Filtering on the edge

Grinders get a performance boost from a technology popular for EDMs.

Grinders get a performance boost from a technology popular for EDMs.

Take a filtration system typically associated with EDM, put it on a grinding machine, and what do you get? Grinding wheels that cut cleaner and cooler and last longer to boot. The technology, edge filtration, has been an affective method for filtering out the sludge byproduct of the EDM process for a number of years.

But now it's finding a niche in grinding applications. Companies grinding tungsten carbide, for instance, are discovering that edge-filtering systems can greatly improve a grinding machine's overall performance.

What sets edge-filtration apart from other methods is that it traps much smaller particles. The key is the filter medium, says Irvin Kaage, president of Transor Filter USA Inc., Elk Grove Village, Ill., whose system filters down to 1 micron.

Rather than being a consumable, like a cartridge or pre-coat, the medium is a special wafer-shaped filter disk. Thousands of these disks are compressed on a 3-ft rod. A typical Transor filtering vessel houses up 61 of these assemblies, or elements as they are called. When dirty oil is introduced, it is forced through the center of the elements, and all particles larger than 1 micron are trapped on the edge of the filter. Clean oil passes between disks and back to the machine.

Benefits of 1-micron filtration
With oil filtered to 1 micron, grinding wheels cut better and last longer. This is because when particulates build up in grinding wheels, they rub or smear instead of cutting clean, which slows the machining process and may ruin surface finishes, says Kaage.

Dirty oil already coursing through a machine's coolant system sprays the wheel with 10 to 20-micron-size particles before actual grinding even starts. According to Kaage, it's like washing your car with dirty water. Clean grinding wheels cut more efficiently, letting companies such as SGS in Cuyahoga Falls, Ohio, shave minutes off cycle times.

SGS manufactures cutting tools, and one of its three divisions has 11 Transor filter systems on Walter grinders. The division, which produces carbide end mills, achieves faster fluting speeds and feedrates, all while running wheels at slower-than-usual rates. "Basically," says Greg Adams, director of manufacturing at SGS, "we decreased cycle times by simply adjusting speeds and feeds without having to change wheel types." With the Transor system, he adds, the shop experienced an immediate increase in productivity.

Another benefit of the Transor system is easy maintenance. Besides keeping grinders looking brand new, the system's elements are not replaced every time they are full. Rather, an automated backflushing procedure cleans them once a shift.

During cleaning, compressed air, which is introduced at the top of the vessel, travels down the center of each element and through the filter disks. This blows sludge and debris off, forcing them to the bottom of the vessel and out to a paper-lined sludge box. Shops can then easily remove the mostly carbide sludge for reclamation.

Reclaiming carbide from a cartridge-type filtering system is more involved, says Kaage, requiring disassembly and physical scraping off of the sludge. Once a shop factors in the labor, it will find that the reclamation may not be worth the effort. And when shops using a precoat-type system conduct a backflush, the carbide mixes with precoat.

Adams, but these have several motors that generate heat. In addition, his shop had to filter smaller particles because carbide and diamond grit from the wheel got back to the machine, causing wear and tear.

When a centrifuge system's bowl is full, shops must shut down the unit to clean out the bowl. This procedure is not only messy, explains Kaage, but it also entails production downtime. And because oil is filtered to only 10 or 20 microns, dirt and grime can still collect within the machine, gunking up scales, valves, pumps, and sometimes spindles.

What most shops are using
Typical OEMs and other shops machining carbide, which has sub-micron particles, use cartridge-type filter systems, says Kaage. "But a cartridge system that filters to 5 microns (nominal) may provide 10 to 15-micron filtration when it is brand new," he adds. What this means is that the filter must first build up some debris and dirt that bands together, which actually aids in the filtering process. Until this occurs, the filter is not operating to its full potential, and particles bigger than 5 microns are passing back into the system.

Other shops may use a centrifuge system. These can filter down to 10 microns but may have drawbacks. For instance, SGS has always used centrifuge units, says

Operational guidelines
Shops using the Transor Filter have to adhere to a couple of guidelines, or they will not reap the full benefits of edge filtration. One has to do with the type of oil used, and the other involves grinding HSS.

According to Kaage, common grinding oils vary in viscosity from 45 to 300 SUS and in flash-point temperatures from 180° to 435°. The Transor system can filter any oils within these ranges; however, the rate at which it does varies greatly depending on the oil type.

Transor strongly recommends running a light oil in its system. The reason is that if a shop's machine requires a coolant flow of 30 gal/min, the Transor Filter must clean at the same rate. For a low-viscous oil, a single-filter Transor vessel will deliver 30 gal/min, but the same system with higher-viscous oil may filter at only 10 gal/min. Technically, shops can run any type oil up to 100 SUS before they have to use a larger system to achieve the desired coolant flowrate.

When most shops chose a coolant, traditionally, their main concern is safety, so they opt for oils with high flash points, says Kaage. What is overlooked is that a high flash point means high viscosity.

These thicker oils, which tend to retain heat longer than thinner oils, cause machine pumps to work harder and generate even more heat. "Shops create more of the problem they are trying to remedy," comments Kaage.

SGS's oil, for example, was of a higher viscosity than Transor recommends, so the company switched to a lighter-viscosity oil. And according to Adams, they've had no problems with smoke or burning, even when grinding 1-in.-diameter, four-flute carbide end mills.

"The Transor system is not dedicated exclusively to tung-sten-carbide applications," says Kaage. "It also works well for grinding HSS, but a standard magnetic separator is required for the type of particles the material generates."

HSS produces a by-product similar in shape to a hairball, explains Kaage. They are floating groups of particles linked together, which must be removed before they enter the Transor Filter vessel. These hairball-like chips fill the vessel, forming large clumps that aren't removed during normal backflushing. Using the magnetic separator ensures the performance of the system and 1-micron filtration.

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