Tech Talk
How clean must the "Cutting Fluid" be for a given applications?
Generally 8 to 12 micron (0.0003 to 0.0005 in) filtration is adequate for general purpose grinding. If surface finish requirements are critical then filtration to 1 micron (0.00004 in) or less is essential. A rule of thumb is that filtration should be carried out to 1/20th to 1/50th the size of the abrasive grain used in the grinding wheel. When using superabrasives in resin bonds, it is suggested to filter to 1/100th the size of the superabrasive grain used in the grinding wheel. Particulate in the cutting fluid acts as an abrasive on the resin bond of the grinding wheel and, as the amount of particulate builds up in the fluid, a slurrying action occurs which can drastically reduce the life of the grinding wheel. It must be realized that filtration down to very fine particulate sizes requires special methods. Magnetic separation is satisfactory for magnetic materials where only a small amount of swarf is to be produced. Paper filtration is the preferred system as it can cope with both large volumes of fluid, as well as very large volumes of swarf. Cyclone and centrifuge type filtration requires a great deal of energy, which generally ends up in the fluid as unwanted heat, and also takes a significant time to achieve good filtration. Diatomaceous earth can be used for extra fine filtration, but requires a great deal of attention to maintain the system. Also, very fine filtration methods can strip the essential additives in the cutting fluid, which are purposely in place to assist in the grinding process. Large volume systems are preferable as they allow some settling of the fluid so entrapped air can escape, reducing the foaming and allowing the fluid to cool. A rule of thumb is that for the maximum flow of the system per minute, a tank capacity 10 to 25 times that should be used. For example, if the main cutting fluid pump delivers a maximum flow of 6 litres/s (95 U.S. gpm) then the tank capacity should be between 3600 and 9000 liters (950 and 2375 U.S. gallons).
Refrigeration of the cutting fluid is particularly important in high production grinding, creep-feed grinding, or grinding with superabrasives applications. A refrigeration unit is essential as it can be accurately controlled to within two or three degrees of temperature, whereas a cooler/radiator can bring the temperature of the fluid ten to fifteen degrees below the ambient temperature of teh air being blown across the cooling fins. Therefore, if the ambient temperature fluctuates then so does the temperature of the fluid. Cooler/radiators also blow enormous amounts of air around the shop environment and generate a great deal of low frequency noise.
Maintaining a constant and cool temperature of the cutting fluid not only is good for the grinding process, but also increases the life of the cutting fluid. Bacterial growth can run rampant through water based cutting fluids. Over a period of only a few days, in a poorly disciplined shop, a cutting fluid can become contaminated with tramp oils, beverages, litter, and waste matter. At temperatures above 28C (83F) degrees bacterial growth accelerates and the fluid smells foul, particualrly if left to settle over a weekend. The properties of the cutting fluid are seriuosly depleted, however, the risk of dermatitis and infection in the form of open sores or wounds to the operators is much worse. Poor management of cutting fluids not only is bad for the grinding process, but also presents a very serious health risk to personel.
