Successful high-speed machining requires attention to tooling, spindle and machine tool dynamics Tool technology and machine tool control capabilities have made high-speed machining more widely used in the manufacture of aerospace components. Although high-speed machining technology is particularly suitable for the processing of aluminum alloys, it also has its place in the processing of composite materials and hard metal materials.
Competitive pressures are continually forcing manufacturers to machine parts in a more efficient manner. At the same time, aerospace structural parts manufacturers also need components with higher strength, lighter weight and tighter tolerance requirements. High-speed machining technology allows manufacturers to reduce cycle times while also producing parts that are more compact and thinner than ever.
According to Wayne Reilly, processing manager at Haas, many people are not rigorous in using the term high-speed machining. In Reilly's view, while some people think that any machine with a spindle speed of more than 10,000 rpm is processed at a high speed, others have a more complicated definition. He said, “It actually depends on the context in which the term is used. Tool manufacturers may define it as speed, and machine tool builders may define it as some look-ahead blocks in CNC systems. High-speed machining The trend in technology is to use faster cutting speeds, feed rates and lighter loads, while conventional machining typically uses low-speed cutting with heavy loads and deep cut depths.†For example, the vertical machining center from Haas (VMC) provides up to 30,000 rpm rated spindle speed and 30 hp (22.4 kW) drive system power rating for high speed machining.
Randy Von Moll, manager of aluminum processing platforms at MAG Cincinnati, said, "I don't like the term high-efficiency machining in isolation from the spindle speed." His definition includes the dynamic response of the machine in addition to the spindle speed. He used five parameters to define efficient machining: 1 spindle speed; 2 spindle power; 3 high feed rate and tool path rate; 4 high acceleration and deceleration; 5 high precision. The latter three conditions specifically define the dynamic response of the machine rather than the spindle characteristics. Von Moll said, "In order to cut alloy materials more efficiently (such as aluminum alloys), it is really necessary to combine high-performance spindles with the high dynamic response of the machine."
If the aerospace parts are divided into two categories: "thin plate parts" and "thick plate parts", he believes that high-speed machining can be defined as: for thin plates with a thickness of less than 50mm, the spindle speed is 30,000 rpm and the rated power is 80 hp ( 60kW); for thick plate parts with a thickness of 50mm or more, the spindle speed is 18000rpm and the rated power is 135 horsepower (100kW).
Von Moll explained, “The maximum dynamic response parameters of the machine tool are not much different when cutting thin plates and thick plates. For both workpieces, the acceleration/deceleration should be around 0.5g and should be provided as fast as possible. (non-cutting) reciprocating motion, at least 1500ipm (38m/min)."
Acceleration/deceleration has a large influence on the cutting time when machining complex cavity-like workpieces, because the tool must change direction several times during machining.
The reciprocating time of the machine will affect the cutting time, especially the auxiliary time (the auxiliary time can account for 20% of the total machining cycle time when cutting the aluminum alloy). The assist time includes the positioning time of the tool when cutting a new workpiece or the time the tool moves to the tool changer. According to lean manufacturing, assist time is a waste that needs to be eliminated. A few years ago, Cincinnati introduced the HyperMach vertical copy milling machine line that combines fast reciprocating movement with high acceleration/deceleration. The fast reciprocating speed of these machines is up to 4000ipm (101m/min), which is designed to shorten the auxiliary time. HyperMach's X, Y, and Z axis strokes are 33m, 3500mm, and 1250mm, respectively, and are equipped with additional A, B, or C axes. The spindle speed of the machine is up to 30,000 rpm. Most HyperMach vertical profile milling machines are equipped with two independent spindles on a common X-axis gantry structure. In order to cope with the market demand for improving the processing efficiency of large workpieces (up to 2000mm × 4000mm), Cincinnati will exhibit and demonstrate the HyperMach horizontal series at IMTS 2008 (Chicago International Manufacturing Technology Exhibition 2008).
“Cutting small chips and processing them as quickly as possible†is the definition of high-speed machining by Makino design engineer Alan Hollatz. He believes that high-speed, small-depth cutting methods can reduce the heat of cutting into the workpiece or tool, and the cutting force of the workpiece and machine tool is also small. Conventional low-speed, large-depth machining methods tend to deform workpieces with wall thicknesses as thin as 0.030" (0.76 mm) in modern designs. Smaller cutting forces also mean lower workpiece clamping requirements.
