By Charles Bates
PAO-based synthetic grinding oils reportedly provide lubricity levels 5X higher than those of mineral-type fluids. (Photo courtesy of Rollomatic Inc.)
While synthetic-oil products, like SintoGrind and IonoGrind, increase grinding-wheel life over 50%, they also support both grinding and erosion processes on such machines as the Helitronic Diamond from Walter Grinders.
Reportedly, synthetic oils last as long as the machines they run in with proper filtration.
Only a few years have passed since many U.S. cutting-tool shops converted from grinding with water-based emulsion fluids to running straight refined oils. However, switching from such refined mineral-based oil products, which typically start as crude oil, to 100% pure synthetics is happening much faster. The reason? Synthetics offer benefits above and beyond those of standard mineral-based oil products.
For instance, synthetics reportedly provide lubricity levels 5 higher than those of mineral-based fluids and 10 those of water-emulsion coolants. They also evaporate 60% slower than mineral fluids, last practically forever, prevent bacteria build up, and eliminate cobalt/carbide leaching.
High lubricity increases grindingwheel life and keeps heat from transferring to workpieces for generating crack-free, fine surface finishes. While mineral-type oils boost wheel life as much as 50%, synthetics improve it even more than that, which is a big deal for shops using grinding wheels that can cost as much as $10,000 each.
Water-based fluids transfer a lot of heat into workpieces, but synthetics wash it away with the grinding swarf. Water effectively removes heat, however, it does boil. When this happens, it turns to steam, and there's no agent to carry away grinding heat. Oils, on the other hand, don't boil.
Oils, both synthetic and mineral, are good for cutter-grinding machine tools. Oils lubricate machine elements, don't damage slideways, prevent rust, and keep "black layers" from forming when grinding hard metals.
"With submicron filtration and keeping them contaminant free, synthetic oils will last as long as the machines they run in," says Peter Knowles, president of Hirschmann Engineering USA Inc., a Buffalo Grove, Ill., company that distributes SintoGrind TTS, a 100% pure synthetic hydrocarbon oil developed by Oel-Held GmbH of Stuttgart, Germany.
SintoGrind comes from polyalphaolefins (PAOs). About four companies produce PAOs by combining a mixture of gases with a catalyst, under heat and pressure, to form a liquid. The process is similar to catching the steam from a tea kettle and turning it back into a liquid, says Knowles.
Because of its molecular structure, the PAO liquid is pure and carries no unwanted molecular chains such as aromatics and polycyclic aromatic hydrocarbons, often found in mineral-based products from crude oil. Even after double and triple refining, uneven molecular chains found in all mineral-based fluids break down from grinding's high heat and pressure. Both gradually deteriorate these oils (cobalt leaching), forcing shops to periodically change fluid.
Applications for SintoGrind include high-speed grinding of carbide, high-speed steels, and stainless using diamond grinding wheels. The product is especially well-suited for cutter-grinding shops working with a lot of carbide because there's no carbide leaching into grinding swarf and then coming into contact with machine operators.
Besides machines, oils are also good for operators. Oil doesn't contain biocides, nor is it attacked by bacteria, fungi, or yeast, so there are no hazardous materials to contend with and no danger of nitrous amines forming, which are considered cancerous.
Also favorable for operators, SintoGrind barely foams or mists. With water-based products, shops supply up to 95% of the water used, and suppliers only about 10%. Soft water causes foaming, while hard water corrodes. But water quality is not an issue when grinding with oil.
Even though SintoGrind is a synthetic, it is still a hydrocarbon and will burn if made hot enough. But its flash point is extremely high, says Knowles.
Machines made for oil
Today, most cutting-tool grinders come equipped for running mineralbased and synthetic oils. Such machines are fully encapsulated and include necessary features such as suction mechanisms, suction flaps, fire extinguishers, flowcontrol instruments, and oilresistant components.
Suction devices suck out oil gas, mist, and smoke to prevent operator irritation. For cleaning the air, systems with deflectors and filtering mats as well as electrostatic filter systems with additional activated carbon filters work well. Shops with large central-ventilation systems use demisters that spray oil on wire nettings to filter the air.
Suction flaps attach to the tops of machines and carry off energy in the rare instance of an explosion. The flaps then close automatically to stop suction. Also, because oil is flammable, oil-ready grinders have automatic fire extinguishers. In addition, shops should flood grinding wheels and workpieces with oil from a fan-type nozzle with 2 to 3-bar pressure to hinder the chances of explosions.
Flow-control instruments immediately switch off machines if oil flow fails, and level-control instruments prevent mixtures of oil and air from pumping into grinding-contact zones. All hose pipes, cables, and seals, as well as stop switches made from plastic, elastomer, or rubber within the grinding zone, should be oil resistant.
Because of oil's lower heat capacity as compared with water-mixable cooling lubricants, shops should double the amount used, but if they run an equal amount, built-in cooling units are a must.