The greening of machining: A holistic approach to machine tool design reduces metal cutting’s environmental impact

Metal cutting has traditionally been a somewhat messy, smelly, hot process involving various oils, liquid coolants, part washing, chip drying, smoke and mist, where energy consumption and floor space take a back seat to metal removal rates and part outputs. But machining is being quietly reinvented as a green process that delivers environmental advantages, reduces energy use, improves part quality, cuts costs and improves workplace safety. Breakthroughs in “green” machine tool design and the machining process itself are reducing plant and operational costs by millions of dollars, while improving plant air quality, part quality and part costs.

The SPECHT horizontal machining centre introduced in 2010 by MAG, a machine tool and systems provider, is an example of this new machine design. Engineered from the ground up to be green, the SPECHT cuts dry, wet or with MQL (minimum quantity lubrication), eliminates warm-up time, conserves space with a 1.8-m wide footprint, sleeps when idle, and minimizes coolant use and air extraction.

Space-saving design reduces plant overhead
The chassis of the SPECHT uses a non-cantilevered box-in-box structure that gives it Y-axis stiffness with low moving mass throughout to allow high acceleration/deceleration rates with reduced drive energy consumption. Most plants have adequate ceiling height, but not always width. The SPECHT design allows the tool changer to be placed overhead to minimize machine width, and a smaller machine allows a smaller plant to be built, conserving building materials. The tool disc and changing unit also allows direct tool change without a cam box or swapper arm, eliminating extra parts.

Minimize and eliminate coolant
One of the objectives behind the SPECHT’s design is the elimination of coolant, which constitutes about eight to 16 percent of the cost in metal cutting. Coolant serves a number of needs, so those needs were systematically reduced or eliminated during the machine’s design. One of coolant’s functions is temperature stabilization of the machine and workpiece, so a design goal was thermal stability. The machine frame was extensively optimized using finite element analysis, modal analysis, thermal and air flow modelling. The result is a thermally symmetrical structure that, coupled with temperature sensors, glass scales and thermal compensation algorithms in the control, reduces or eliminates energy-wasting machine warm-up. Minimizing warm-up reduces part-probing needs, too, which provides more time for chip cutting.

Steeply angled chip sheds and interior guarding ensure a free flow of chips away from the machine bed without coolant flushing. The result is a machine that uses 40 to 80 percent less coolant than legacy designs. An optional variable frequency drive coolant pump controls flow rate from 100 to 40 percent (20 to eight litres per minute). The coolant rate is optimized for each tool, and the flow is dialed back when not needed. This reduces pump energy use and the need to chill the coolant. The standard machine design uses a drum-style, self-cleaning coolant filter, eliminating replacement media. The SPECHT machines designed for flume coolant handling utilize a high-pressure coolant filter with an integrated back flushing filter. In addition to requiring less coolant, SPECHT machines have an automatic central lube system using NGLI-00 grease, which minimizes the loss of lube oil into coolant, increasing coolant life.

The SPECHT’s optional MQL system eliminates coolant altogether. MQL is a nearly dry system that delivers a fine through-tool oil mist to the interface of the tool and work surface. The amount delivered is tailored for each tool and operation to produce optimum lubricity, so tool life is improved. MQL eliminates plant-wide coolant systems and pumps, and all the energy it takes to run them. It also eliminates coolant disposal costs, while improving worker health by providing a safer environment.

At Ford Motor Company’s Van Dyke Transmission plant in Sterling Heights, Mich., the installation of MQL machines netted a seven-figure savings in initial cost, while the machines are producing parts at lower variable cost and improving plant air quality. More importantly, the quality level of parts machined with MQL is equal to, or better than, comparable wet machining operations. The plant currently uses more than 120 MAG CNC machines, including 52 SPECHT horizontal machining systems, equipped with MQL.

The steeply angled interior of the machines at Van Dyke Transmission, coupled with a chip evacuation system, eliminates the need for chip-flushing coolant and the resulting cost for pumps, filter media and chip drying. The machining envelope is kept under negative air pressure, with chips and oil mist pulled out in an air stream, then through a centrifuge and filter system. Dry chips collect in a hopper, and clean air is returned to the plant or back to the machine enclosure. As added bonuses, tool life is increased with MQL and the scrap rate on dry aluminum chips garners an extra $.15 per pound over wet, demonstrating the cascade of cost savings when flood coolant is taken out of machining.

Minimize raw energy consumption
To minimize air extraction needs, the SPECHT’s machining envelope is designed to be as low-volume as possible for a machine of this class, and location of the air intakes is optimized in software. Coupled with the chip-shedding interior, this reduces air extraction from one-half to one-fifth of MAG legacy machines, and the extraction ratio of MQL/wet machining has been reduced from 2.5/1 to 1.4/1.

The SPECHT electrical system uses regenerative drives that, during braking, produce electricity for storage in capacitors or return to the power grid. Low-energy, eight-watt magnetic coils are used in the machine’s valves versus the traditional 30-watt, and the control is cooled via heat exchanger and the machine’s single chiller. A cycle-time regulator aids energy management by balancing cycle times and variation of machines to match production volumes. In addition, a user-configurable “sleep” mode combines with the thermal management and compensation strategy to minimize or eliminate warm-up periods.

The SPECHT control includes an ECO mode that optimizes energy consumption based on performance characteristics selected by the operator. ECO mode shows where energy is being consumed during a cycle and where it can be reduced, allowing the operator to fine tune axis speeds, tool change times, air extraction, coolant use, chip conveyance and more to optimize energy use versus required cycle times.

Eliminate hydraulics
The use of hydraulics on the current SPECHT is minimized through use of more accurate electromechanical systems, and the hydraulic system itself uses an accumulator and pressure-reducing valves to minimize pump size/energy use. MAG’s overall engineering philosophy is to eliminate, or at least minimize, hydraulics on all of its machines by replacing them with modular, more flexible electromechanical systems. These systems provide better closed-loop feedback and limit energy use to on-demand, with no wasteful pumping and cooling of hydraulic fluid. The cost of replenishing, disposing and recycling oil is also eliminated, as are leak points and spills.

Conclusion
As a result of these design steps, all SPECHT configurations have reduced energy consumption over legacy designs, with the linear motor configuration producing a 10 percent energy savings in operation and more than 60 percent in “sleep” mode.

Peter Mosher is a product leader at MAG (www.mag-ias.com), a machine tool and systems company.