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.

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.

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.

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.

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Milling power

The power consumed during a milling operation can be categorized as that consumed by the main spindle and by the feed drives. During rough facing operations with a paraxial feed rate, the feed motors in our example consume 200 W on average. The main spindle reaches its rated power during this phase of operation, consuming 19 kW.

Now consider a typical milling operation, machining a circular pocket with a roughing and finishing cycle. The mean power consumption of the feed drives during this operation is 100 W in our example, while the main spindle needs 1.5 kW of power.

It is clear feed drives account for only a small portion of total power that a CNC consumes, so making them more efficient does little to boost energy efficiency. The spindle, on the other hand, can significantly impact energy consumption. If it operates far below its rated power, the intrinsic losses rise in proportion and hamper overall energy balance. If the spindle limits the metal removal rate during milling, the whole operation takes longer. There is a drop in energy efficiency because of the base load of the auxiliary components.

Moreover, making the milling process more efficient will boost its energy efficiency as well. One approach, for example, might be to use synchronous rather than asynchronous motors.

CNC systems also have an opportunity to benefit from the efficiency of regenerative supply modules. Every acceleration of a drive eventually demands a braking action. Regenerative drives convert kinetic energy from the moving masses back to electrical energy. Power supply modules for CNC available today typically are designed both for regenerative and nonregenerative braking. In a nonregenerative supply, the kinetic energy released by the braking process is converted to heat by braking resistors. A regenerative drive returns this energy to the power grid rather than dissipating it. However, the path required for returning the energy and the necessary components for smoothing the grid power generate losses even when the drives need no power. The power loss rises slightly even when power is not being regenerated.

A regenerative supply module operates more efficiently than a nonregenerative module when the regenerated energy more than compensates the higher power loss. Therefore, whether a regenerative drive makes sense or not depends on the expected machining operations.

For example, the number of tool changes affects energy consumption. In one example, a tool change operation periodically interrupts a 15 kW milling operation. The spindle consumes 60 kW during its start-up. The regenerative supply briefly returns 48 kW to the grid, but a nonregenerative supply converts the kinetic energy to heat.

Metal cutting, of course, requires a lot of power. Thus the mean of the input power consumption drops when there are more frequent tool changes. Regenerative supply modules work more efficiently when there are frequent interruptions. In one case, regenerative modules worked well as soon as the time interval between tool changes was 100 sec or less (about 0.6 tool changes/min). Regenerative supplies then, often prove to be the best choice in processes experiencing numerous tool changes per minute. On the other hand, contour milling, which is often characterized by infrequent tool changes, is probably better off with nonregenerative systems.

It is worth noting that though switching off auxiliary components can save energy, this practice can also have the opposite effect. Consider what happens with the sudden removal of waste heat from auxiliary components or the removal of the temperature-stabilizing effect of work media. These actions can cause thermal displacements in the machine frame which in turn can throw off tolerances and generate scrap parts. So switching off auxiliary devices saves the most energy on machines where their operation has little impact on thrermals.

The CNC can serve as the central control unit for managing energy use of machine tools and auxiliary components. Delay times can be assigned to events so that, for example, motors can be locked and disconnected from electrical current after a stop. Similar functions can be programmed for deactivating auxiliary devices, axes, light in the workspace, and so forth.

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Similarly, the time period when the machine tool isn't cutting impacts its energy efficiency. So minimizing tool setup time helps boost energy efficiency. In that regard, use of touch probes rather than a dial gauge for workpiece measurements can speed setup times.

To understand the difference, consider two example tasks. The first consists of aligning the workpiece blank parallel to the axes, setting the datum in the working place at a corner, and setting the tool axis datum at the top surface of the blank. The second task consists of aligning a workpiece paraxially using two holes, setting the datum of the working plane in the center of the first hole, and setting the tool axis datum at the top surface of the blank.

As an informal experiment, technicians did both tasks first using a dial gauge and then a touch probe. In the case of the first task, use of a touch probe saved about four minutes, which amounted to 72% of the time needed for the same task with a dial gauge. Assuming ten such setups daily and 220 working days, use of a touch probe for this setup task saves about 580 kW-hr annually. Results for the second task were similar. Setup using a touch probe saved about five minutes, 77% of the time needed for dial gauging. Assuming the same ten daily setups and 220 working days, touch probing saves 730 kW-hr annually on this task.

Energy-efficient servos

Spindle motors and feed axes are under closed-loop control. And even a small disturbance in their feedback signals can cause large fluctuations in motor current. There are subtilties associated with position feedback that can affect energy efficiency. Position encoder feedback is often interpolated to improve resolution. The interpolation includes short-range deviations within one signal period (interpolation error) of about 0.5% of the signal period. If the frequency of the interpolation error increases, at some point the feed drive can no longer follow the error curve. However, the interpolation error still generates additional current components. Therefore, if the torque remains constant, the motor consumes more energy and its efficiency worsens. The additional energy required gets converted to heat. A cooling system must dissipate this heat, and the cooling system, of course, also consumes energy.

Consequently, encoders with a high signal quality will bring more energy efficiency, as will those with a resolution high enough to eliminate the need for interpolation.

Use of encoders can also affect energy efficiency by potentially eliminating scrapped parts. One primary cause of parts that are bad is the thermal drift of feed axes running on recirculating ball screws. The temperature distribution along ball screws can change rapidly as feed rates and moving forces change. The effect is pronounced on machines running semi-closed loops where there are no linear encoders along the feed axes. It is not unusual to see changes in length of 100 µm/m within 20 minutes. This drift can cause significant flaws in a workpiece.

Use of a linear encoder for measuring slide position can head off such difficulties. Then a rise in ball screw temperature won't influence accuracy, simply because the position control loop compensates for mechanical errors in the drive.

Consider an example of a coupling lever made twice from the same workpiece blank. The second workpiece is simply machined 10 mm below the first. Between the two machining operations, twenty machining cycles for the same part are executed above the blank.

In a semi-closed-loop operation, the contour of the second workpiece deviates from the contour of the first workpiece slightly. In our informal experiment, there was an offset between the two of 44 µm. Incorporating a linear encoder in a closed loop eliminates the edge.

In this example, it took two hours to manufacture 22 coupling levers. The bore of two holes at a distance of 350 mm must hold a tolerance of IT7, which allows an error of ±28 µm. To manufacture 22 good parts in a semi-closed loop, the machine must first run the NC program cyclically for 25 minutes to ensure compliance with the IT7 tolerance. During warm-up, the energy consumption is only about 10% below the value during milling.

Consequently, energy costs-per-good-part in the semi-closed loop are 19% higher than for manufacturing 22 parts in a closed loop with linear encoders. If 50 parts are manufactured on a milling machine in the semi-closed loop with a preceding warm-up phase, the 8 kW dissipated during milling for 220 working days works out to an additional energy requirement of 660 kW-hr.