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Fast vs. Slow GrindingLately the focus has been on high-speed grinding and machining. The theory is that a grinding wheel, which is literally millions of tiny cutting tools, will remove more material the faster it is rotated. We have all read about research initiatives using special wheels at 30,000 sfpm or more. But the higher the wheel speeds, the higher the temperatures in the grinding zone. So along with the high-speed spindles and wheels, high-pressure coolant systems with special delivery nozzles had to be developed to overcome the air turbulence caused by the high-speed wheel. This is all very expensive and difficult to justify in a production-oriented environment where today's profit margins are slim.
Has anyone ever considered that maybe we are going in the wrong direction? Slow speed grinding has been discussed in the past, and some experimentation was done years ago. But it never really caught on, perhaps because people had a perception problem with the word "slow," relating it to the feed and production rates rather than simple wheel speed. Add that most older grinding machines have no precise control over wheel speeds, especially in the 1000 to 2500 sfpm range, and you have a highly efficient grinding process that has been largely ignored.
A new technology (CoolCut, developed by Machine Tool Specialists) has been developed that might change all that. By making electrical and mechanical modifications to existing grinding machines, the technology provides superior control and high cutting rates at slow wheel speeds. It is particularly useful for hard materials, such as carbide and ceramic.
Grinding Without HeatAs an example of what the new technology can do, a conventional 6 x 18 surface grinding machine equipped with the new technology and a simple vise on the magnetic chuck was used to plunge a series of seven slots in increments of 0.050 in. deep to 0.350 in. deep. This was done using a diamond wheel (0.063 in. wide) with a special composite-resin bond, in a standard 3à4-in. square by 1à4-in. thick tungsten carbide turning insert. Table 1 shows the actual grind time associated with each depth of cut.
Not only can this process yield significant advantages in stock removal rates compared to conventional grinding processes, but it accomplishes this removal without introducing heat to the wheel or work zone. In the example above, a rib barely 0.010-in. wide was left between the ground slots. No burning, cracking or fracturing occurred that would be associated with heat. In fact, an independent metallurgical laboratory found no damage at all.
The process has been compared to creepfeed grinding, and there are similarities. However, creepfeed grinding machines are generally designed with higher stiffness than conventional machines. Often, polymer or concrete is used to reinforce the machine beds and dampen vibration. Such machines can use conventional aluminum oxide, cubic boron nitride (CBN), ceramic bond or even diamond wheels running at high speeds, and often having to incorporate expensive high-pressure coolant delivery and filtration systems. Dressing is often done continuously to maintain the wheel form, closed loop servosystems and CNC control feed rates.
In slow speed grinding, CBN or resin-bonded diamond wheels are used, and machine stiffness is less critical. There is no need for high pressure coolant systems. A simple plastic-link coolant nozzle with a small stream of coolant is all that is necessary. Feed can be controlled by CNC, hydraulics or even by hand.
The typical grinding process generates heat at the point of the cut, just like any machining process. High amounts of energy are converted into heat, resulting in high temperatures where the cutting takes place. Heat flows into the grinding wheel, the chip that is removed, and the work. Too much heat, and the wheel picks up molten particles of the material and glazes over, causing more friction and more heat. This is when the wheel needs to be dressed, removing these particles. Damage to the workpiece is demonstrated by burn marks and cracks that often do not show until later inspections.
With the new technology, the abrasive grains penetrate the workpiece more easily at slow speeds and generate less friction. The cutting is much more efficient because the horsepower consumed is turned into cutting energy rather than heat energy. The material is cut, not deformed. CBN and diamond wheels with a special formulated composite-resin bond are effective in slow speed operations because they are capable of penetrating the workpiece surface without rapid friction wear of the abrasive grains.
Slower is FasterThe new technology works because the relationship between the wheel speed, wheel type and material to be cut are predetermined. On a machined equipped with the new technology, wheel speeds are not marked in rpm but are labeled incrementally on a speed potentiometer. For instance, to cut carbide with a diamond wheel, the wheel speed would be set at a predetermined number, such as 1, 2, etc. Variation from these speeds will cause poor grinding results and excessive wheel breakdown. Proprietary electronics drive the wheel at the proper critical speeds and maintain the appropriate feed rate for each particular application.
The technology can be adapted to virtually any conventional grinding machine. It works best on well-maintained machines with good slide systems and well-balanced wheel spindles. Automatic feed systems are a plus, but they must often be modified to increase the feed rates to speeds for which they were not originally designed. Electronic or CNC systems are best since they offer a highly accurate means of control and consistency.
In a recent test, a standard Okamoto surface grinder was modified using the new technology. Using a specially formulated diamond wheel, the technology was able to double the rate of stock removal at the same feed rate that was being previously used to grind conventionally. Again, this was achieved without damage to the wheel or workpiece-effectively doubling production. The material was a hard wear ceramic coating on cast steel in the Rc70 range, requiring diamond wheels.
Faster is not always faster. Sometimes slower can be faster. Slower can also be more cost-effective. For manufacturers still grinding hard-to-grind materials conventionally, slow grinding is worth a second look.