New MTC technique offers dramatic improvement in CNC machine tool stability

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Introduction

Melbourne Australia based machine tool builder ANCA has been granted a U.S. patent for Motor Temperature Control (MTC), a technique that dramatically improves the stability of precision machine tools.

Problem to be solved

A key element of machine tools is the spindle which holds and drives the cutting tool which machines the workpiece. Any dimensional errors in the spindle will be reflected as dimensional errors in the workpiece. A common technique in modern machine tools is to use motorized spindles where the driving motor is an integral part of the spindle. ANCA’s patent is a way of maintaining the motorized spindle at a constant temperature. A significant problem in the use of precision machine tools is the impact of changes of temperature particularly in the temperature of the spindle. Changes in temperature of the machine tool can be caused by changes in ambient temperature, coolant temperature, run time, spindle rpm and spindle load.

Solution

ANCA’s solution is to actively control spindle temperature by varying the electrical losses in the motor element while simultaneously maintaining spindle motor speed and load. This is accomplished by using special firmware in the spindle drive amplifier. Although simple in concept, this in practice is a major step forward in manufacturing technology.

Examples of the application of MTC

Faster Warm Up time

Figure 1A near universal approach to precision machining is to run the machine in a warmup cycle to bring all the elements of the machine to some level of thermal stability. Typically, this warmup cycle will take approximately half an hour. Using MTC, this time can be reduced to around 5 minutes. Figure 1 shows the impact of MTC on actively heating the spindle to reach working temperature quickly.

Stability in batch grinding of endmills

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Figure 2

Figure 2 shows the results in grinding two batches of 6mm diameter endmills, with and without MTC.

Grinding a Skiving Cutter.

Figure 3

Electric vehicles incorporate a high-speed gear box in their drivetrain. The accuracy of the components of this gear box will impact on lifetime, efficiency, and noise of the electric vehicles. Figure 3 shows a typical skiving cutter which is used to manufacture internal and external gears and will determine their accuracy. ANCA manufactures a special machine, the GCX, for grinding this class of tools. MTC is a key enabling technology for this machine to produce cutters to AAA class. Firstly, it is used to control the temperature of a dressing spindle which is used to create a high accuracy profile on the grinding wheel. MTC is also being used to control the main grinding spindle temperature during up to 12 hours of continuous grinding. Figure 4 shows the measurement results for contour accuracy of a tooth and index accuracy for all the teeth.

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Figure 4

Summary

Motor Temperature Control (MTC) is a major innovation in machine tool technology. It improves the accuracy and stability by controlling the temperature of motorized elements of the machine by special software in the spindle drive amplifier. It drastically reduces the time required for a machine to reach working temperature.

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