Grinding fluids won't go away, despite talk of dry machining and dry grinding, but they can be tailored to the application and material
By Stuart C. Salmon, PhD, FSME, President, Advanced Manufacturing Science & Technology, Rossford, OH
Competition between the large-chip processes (milling, turning, and broaching) and the small-chip processes of abrasive machining is ongoing in metalworking. Yet, given a ceramic or a whisker-reinforced metal--or a fiber-reinforced polymer--there's no alternative to grinding. Grinding will always be wet, whether oil, water or glycols are used. Grinding dry, or with micro-lubrication, has nothing like the impact on the grinding or abrasive machining process that dry operation has on machining processes that make large chips.
The different morphology of the very small chips generated by the grinding process can be illustrated by a tensile test used to measure the strength of a material. The test uses a specimen approximately 0.500" (12.7 mm) in diameter. If the diameter of the specimen is significantly reduced, the strength of the material appears substantially greater. The tensile strength of a material increases as the whisker diameter decreases. Remember, the individual grain depth of cut of a grinding wheel may be a factor of 1/10 to 1/1000 the size of the glass filaments measured in Table 1.
|Table 1: Filament Strength vs. Diameter
|Diameter of Glass Filament (um)
||Measure Tensile Strength (kg/mm2)
Large-chip making processes may be moving away from the traditional flood coolant systems toward microlubrication, using higher cutting speeds and multicoated tools, a combination that works well in both hard and soft materials. Grinding dry, however--in anything other than a few resin or rubber-bonded-wheel applications--will not be successful. Dry, or even quasi-dry grinding and honing may be at the bottom of the "dry machining" applications list, but the good news is that we can now design the best fluid for each application--best for the environment and best for production performance.
Frictional heat plays a large part in the very high-energy process of grinding. So does the thermal conductivity of the grain and bond, the size and shape of the abrasive grain, and, most significantly, the lubricating and cooling capabilities of a cutting fluid. Forward-thinking cutting-fluid manufacturers are therefore developing families of fluids suited to specific grinding processes, related abrasive-machining processes, and specific workpiece materials.
Selecting the proper fluid is critical, and once the fluid is in the machine, proper application is vital. Many users pay a premium to get a grinding fluid with specific properties, but never realize that potential because they use the wrong application method.
Much has been made of air barriers around grinding wheels, and users anxious to use the right method often ask: "At what pressure should I apply the fluid?" The question implies that some impenetrable layer of air exists around a wheel, so you need an extraordinarily high pressure to penetrate the air barrier, pressurize the fluid, and force it into the arc of cut.
It's the grinding wheel that carries fluid into and around the arc of cut. Fluid must be applied to the periphery of the wheel at the point where it enters the arc of cut. The simplest way to apply fluid properly is to design a fluid application nozzle that will ensure that the fluid velocity exiting the nozzle equals the surface speed of the grinding wheel.
It's important to create a laminar flow of fluid by streamlining the flow in the system, and to eliminate tortuous paths through pipes, elbows, and valves. The smoother the flow from the pump to the grinding wheel periphery, the better. Given the proper application method, fluid performance can be measured more effectively in the areas of surface finish, stock removal rate, wheel life, total power, and the normal force between the grinding wheel and the workpiece being ground.
Companies endeavoring to machine dry have generated a lot of excitement. "Eliminate cutting fluids and save 16% of your manufacturing costs!" says one paper, citing the experience at a Daimler-Benz plant in Germany in 1991. That 16% figure has percolated throughout the metalworking industry in the United States, and it's quoted repeatedly. The 16% number is accurate for that Daimler-Benz application, but for industry overall the figure is more like 3 to 5%.
We need to remember that the European effort to cut fluid usage came from a concern for the environment rather than having productivity enhancement as a goal. Warnings were sounded in technical papers, as well as in the industrial press, that machining dry--by simply turning off the coolant--would increase the cost of cutting tools because tool life decreases as tool-tip temperature rises. So going dry involves a tradeoff.
Dry machining has other disadvantages as well. They include:
- production and accumulation of dust around the machine tool's gaging, location, and wear surfaces;
- generation of fumes and odors reminiscent of a welding booth;
- accumulation of chips in the machine enclosure;
- no protection from rust on newly machined ferrous surfaces;
- production of clumps of hot chips that cause hot spots around the machine structure, producing thermal irregularities.
The message that emerges is that today, though machining dry is likely to be costly, the cost is justified by the benefits dry machining provides for the environment. However, these disadvantages are being eliminated by new machine designs, multicoated and hard-lubricant coatings on cutting tools, substitution of microlubrication for cutting totally dry, and cryogenics. All of these developments benefit those who make large chips.
