Understanding MQL": machining with minimum quantity lubricant can save money and improve both tool life and part finish. But it can also involve chang

We have all been conditioned to keep cars cool by filling the radiator with coolant. Much the same preconditioning has been applied to manufacturing, where coolant is routinely used to address the thermal stability of the machine tool, cutting tool and workpiece, and also to remove chips. But is coolant as essential to every machining process as it is to a car?

The cost of coolant is approximately 15 percent of the life-cycle operational cost of a machining process. This cost continues to rise. It includes the costs associated with procurement, filtration, separation, disposal and record keeping for the EPA. Already the costs for disposal of coolant are higher than the initial cost of the coolant, and they are still rising. Even stricter regulations are under consideration for coolant usage, disposal and worker protection. As a result of all of this, coolant in wet machining operations is a crucial economic issue. An alternative, machining with "minimum quantity lubricant," or MQL, is gaining acceptance as a cost-saving and environmentally friendly option in place of some wet machining processes.

MQL permits dramatic cuts in coolant costs, while protecting workers and the environment. It also delivers improved tool life and surface finish even though tool life is often the reason why wet machining is applied. MQL can deliver better life for two reasons: (1) the optimum concentration of lubrication can be specified for a given operation, and (2) silicon particle contamination suspended in the cutting fluid is eliminated MQL machining processes can be adapted for cast iron as well as aluminum. Challenges (all covered below) include precisely controlling lubricant mixing systems, maintaining thermal stability, choosing the appropriate cutting tools and removing the chips.

Control of Lubricant

Control over the amount of lubricant dispensed is important because different processes require different amounts of lubricity. For example, milling is a surface operation, and it requires a minimum amount of lubricity. Deep-hole drilling is a different operation requiring a different level of lubricity. And yet a third level of lubricity is required for tapping and thread cutting operations because of their high surface pressure.

The objective of an MQL mixing system is to deliver a precise amount of aerosol. That is, the diameter of the aerosol particulates is held to a precise tolerance to maintain optimum wetting and lubrication properties. In machines designed for MQL, lubricity can be controlled using a parameter in the part program that varies the aerosol's amount and duration. Early attempts to apply a mixture of oil and air in this way failed because demixing of the oil and air occurred at high speeds. However, new systems have proven as effective as wet operations at maintaining lubricity. One example is Cross Huller's "Specht Duo," a two-spindle CNC production module built for wet or MQL operations, which features a precision dosing system. The dosing system is integrated into the motorized spindle housing.

The CNC program controls the dosing valve that provides the precise amount of lubricant. The lubricant is mixed with air to form the desired air/oil aerosol mixture. It is then fed to the cutting edge through ducts in the tool. Because of the short distance between the aerosol generation point and the cutting surfaces, optimal cooling and lubrication conditions for a given machining process and cutting tool can be achieved and maintained. The aerosol is switched off as the spindle traverses from the position of one machined hole to another. This eliminates oil buildup on the workpiece and on the surface of the machine, minimizing the need for operator intervention to clean the machine. Because the chips produced with the MQL system remain essentially dry, the need for time-consuming and costly coolant recovery operations is eliminated.

Silicon Contamination

Along with the ability to control lubricity, MQL improves tool life and surface finish by eliminating abrasive silicon particles suspended in the coolant. Aluminum workpieces contain approximately 13 percent silicon, which reduces tool life and leads to poor surface finishes. Fine aluminum/silicon particles can become suspended in wet machining coolant. While filtration systems eliminate 40-micron particles, particles smaller that 40 microns pass through the system to get recycled with the coolant.