 to 120 m/s is the best - however, it can go even faster. They are also required to be run with coolant.
Cost: Like Resin bond wheels, the cost of a vitrified bond wheel is based on CBN concentration and wheel size, as well as the usable abrasives layer. These are the most expensive among all bonds, although the wheel core could be reused and resegmented with a new abrasives layer as long as it’s made of steel. Grinding wheel manufacturers offer different core materials to meet the requirements for various operations.
Depending on which core material is used, both the cost and weight of the wheel will be affected. Here are some examples of core materials:
Aluminum is typically used for processes where the wheel’s core won’t be reused. This will lower the cost and weight of the wheel. Although in some rare cases, aluminum hubs can be re-used.
Carbon fiber is used when lower weights are necessary and it can also add a dampening property to create smoother grinding. It can be reused, however, this is the most expensive core material.
Titanium is an alternative material for carbon fiber, showing similar features and being more available in the market, this type of core can be reused as well.
Vitrified cores are used for processes where thermal expansion needs to be kept under control and/or part tolerance is very tight. The core is similar in weight to a conventional core and usually cannot be spun at more than 60 m/s. These types of core cannot be reused.
Steel cores are the most popular because they are very versatile and can be reused multiple times.
Features: With an electroplated bond, CBN is held in place by a mechanical method using an electroplating layer. This allows for maximum grain exposure, but also the shortest life because it is only a single layer of abrasive. These have the highest removal rates because of the grain exposure. With this high grain exposure, super smooth finishes can be challenging to achieve. High temperatures will affect this bond leading to the grain being more easily released, although there are new improvements which allow for better grain retention. Coating the wheel with Ti2 can help dissipate the heat and translate to better grain retention and longer wheel life. These type of wheels are very aggressive and cut very well when new, but that cutting rate decreases with time. This type of wheel has the biggest change in surface roughness (delta ∆) among all three bonds.
Grinding Conditions: The core has to be very precise and not allow excessive runout. They can be used in either wet or dry conditions. When grinding wet, oil is recommended, however water based coolants can also be used. These wheels require speeds of approximately 30 m/s for dry conditions and 60 m/s when run wet. Note, though, running a dry process may have a negative effect on wheel life.
Cost: This type of bond is the least expensive among all three options due to the single layer design. There is a new technology that can allow for multiple layers, which would increase the cost and complexity. The core of electroplated wheels can be reused and they can be made of different materials, depending on the application. In some cases the core is made of one material and has an outer ring made of steel which is attached to the core. The steel ring will help ensure good electroplating. For example, cores made of Titanium can have an outer ring of steel added to allow for good electroplating. This will help to reduce the weight drastically, however, the price will increase compared to a regular steel core.
It is also important to mention some key advantages of conventional wheels such as different grain technology (Alox vs ceramic); less requirements to dress, like the need to have only a single point dresser (in most cases), etc.; which need to be considered. The purchase price will always be the lowest with a conventional grinding wheel. Additionally, the wheel will never be affected by rust.
Since most CBN wheels require some type of steel core, this core can tend to rust and depending on the humidity, can affect the performance and shelf life of the wheel. For larger CBN wheels, the steel core can also add considerable weight to the spindle, which needs to be checked with the machine manufacturer in order to ensure safe operational conditions and to avoid premature wearing of the spindle.
When the conditions are met for specific bonds, CBN grinding wheels can increase productivity. Each CBN bonding system requires different operating conditions. Sometimes those conditions can be met by the same machine that handles conventional wheels. To ensure a successful transition from conventional wheels to CBN, it is strongly advised to consult the grinding wheel and/or machine manufacturer.
This article was written by Augusto Naruse, Norton Saint-Gobain Abrasives (Worcester, MA). For more information, visit here .
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