The energy efficient machine tool

When the topic of energy efficiency comes up, energy-efficient machine tools don't typically spring to mind. Yet machine tools contain motors and auxiliary components whose energy consumption varies widely during machining operations. Several means can be employed to reduce the amount of energy these machines consume.

For example, the main spindle drive and coolant system work near their rated power during roughing operations with high stock-removal rates. But they consume much less power during finishing operations. On another note, use of energy management functions can let controls switch off power consumers during phases of machine operation that aren't productive. Similarly, measures that minimize setup time also boost energy efficiency because they shorten phases of operation that don't produce product. Thus the ability to manufacture with accuracy starting with the first part is actually an energy-efficiency strategy.

Here's how the total 13 kW power consumption for roughing and finishing an example metal housing part example broke out. In both operations, the power requirement for the spindle motor dwarfs that of the feed drives.

Specifically, consider the milling process and its energy demands. These demands can be divided into cooling the lubricant, generating compressed air, powering auxiliary components on the milling machine, and powering the CNC control package with the main spindle and feed-axis motors. To this consumption one must add energy for lighting, ventilation and air conditioning.

To understand the possibilities for energy savings, it is helpful to consider a specific example. Suppose we are milling an aluminum housing measuring 150×50×25 mm. The machining center has a work envelope of 850×700×500 mm. The mean total power consumption is 13 kW for roughing and 74 kW for finishing. The power balance during roughing and finishing provides more details about how different systems consume power.

In this example it takes 5.1 kW to temperature-stabilize and pump the cooling lubricant. One possibility for saving energy would be dry machining. But going without cooling lubricant can significantly boost the scrap rate in many cases and thus defeats the purpose of saving energy.

To determine power efficiency, Heidenhain technicians measure power going into the supply module of a typical machining systems, as portrayed in the diagram.

Power for compressed air varies only slightly through production readiness, roughing, and finishing. In this case it averages 13 kW. The air serves to lubricate the spindle, tool changing, and cleaning the workpiece. It is also used in small quantities for sealing.

Of course, several parts of the machine consume electricity. Electrical consumers include the CNC control with main spindle and feed axis motors as well as numerous auxiliary components such as the pallet changer, coolers, hydraulics, and automation components. The power consumed by auxiliary components is relatively constant. In the case of our example, it varies throughout preparation, roughing and finishing by only 600 W. The power that auxiliary components consume largely determines the amount of energy consumed during production readiness operations. So there can be substantial energy savings through selectively disabling auxiliary components when they aren't needed.

Tests at Heidenhain revealed feed axes power consumption was insignificant compared with spindle power consumption during rough facing operations. (Spindle speed was 4,100 rpm, infeed depth, 4 mm.) Feed axes power was a bit more significant during circular pocket milling. (Circular pocke D=20mm, end mill D-10mm, lateral allowance of 5 mm, infeed depth 2x5 mm, spindle speed 16,000 rpm.)

Components under CNC direction include the feed axis motors and the main spindle. They account for just 27% of the power consumed in this example. The mean power consumed by the feed motors is 250 W and is largely determined by the holding power of the vertical axis. Short peak levels of power get consumed only during acceleration and braking.