Minimum quantity economically reasonable alternative wet machining.
Cross Hüller North America
Sterling Heights, Mich.
It's common practice in manufacturing to rely on coolants to keep the machine, cutting tools, and workpieces thermally stable and undamaged. Coolant also washes away chips. But coolant has significant costs. These include the costs for buying, filtering, separating, and disposing of it, not to mention EPA documentation requirements. together they make 15% of the entire life cycle cost of a machine tool.
Tighter EPA rules are on the horizon for using and disposing of coolant, along with protecting workers from it. These rules will surely raise the cost of using coolant. (It already costs more to get rid of coolant than it does to buy it.)
A new approach, Minimum Quantity Lubricant (MQL) machining, could change the situation. Though primarily used for aluminum, MQL is an economical, environmentally friendly alternative to wet machining.
USING JUST ENOUGH
The goal of MQL is to use just enough coolant and lubricant to minimize friction and heat. The diameter of the aerosol particulates, however, must be within tight tolerances for optimum wetting and lubrication. Although early attempts at applying oil and air failed because they separate at high speeds, newer MQL machines are as effective as wet operations at maintaining lubricity. The Cross Hller Specht duo, for example, a high-performance, two-spindle CNC module built for wet and MQL operations, features a precision dosing system. It is built into the motorized spindle housing for rapid responses.
In CNC machines designed for MQL, the software for a specific part controls the amount and duration of aerosol spray. This is important because different parts are made using a variety of machining processes, and each needs a different amount of lubrication. Milling, for example, is a surface operation and requires a minimum amount of lubricity. But tapping and thread cutting call for much more because of the high surface pressures involved.
In the Specht duo, the CNC program controls a dosing valve, which meters out precise amounts of lubricant. The lubricant is then mixed with air to form the required aerosol, which is fed through ducts in the cutting tool to the cutting edge. It's such a short distance between the dosing valve and the cutting surfaces, the aerosol stays cool and fully mixed for good cooling and lubrication.
To minimize the amount of lubricant used, the aerosol switches off as the spindle moves from one hole to another. This also eliminates oil buildup on the workpiece and the machine, making clean up faster. Chips produced with the MQL system remain essentially dry, so time-consuming and costly coolant-recovery operations are unnecessary.
Coolants for wet machining are typically water-based emulsions with 6 to 8% oil. Oil concentrations are not optimized for any specific tool and are never really an optimum concentration because constant evaporation and replenishment makes it either too diluted or too concentrated. In MQL, on the other hand, all the fluid in the aerosol is oil for superior lubricity and less wear on the tools.
KEEPING SILICON OUT
Tool life and surface finish with MQL are also improved because there are no abrasive silicon particles suspended in the coolant. Aluminum contains about 13% silicon, which is made from a quartzlike material and is extremely abrasive. In wet machining, silicon is practically always present in the recycled coolant despite filtration systems that eliminate 40- m particles. It's the silicon particles smaller than 40 m passing through filters that shorten tool life.
KEEPING THINGS COOL
There are considerations engineers should be aware of when designing for MQL. One of the most important is to maintain thermal stability and introduce as little heat into the machining process as possible. For example, changing the sequence of machining operations affects how much heat goes into the part. And temperature-compensation algorithms let the CNC machines calculate how a thermal load will affect machining accuracy and make adjustments that turn out parts and features within tolerances despite the heat.
Another way to minimize heat is to keep incoming parts quarantined until their temperatures stabilize. For example, at one plant, aluminum wheel knuckles are machined from parts received directly from a foundry next door. A probe measures the incoming parts' temperature. If it is too high, the parts are kept in a queue until cool enough for machining. Once the knuckles reach the right temperature, robots
load them into machine tool.
If inventory is too small for quarantining parts, experts can develop a temperature compensation algorithm for a specific part. Complex parts may not expand uniformly when heated. To derive a temperature-compensation algorithm, a part is artificially heated to determine how it expands. A look-up table is developed and then included with the part program. Based on temperature readings from a probe, the program adjusts where the machine tool cuts specific features.
In wet machining, a part is normally rough machined then finish machined. For example, pockets and cavities, as well as threading and tapping operations, are done first, then finishing operations are run.
In MQL machining, however, designers should use rough machining only for what is essential to shape the part because it generates so much heat. The part is then finish machined before it can heat up. For example, bearing bores and dowel holes with a tight positional tolerance are finished relatively early in the process, versus later in a normal wet process. Finally, miscellaneous operations relatively immune to heat and in no need of much cooling, such as drilling and tapping, are handled after finish machining.
TOOL AND CHIP REMOVAL
Some MQL processes, including milling, drilling, and tapping, as well as finish cam boring and finish machining of valve seats and guides, call for cutting tools with lubrication ducts. These ducts transfer aerosol lubricants specified by the part program to the cutting edge of the tool. The duct needs to be properly tuned because any abrupt changes in the duct's diameter or blind endings will prevent the free flow of aerosol. This causes the aerosol to turn into large globules of oil and it looses its lubricity properties.
There are a variety of techniques and systems for effectively removing chips that don't rely on coolant. For example, stainless-steel chips cut at steep angles eliminate nests of chips. Vacuum systems can recover fine particulates (mists and dusts), and conveyors can remove chips from machines. Ultimately, however, the best way to minimize machine-tool contamination is by using MQL and a responsive mixing system. This eliminates the need to recover any coolant. It also means never manually cleaning chips off the walls of the machine.
PUTTING MQL TO WORK
Jumping headlong into MQL can lead to a variety of machining problems related to heat, tool life, and chip removal. However, if you use tools designed for MQL and systems and strategies to control heat, you can replace coolants. Some times it takes special tooling with highperformance coatings, heat-resistant materials, and lubricant ducts, as well as chip evacuation systems.
With properly designed tools, however, you can use the same feed rates and speeds you used with wet machining. And tool life should stay the same.
MQL machining reduces costs and protects the environment. In the final analysis, as long as the mixing and dispersion of oil is precisely controlled, part quality remains as good as or better than wet machining processes.