The energy efficient machine tool

Sep 1, 2011 12:00 PM,

Dr. Johannes Heidenhain GmbH Heidenhain Corp. Schaumburg, Ill.

Even metal working operations can be optimized for energy efficiency with no adverse impact on productivity.

Page 3 of 3

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.

Touch probing saved over 70% of the time normally needed to gauge dimensions manually with dial calipers in two test parts done as examples at Heidenhain.

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.

Recirculating ball screws get hot, as revealed in this thermogram taken by Heidenhain personnel. Use of a linear encoder for feedback of slide position heads off errors caused by mechanical displacement as caused, for example, by heating.

Angle encoders can make a difference in energy efficiency. Plots of motor current at two different shaft speeds show the currents measured with A, a low-interpolation-error optical encoder, and B, a high-interpolation-error modular magnetic encoder.

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.