Aspects of Energy Efficiency in Machine Tools

Discussions of the efficient use of energy have become more frequent in many sectors of industry. Machine tools comprise numerous motors and auxiliary components whose energy consumption can vary strongly during machining. The main spindle drive, for example, and the coolant system work near their rated power during roughing with a high stock-removal rate, while the power consumption during finishing is significantly lower. There is a very close interdependence between the individual components and subassemblies of a machine tool and aspects of productivity and quality. From a detailed examination of manufacturing processes to the power consumption of individual components, potential for savings can be evaluated and measures can be defined for the efficient use of energy.

In many branches of the investment goods industry, energy efficiency has developed into an important product characteristic. Geared motors for materials handling technology have been divided into efficiency classes for some time now. A multitude of ideas has been tried for increasing energy efficiency in manufacturing with machine tools. Potential savings result with regard to the base load of machine tools that require energy consumption even in nonproductive phases. The base load is determined substantially by the auxiliary components of a machine. Besides the use of energy-efficient motors in the auxiliary components, many possibilities for reducing the base load can be found in proper energy management. With energy management, consumers are specifically switched off by the machine control in nonproductive phases.

Measures to support the operator during setup also increase energy efficiency, because they shorten nonproductive phases and reduce the influence of the base load. Scrap inevitably increases energy consumption per good part. Manufacturing with accuracy starting from the very first part can therefore become a decisive factor for the energy efficiency of a machine tool. Machine designs with balanced thermal behavior and precise position measuring technology have a distinct advantage here.

Energy demand during milling

The power requirements of a milling process are divided into the following consumer groups:

• Cooling lubricant processing
• Compressed air generation
• Electrically powered auxiliary components of the milling machine
• CNC control package with main spindle and feed axis motors

The proportionally calculated energy for lighting, ventilation and air conditioning is added to these groups. The energy demand of a milling process strongly depends on the size of the milling machine and the machining task.

In this example, an aluminum housing with dimensions of 150 mm x 50 mm x 25 mm is to be milled on a machining center with a work envelope of 850 mm x 700 mm x 500 mm. The mean total power consumption of all the above-mentioned consumer groups is 13 kW for roughing and 7.4 kW for fi nishing. The power balance during roughing and finishing provide more detailed information on the distribution of energy consumption among the individual consumer groups.

The cooling lubricant is prepared centrally away from the milling machine (pumping, temperature stabilization). For roughing, this has a mean power requirement of 5.1 kW. For finishing, the mean power requirement decreases to 1.5 kW. Production readiness consumes almost no power. Dry machining offers great potential here for increasing energy and resource efficiency. In many milling applications, however, doing without the cooling lubricant can significantly increase the scrap rate and therefore increase mean energy consumption.

The mean compressed air power changes only slightly in the phases of production readiness, roughing, and finishing. It averages approx. 1.3 kW. Compressed air is required for minimum lubrication of the spindle, tool changing and cleaning the workpiece. In small quantities it is required as sealing air (spindle, tool measurement, linear encoders).

The electrical consumers of the machine include the CNC control with main spindle and feed axis motors as well as numerous auxiliary components (pallet changer, cooling, hydraulics, automation). The power consumption of the auxiliary components varies in the production conditions of readiness, roughing and finishing by only 600 W. With a power consumption of 2.5 kW, the auxiliary components largely determine energy consumption in the production readiness condition. A requirement-oriented deactivation of auxiliary components therefore offers substantial potential energy savings.
The CNC control package with feed axis motors and main spindle require in this example just 27 % of the total power requirement. In both cases, the mean power consumption of the feed motors is 250 W and is largely determined by the holding power of the vertical axis. Short peak values occur only in the accelerating and braking processes.

Energy efficiency of the drive components

Spindle and feed-axis motors are among the central components of a machine tool. The energy efficiency of a drive component depends on the ratio of delivered power to consumed power and is therefore reflected in the efficiency. The network of drives on a machine tool converts consumed electrical energy to delivered mechanical power. The components of the drive network are a power supply module, drive modules, motors and the mechanical components. Data on efficiency typically refer to the rated power. In other rated values, the efficiency of individual components can vary significantly. HEIDENHAIN supply modules and drive modules attain efficiency values of over 95 %.

Power consumption during milling

Here the power consumption of a main spindle and the feed drives is itemized.

Example 1: Rough facing

During rough facing with paraxial feed rate the feed motors have a mean power consumption of 200 W. The main spindle reaches it rated power at approx. 19 kW.

Example 2: Circular pocket

The circular pocket is machined with a roughing and a finishing cycle. The mean power of the feed drives here is 100 W. The main spindle needs 1.5 kW of power.

Conclusion

Feed drives contribute only a small share of the total power of a CNC and can therefore contribute little to increasing the energy efficiency. On the other hand, the selection of the spindle can have a significant effect on energy consumption. If a spindle drive operates far below its rated power, the intrinsic losses of the drive increase in proportion, with negative effects on the energy balance. If the spindle limits the maximum possible metal removal rate of a milling process, the milling process inevitably takes longer. The result: energy efficiency decreases due to the base load generated by the auxiliary components. There is also potential for more efficient design of milling processes in the consideration of the efficiency of spindle motors, for example by using synchronous instead of asynchronous motors.

Efficiency of regenerative supply modules

Every acceleration process of a drive requires a braking process in return. The energy from the drives’ moving masses is largely reconverted to electrical energy.

The supply modules of the CNC controls from HEIDENHAIN are designed both for regenerative and nonregenerative braking. In a nonregenerative supply module, the kinetic energy released by the braking process is converted to heat by the braking resistors. A regenerative supply module returns this e