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Oil Removal for Cost Savings and Improved Parts Quality

June 23, 2021
Whether it's managing tramp oil in coolants or accumulated oil from parts washing, machining operations benefit from efficient, well designed oil-removal systems.

CNC machine shops are constantly seeking new ways to reduce costs and improve finished part quality. Many of them may be overlooking a significant opportunity to accomplish both mandates by focusing on improving the process of removing oil generated in various stages of machining. Whether “tramp” oil finds its way into coolant, or accumulates in parts washing baths, oil residues can degrade the performance of machining operations.

Tramp oil in coolant can clog lines and spray nozzles, shorten tool life, stain workpieces, and lead to inaccurate tolerances. The cost of replenishment and disposal of coolant is substantial. In parts washing, oil that is not removed accumulates and contaminates wash solutions, resulting in frequent bath recharging, and poor adhesion during painting or plating.

There also are significant environmental treatment and disposal costs to consider.

Fortunately, the solutions to removing oil are similar regardless of its source. Oil that accumulates in tanks and sumps can be economically and efficiently removed through carefully designed, continuous skimming and/or separation systems. When needed, these systems can be customized to meet the requirements of specific equipment and various configurations and sizes of coolant reservoirs from individual machine sumps to large central systems.

Removing tramp oil extends coolant life -- Tramp oils (hydraulic fluids, lubricating oil and greases, and protective metal coatings) can be generated by continuous small leaks from equipment, or a one-time occurrence, such as a hydraulic line break.

However, for machine tools to perform at peak efficiency, the coolant must be free from contamination. When tramp oils combine with other contaminants in coolant, they may form a sticky substance that clogs lines and sprays. When oil-contaminated coolant hits the tool, the cooling effect of the coolant is reduced, which shortens tool life and causes product staining, inaccurate tolerances, and a smoky atmosphere in the plant.

Tramp oils in coolant also are a breeding ground for bacteria and may cause problems ranging from unpleasant odors to employee skin rashes. If not removed, bacteria also slowly destroys the coolant.

Removing tramp oil extends the service life of the coolant, which of course improves operating costs and enhances productivity.

One cost-effective and simple method to combat tramp oil is a surface-oil skimmer that mounts on the coolant reservoir and continually draws off surface oil. For this to be an option, the tramp oil must separate from the coolant and gather on the surface.

The most efficient type of oil skimmer uses a Free-Floating Collector Tube that actively and continuously removes the oil and grease as it rises to the surface of the water. As the tube moves across the surface, oil adheres to the outside; then, the tube is drawn through a series of ceramic scrapers that constantly remove the oil, which drains by gravity into a collection vessel. The tube skimmer operates continuously, with minimal attention or maintenance. Tube skimmers are available in sizes and configurations to fit all applications, from small individual sumps to large holding tanks.

For applications where the tramp oil does not easily rise to the surface, minor modifications can be made to the coolant reservoir to increase tramp-oil separation prior to skimming. This can include reducing water turbulence, preventing coolant from plunging into the reservoir so tramp oil does not break into smaller droplets, and installing barriers that act as baffles.

However, if the oil is emulsified or entrained in the water, it first must be separated before skimming. In this case, the best option is a system that accomplishes both steps, such as the Separate and Skim (SAS) oil/water-separation system from Oil Skimmers Inc. For over 50 years, the company has created skimmers and oil-separation equipment that address applications from small individual machining centers to large central systems.

In some instances, space or access also may be limited, preventing installation of an oil skimmer on or next to the coolant sump. The compact CoolSkim, also from Oil Skimmers, provides a method of moving the coolant to a uniquely designed separator that facilitates the separation and removal of the oil, with the clean coolant being returned to the original sump/tank.

Extending wash solutions -- Another common process for machine shops is part washing to remove oils, coolant, and other debris that accumulate during the various stages of the manufacturing process. If this oil is not adequately removed it may result in poor adhesion during subsequent painting or plating steps.