Dry machining may not be entirely dry; some fluid may be used, but it's a far cry from flood methods. Flow rates with those methods are typically 5100 gpm (0.3 to 6 L/sec) flow rate, whereas with microlubrication the rate approaches 0.02 gpm (0.001 L/sec.).
We are at the dawn of a new era in machining technology.
Developing a "green" cutting fluid for metal machining was the initial goal of a group of some 60 cutting-fluid-related companies under contract to the Environmental Protection Agency (EPA). Under the auspices of Cincinnati's Institute for Advanced Manufacturing Sciences (IAMS), the group initially intended to develop an environmentally friendly fluid. Once the fluid is contaminated by metal swarf, however, the constituents of that swarf, suspended in the fluid, are likely to put that green fluid in the hazardous waste category.
As the IAMS research program expanded into all aspects of cutting fluids, users, particularly the Boeing Co., insisted that the document encompass a machinability section. Chemists have always formulated grinding fluids with an eye to health, safety, and environmental impact. Benefiting the actual grinding process was almost a second thought. The IAMS group thought that if fluids were to be evaluated with respect to their impact on health, safety, and the environment, then surely their efficiency in fulfilling their main purpose, lubricating and cooling the machining process, should also be measured and evaluated.
In this test, the S factor is demonstrated. It's the length of time a grinding wheel can function without experiencing a catastrophic breakdown of the wheel's vitrified bond system. These data indicate that the wheel broke down after about 280 seconds of grinding.
IAMS took on the task of evaluating fluids for milling, General Motors drilling, Boeing turning, and Master Chemical Corp. grinding. Master Chemical Corp. bought a new CNC grinding machine and instrumented it with piezo-electric dynamometry for tests at the Edison Industrial Systems Center (EISC) in Toledo, OH.
Grinding tests on 4140 steel carried out at EISC contradicted the accepted industry view that increasing the concentration of the fluid would directly and proportionately improve the grinding operation. In tests on a number of water-based fluids to determine the relationship between G-ratio (wheel life) and fluid concentration, researchers found that at low concentrations--around 5%--a fluid ground well and wheel life was acceptable. As the concentration increased to 7.5 to 8%, however, wheel life decreased. From 8% to10% and higher, wheel life increased. The trough was quite shallow for some fluids, but for others, it was so deep that at around 7.5% the fluid would not grind at all. In fact, the conditions of force and power proved so severe that at 7.5% the 7-hp (5-kW) machine would stall, having ground successfully at 5% and at 10%.
If fluids perform as well at 5% as they do at 10%, why use higher concentrations? Generally, water-based fluids are more susceptible to bacterial growth at lower concentrations. Higher concentrations contain more biocide, so there are fewer bacterial problems.
Other fluid-performance measurements made at EISC make it easy for users to customize their fluids for a given material and operation. The delta rate is the rate of increase in normal force between the grinding wheel and workpiece as the process continues. A steep in-crease in force will usually not be acceptable unless the initial level of normal force is very low. The S factor is the length of time a grinding wheel grinds without catastrophic breakdown of the vitrified bond system. Users grinding large steel mill rolls would want to pick a fluid with a high S factor. If users are grinding ID bearing races, they would not be concerned about using too low an S factor, because the wheel is dressed frequently to ensure the most accurate form. The delta rate would be judged on the basis of the length of the grinding cycle.
Now that performance data can be generated to design the best fluid for each application, from both the environmental and performance standpoint, manufacturers can ask suppliers for a fluid matched exactly to their needs.
The delta rate quanitifies the rate of increase innormal force between a workpiece and grinding wheel during a grinding process. In the test shown, the slope shows an increase in the normal griding force during a reciprocating grinding process.
The challenge for chemists at this point is to design environmentally safe, highly productive, robust fluids with high lubricity, EP (extra pressure) additives, and excellent cooling capabilities. These fluids will likely be chemically active, with wetting agents that don't foam. New fluids with glycol-based chemistry will usher in a new era. Ester oils will be less toxic and more acceptable for grinding with cBN. Alcohol-based fluids will use evaporation as a cooling system. Each fluid will be tailored to the needs of the operation.
As makers of large chips move toward microlubrication, higher speeds, and multicoated tools, makers of very small chips are moving from cylindrical grinding to hard turning. At the same time, creepfeed grinding is displacing many broaching and milling operations.
Grinding is materials-driven. The harder the material is to machine, the more creepfeed grinding and high-speed superabrasives come into their own. Customizing the grinding fluid for the specific application and combination of abrasive and material makes creepfeed and superabrasives even more productive. As advanced materials are developed, the science of grinding and abrasive machining will be positioned to perform productively, predictably, and in an environmentally friendly manner.