High-speed machining in the aviation industry (2)

Hollatz recommends that high-speed cutting should be used when finishing aluminum alloys. “If the spindle is rated at 30,000 rpm, we will try to run at full speed. At the same time, we will also limit the diameter of the tool used. Considering the centrifugal force caused by the tool imbalance The higher the machine speed, the smaller the tool diameter." As an example, a large machine tool (33,000 rpm, motor power 107 hp [80 kW]) produced by Makino is not recommended for any tool larger than 50 mm. For most cutting operations, tools with a diameter of 25 mm or less have the highest cutting efficiency.

Like most machine tool suppliers, Hollatz recommends a hollow short taper shank (HSK shank) for higher spindle speeds than CAT shank. He pointed out that CAT tool holders may cause accuracy problems in the Z direction during high speed machining. At the time of machining at high speeds, there have been extreme cases where the CAT holder caught the spindle. The design of the HSK tool holder is characterized by the use of a double contact between the taper and the end face, so that the accuracy in the Z direction can be controlled. “When the spindle speed is below 20,000 rpm, the CAT holder can be used, but when the speed is up to 30,000 rpm, there is no choice but to use the HSK holder.”

Another key factor in high-speed machining is the CNC controller and its ability to precisely control machine motion at high speeds. The controller with “forward-looking” function can control the current speed and acceleration/deceleration of the tool according to the position that the tool will reach. This function is just as important as the high-speed drive spindle.

According to Hollatz, the standard "forward-looking" function of Makino machine controllers has more than 60-80 G-code modules. The Super GI.4 controller package is specifically designed for high speed machining with over 180-250 modules. For the same tool path, Super GI.4 is 15%-30% faster than the SGI.3 controller it replaces.

According to Reilly, processing manager at Haas, Haas machines offer an option for high-speed machining control. Haas' high-speed machining control module allows for higher feed rates and more complex toolpaths without the machine having a downtime. The Haas machine uses a motion algorithm called “pre-interpolation acceleration” and combines with the full “forward-looking” function of up to 80 modules. Its high-speed machining control module provides contours up to 500ipm (13m/min). Feed motion without the risk of programmed tool path distortion. “The biggest benefit of doing this is that it is 'forward-looking' when executing the program, and it keeps moving as fast as possible when there is any change in direction of motion.” Reilly explained, “If the direction of motion does not change much, the speed of movement is also There is almost no need to change. The change in speed is proportional to the change in direction."

In the aerospace industry, as new aircraft use more composite materials to reduce weight, the need for processing composite materials is becoming increasingly urgent. Boeing 787 aircraft using synthetic materials to make fuselage and wings is a typical example of this trend. The high-speed machining of aluminum alloys will soon become a standard process, and it seems to be meaningful to apply high-speed machining to other commonly used aerospace materials, and of course, composite materials are no exception. “When the composite parts are manufactured using the near-net forming process, machining is required to meet the accuracy requirements of the mating, joining and recessing parts,” explains Jeff Crick, Cincinnati's Composites Processing Platform Manager. “For example, using layering. Machining can create an access hole on the surface of the wing, but can only achieve an accuracy of about ±0.5mm (the lamination process can only achieve this accuracy). In order to achieve higher precision in the required part, secondary processing is required ( Such as machining), just like finishing aluminum, titanium or steel."

According to Crick, the power and torque required to machine composites at high speeds is small compared to machining aluminum alloys. The machine itself does not need to be as thick and strong as a machine that cuts titanium, but still needs to be rigid enough to overcome vibration and resonance. Most machine spindles range in speed from 10 to 13,000 rpm (although they can operate at higher speeds). For example, a large aerospace component manufacturer in the United States achieved high-speed machining of composite materials with a depth of 0.012-0.016" (0.3-0.4 mm) on a 24,000 rpm machine.

Today, most composite materials are processed using machining units originally designed for metal cutting. Crick believes that the ultimate goal is to create a special machine that is lighter and designed specifically for processing composite materials. One must pay attention to the trend when designing such machines, that is, the size of aerospace composite parts is getting larger and larger. Crick said, "Composite parts can be very large, such as up to 100' (30m) wing cover, and even the entire fuselage components, such as the new Boeing 787 with a cabin section diameter of more than 20' (6m), The length exceeds 30' (9m). In this large structure, the machining tolerances on the joint between one fuselage and the other fuselage are very strict. Other components may be long and have ribs, such as Wings, stringers, pillars and floor beams, etc."

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