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 holder is characterized by the use of a double contact between the taper and the end face, thus controlling the accuracy in the Z direction. “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.â€
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 toolpath, the 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 causing machine downtime and other failures. 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 contour lines 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. 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 alloys, titanium alloys or steel."
According to Crick, the high-speed machining of composites requires less power and torque than machining aluminum alloys. The machine itself does not need to be as thick and strong as a titanium-cut machine, 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. Care must be taken when designing such machines, that is, the size of aerospace composite parts is growing. 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.
In order to process these long, thin and easily bendable parts (Crick visually describes them as "wet noodles"), Cincinnati has developed a special extrusion milling machine. The machine can process aluminum alloys and composite materials. The processing range is 13′×8′ (4×2.4m), the spindle speed is 24000rpm, and 12 tools with a diameter of 25mm are used for machining. The workpiece can be up to 40′. (12m).
“Free-cutting materials (such as aluminum alloys or composites) benefit the most when almost all machining methods and workpiece materials benefit from high-speed machining,†says Haas processing manager Reilly. “Because of high speed, high feed Hard milling technology with small depth of cut, hardened die steel can also benefit from high-speed machining. Titanium alloy, as an increasingly important workpiece material for the aerospace industry, is certainly one of the beneficiaries."
“If aluminum cutting machines are like F1 cars, titanium cutting machines are more like bulldozers,†says Dan Cooper, productivity solutions manager at MAG Maintenance Technologies. “They differ greatly in spindle speed, despite the principle of high-speed machining— - High speed and small depth of cut are sometimes of interest for titanium alloys, especially for thin-walled parts, which are preferably machined using high-speed machining. For example, a user's part has a thickness of 0.030" (0.76 mm) and a height of 3" ( 76mm), such large height thin-walled parts cannot be roughed by old-fashioned traditional processes. Low-speed, large depth-cutting, and high-torque cutting will cause workpiece deformation and tool offset, especially for the processing of new 5553 titanium alloy parts."
Cooper pointed out that the low thermal conductivity, high modulus of elasticity and high strength of titanium alloy make it a difficult material to cut, "although cutting torque and dynamic stiffness may not be important for composite and aluminum alloy processing, but It is very important for titanium alloy processing. Compared with aluminum alloy processing, this will limit the processing speed of titanium alloy."
Cooper prefers to measure high speed machining with surface speed and feed rate instead of spindle speed. Surface speed is a function of spindle speed and tool diameter; feedrate is a function of spindle speed and tool density. The denser the teeth and the higher the surface velocity (SFM), the higher the feed rate, so the design of the tool is critical.
According to Cooper, MAG's new carbide tools can be machined at a surface speed of 390 fpm. “With a tool with a diameter of 25.4 mm and the largest number of sipe, we can only process at 1500 rpm and 2.5 m/min, which is quite high for titanium machining.â€
High-speed machining technology has proven its advantages in aluminum alloy processing, and it is expected to do so when processing new, harder materials like titanium.
“Today, high-speed machining of aluminum alloys is becoming a standard process,†said Rudy Canchola, processing manager at Mazak's West End headquarters and aerospace technology center. For him, the biggest processing challenge at present is high-temperature alloys (such as 15-5 stainless steel, 5553 or 6Al4V titanium alloys), which are used more and more in the aviation industry. Recently, he plans to use a variety of tools on the Mazak machine for cutting tests (including titanium alloy machining tests on the Mazak VCN-510C vertical machining center). Canchola said, “We have proven that the speed of machining titanium alloys with solid carbide end mills can reach 500-600 fpm. We think this is very good.â€
They also processed 15-5 stainless steel on the Mazak Vortex 815-II five-axis machining center with tools from Seco, Ingersoll, Kennameta and Sandvik. Cutting test. The test adopts the method of down milling, and the surface cutting speed reaches 400-600 fpm.
Canchola said, "Most of our machines have the ability to achieve this high surface feed rate. If the user needs to cut this material, we can provide them with the data obtained in these tests."
When machining superalloys, the "forward-looking" function of the machine controller is not as important as when machining aluminum alloys because the cutting speed is not too high. The most important control function is to measure the load on the spindle and shafting and adjust accordingly. The Mazak machine is capable of receiving feedback electrical signals from the servo motor and adjusting the speed to match the cutting conditions and, if necessary, stopping the tool change.
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