Although oil accumulation has long been an issue, parts manufacturers seeking a higher level of product quality often incorporate oil removal solutions in high-end, automated parts washers.

“If the wash is not properly prepared and foreign substances are not removed from the metal in the wash phase, the finish - the ‘dress’ - may not hold properly, and it may not wear well,” said Oil Skimmers vice president Jim Petrucci.

As the wash stage removes the oil, it accumulates in the wash solution. This accumulation contaminates the wash solution, resulting in frequent bath (wash) recharging or improperly cleaned parts. These problems lead to increasing costs that include disposal of the contaminated wash solution, additional use of chemicals, costly downtime, and additional labor.

Even parts-washer OEMs acknowledge the critical importance of a wash that is free of floating oil. One well-established Midwestern supplier of part-washing systems addressed the issue of oil accumulation in the wash by installing an oil skimmer with its equipment. The Oil Skimmers Model 5H is a fully automatic skimmer provided to decrease downtime and increase bath life.

With the skimmer, oil adheres to the outside of a closed-loop tube as it is drawn slowly across the surface of the water, automatically adjusting to changing water levels. The oil-covered tube passes through scrapers that remove the oil. The clean tube then returns to the water surface to collect more oil. The recovered oil flows into a collection container and is virtually water-free.

“With the addition of the Model 5H [to our equipment], we have happier customers. The metals are cleaned of oil, we’ve saved the customer time and money, and the final plating or painting will ultimately adhere better, look better, and last longer on the metal parts,” the OEM’s director of engineering said.

Because the wash is cleaner, the operators also notice better part cleaning, less use of chemicals to combat contaminates, and better salt-spray test results.

Another benefit of using a well-designed oil skimmer is that the process is less labor intensive. “Downtime to recharge the water requires additional labor to restart the bath,” he added. “Plus, we do not have to monitor the wash as often, since we are putting in fewer chemicals. This oil skimmer can work unattended, 24 hours a day, without the need for someone to watch it.”

According to Oil Skimmers’ Petrucci, another parts washer OEM was motivated by increasing environmental regulations to add a skimmer to its equipment late in the design process, despite very little available space for the unit.

“One of the challenges when applying a skimmer is the particular physical envelope available,” according to Petrucci. “If we don’t have an off-the-shelf unit, we have the ability to modify the equipment and mountings to fit the requirements. In this case, our engineering team designed a solution that would enable the machine builder to put a skimmer in that piece of equipment.”

Oil and coolant disposal costs – Whether the tramp solutions are generated in machining or parts washing, CNC operators incur costs for proper treatment and disposal of oil in wastewater and when disposing of coolant.

In the case of coolant, one option is to pay to have the entire coolant mixture hauled away. However, waste haulers typically charge more if the oil has not been removed from the mixture. Some haulers will not take any mixture with oil.

An alternative is to break down the coolant, discharging the water and disposing of the concentrate. Ultrafiltration and distillation are the most common ways to break down the coolant mixture, but each works better if any oil in the mixture is first removed. When free oil is eliminated from the coolant, the distillation process requires less energy; depending on the system, ultrafiltration efficiency increases from 20 to 70%. Certain coolants can be discharged into municipal sanitary sewers if they are oil-free.

With so much to gain, including immediate cost savings, installing an efficient oil removal system far outweighs the initial cost of the equipment.

“Machining operations, including major aerospace and automotive corporations consulting with us on oil removal solutions, usually have one of three motives: to improve the quality of their finished parts, extend the life of coolants and wash solutions, and to meet the increasing environmental standards for treatment and disposal,” Petrucci said.

“Given that the efficient and economical removal of oil can impact all three factors concurrently, it is something machine shops should be reviewing much more closely,” he concluded.

Jeff Elliott is a technical writer in Torrance, Calif.

About the Author

Jeff Elliott

Jeff Elliott is a Torrance, Calif.-based technical writer.  He has researched and written about industrial technologies and issues for the past 20 years.