When considering upgrading a cylindrical grinding process from a conventional vitrified aluminum oxide or ceramic wheel to Vitrified cBN (VitcBN), there are 5 main reasons that can help justify the use of cBN over conventional abrasive grains.
1. Cycle times
Since cBN has a higher hardness factor and can stay sharper for longer periods of time, it can be pushed harder and cut faster and with a reduced frequency of re-dress.
2. Super hard materials
With some hard to grind materials such as M4, cBN makes more sense than conventional grains because sometimes the hardness of the alloy is close to the hardness of the conventional grinding wheel, which can glaze or damage the wheel.
3. Grinding stock
Having a high volume of material to remove may warrant using VitcBN grinding wheels.
4. Reduce costs
The VitcBN wheel reduces the amount of dressing required, and the abrasive layer needed (usually from 3mm to 9mm radius) to grind big batches. This can translate into lower overall operation costs.
5. Thermal stability
The thermal conductivity of the cBN grain is greater than aluminum oxide or ceramic grain, which helps dissipate heat.
Making the switch
To switch from a conventional grinding process to a VitcBN process, it is important to review 3 main areas: machine requirements, wheel considerations, and training.
Address machine requirements
The grinding wheel spindle has several areas to consider.
1. Bearings: The spindle will need rigid bearings to handle a heavier wheel.
2. Max Operating Speed: Spindles for VitcBN require higher speeds than for conventional wheels and usually run between 40 to 120m/s.
3. Horsepower: VitcBN wheels have a steel core that makes them heavier. The motor will require enough horsepower to spin the wheel, and maintain grinding speeds. Carbon fiber has been used as a core material to lower weight, but costs more.
1. Dressing device: VitcBN wheels require a rotary dresser, as there is no other way to dress a VitcBN wheel.
2. Location of dressing device: The conventional process typically does not require a rotary dresser, there may be no place to setup such a device. It should be located in a rigid spot with good access to the VitcBN wheel without interfering with the grinding process.
1. Pump: Coolant flow needs to match the wheel speed and should provide enough pressure to break the air barrier, and ensure optimal coolant delivery.
2. Nozzles: Sometimes specially designed nozzles are required to break the air barrier and deliver the coolant.
3. Coolant: In many cases coolants used with conventional wheels can be used for VitcBN, but it is recommended to work with the coolant manufacturer to come up with best option.
Since cBN is harder than aluminum oxide or ceramic grain, the grinding machine needs to be rigid enough to withstand the grinding forces. In addition to having tight spindles, it is also necessary to have close fitting slides.
Machine guards: When running VitcBN at higher speeds, it’s important to have a guard to protect the operator.
Grinding wheel considerations
If the cylindrical grinding machine cannot spin the wheel fast enough, the VitcBN specification will need to be modified for slower speeds.
If the spindle cannot take the weight of the VitcBN wheel, a smaller wheel will need to be designed, as long as the machine and process can accept that. It may be possible to change the wheel core to a lighter material.
1. Handling: The operator will need to learn the proper way to care for the VitcBN wheel such as the need to store the wheel in a dry area to avoid rust.
2. Training: The operators and engineers will need to learn and determine the optimal conditions for grinding and dressing a VitcBN wheel.
VitcBN wheels are a great option for production grinding due to the potential to increase productivity and part quality, and lower the tool cost if used correctly. However, it requires study and may require adjustments to equipment and company practices.
For information on Norton | Saint-Gobain products, phone 508-795-5000 or visit the company's website.
Related Glossary Terms
Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.
- aluminum oxide
Aluminum oxide, also known as corundum, is used in grinding wheels. The chemical formula is Al2O3. Aluminum oxide is the base for ceramics, which are used in cutting tools for high-speed machining with light chip removal. Aluminum oxide is widely used as coating material applied to carbide substrates by chemical vapor deposition. Coated carbide inserts with Al2O3 layers withstand high cutting speeds, as well as abrasive and crater wear.
Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
- cubic boron nitride ( CBN)
cubic boron nitride ( CBN)
Crystal manufactured from boron nitride under high pressure and temperature. Used to cut hard-to-machine ferrous and nickel-base materials up to 70 HRC. Second hardest material after diamond. See superabrasive tools.
- cylindrical grinding
Grinding operation in which the workpiece is rotated around a fixed axis while the grinding wheel is fed into the outside surface in controlled relation to the axis of rotation. The workpiece is usually cylindrical, but it may be tapered or curvilinear in profile. See centerless grinding; grinding.
Removal of undesirable materials from “loaded” grinding wheels using a single- or multi-point diamond or other tool. The process also exposes unused, sharp abrasive points. See loading; truing.
Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.
- grinding machine
Powers a grinding wheel or other abrasive tool for the purpose of removing metal and finishing workpieces to close tolerances. Provides smooth, square, parallel and accurate workpiece surfaces. When ultrasmooth surfaces and finishes on the order of microns are required, lapping and honing machines (precision grinders that run abrasives with extremely fine, uniform grits) are used. In its “finishing” role, the grinder is perhaps the most widely used machine tool. Various styles are available: bench and pedestal grinders for sharpening lathe bits and drills; surface grinders for producing square, parallel, smooth and accurate parts; cylindrical and centerless grinders; center-hole grinders; form grinders; facemill and endmill grinders; gear-cutting grinders; jig grinders; abrasive belt (backstand, swing-frame, belt-roll) grinders; tool and cutter grinders for sharpening and resharpening cutting tools; carbide grinders; hand-held die grinders; and abrasive cutoff saws.
- grinding wheel
Wheel formed from abrasive material mixed in a suitable matrix. Takes a variety of shapes but falls into two basic categories: one that cuts on its periphery, as in reciprocating grinding, and one that cuts on its side or face, as in tool and cutter grinding.
Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.