With high-speed machining techniques, such as provided by many of the Makino horizontal machines, it is now possible to mill large, thin-walled aircraft structures from one piece of solid aluminum, rather than by the assembly of several smaller sub- components. High-speed machining (HSM) has had a large impact on the design and fabrication of aerospace parts, and HSM techniques have been used to improve the quality of conventionally machined parts as well.

In his presentation to the Makino Aerospace Seminar, Adam Schaut offered definitions of high-speed machining, discussed the benefits and limitations of high-speed machining, and evaluated issues surrounding high-speed versus conventional machining.

HIGH-SPEED MACHINING DEFINED

The concepts of high-speed machining have been around for several decades, but it wasn't until the late eighties or early nineties that HSM became a buzzword in the machining industry, when manufacturers began to see the great benefits HSM enabled.

What is high-speed machining? In the simplest terms, high-speed machining can be defined by any of the following:

• Spindle speed definition (spindle speed exceeding 10,000 RPM). Note that this definition is constantly changing with improvements in spindle capabilities.
• Tool-tip speed definition (cutting speed exceeding 2,500 surface feet per minute in aluminum).
• Bearing speed definition (bearing DN rating exceeding 2,000,000).

High-speed machining can also be defined by a frequency. Schaut and his colleagues at Boeing, St. Louis, prefer the definition given by Dr. Scott Smith, a researcher for the University of North Carolina, Charlotte: "HSM occurs when the tooth pass frequency (RPM / 60 * number of teeth) approaches a substantial fraction of the dominant natural frequency of the system." This definition states that HSM involves more than just high spindle speeds. HSM occurs when the spindle speed is optimized for a particular tool in a particular machine. Confused? Keep reading…

THE BENEFITS OF HIGH-SPEED MACHINING

Schaut describes several benefits to HSM, including shorter machining cycle times; improved surface finish; reduced thermal and mechanical wear (resulting in longer tool life); and proven regions of high stability, which lead to increased material removal rates. These regions of high stability are a direct result of choosing the correct spindle speed for a particular machine and tool configuration.

Another benefit of HSM is that thin-walled, monolithic parts can be machined in a cost-effective manner. While conventional machining speeds could be used to manufacture thin-walled parts, the machining times would make the process cost prohibitive due to the small depths-of-cuts required to control the process. The benefits of machining monolithic parts include reduced part count and assembly time, improved part strength, and most often a reduction in part weight. With this advanced process, even intricate thin-walled parts are machined as one piece with accuracy.

Take, for example, the Boeing F/A-18E/F tactical fighter. Manufacture and assembly of this fighter plane have been greatly improved by high-speed machining processes. High-speed machining allows the large structural components of the F/A-18E/F to be machined in one piece, rather than the conventional way of manufacturing, which required large components to be assembled from smaller sheet metal parts, extrusions, and other sub-components.

Schaut points out one component of this plane by means of example—an avionics shelf. Manufactured using the the HSM method versus conventional methods, the resulting weight is nearly one pound less. All in all, the HSM method accounts for a 73 percent reduction in total costs.

	Tooling	Manufacturing Time	Pieces	Weight
CONVENTIONAL	53	1,028 HOURS	44	10 LBS.
HSM	6	43.9 HOURS	6	9 LBS.

This particular component consisted of 44 individual pieces of sheet metal, took 53 tools to construct, and required 978 hours in design and fabrication with an additional 50 hours of hand assembly. The resulting weight was just under 10 pounds. Using the HSM method, Boeing is able to produce the same component faster and for less cost. In fact, the HSM version consists of only 6 pieces, requires only 6 tools, uses 38.6 hours for design and fabrication, and requires only 5.3 hours for hand assembly.

As another example Schaut cites the speed brake for the Boeing F-15E fighter plane. In the past, this part required the manufacture and assembly of 483 composite parts, 8 machined parts, and 7 honeycomb parts. Through the implementation of high-speed machining processes, the part count has been reduced to only three. The fasteners required for assembly have been reduced from 372 to just 20, and the number of tools and part fixtures needed dropped from 438 to just 6. Given this example, the practical benefits of high-speed machining are clear.

CONTROLLING CHATTER IN THE HSM PROCESS

A primary concern in high-speed machining is chatter. Chatter is a self-excited vibration between the cutting tool and the work-piece. It is the result of a cutting tooth machining over a portion of the work-piece that was machined by a previous tooth of the cutter, which changes the effective chip thickness and resulting cutting forces on the tool. When chatter exists, it creates large cutting forces that may accelerate tool wear, often causing catastrophic tool failure. Chatter results in surface finishes that may be unacceptable, requiring part rework or, worse yet, complete part rejection. The large cutting forces resulting from chatter can also negatively impact the life of machine components.

Chatter becomes particularly problematic in high-speed machining since high cutting speeds results in the loss of process damping. In other words, the wavelengths of work-piece vibrations increase due to the high cutting velocities, which results in the loss of friction or rubbing on the clearance face of the tool.

The key to high-speed machining and the elimination of chatter is in the choice of the spindle speed. The optimum spindle speed is chosen by matching the chatter frequency with the tooth passing frequency of the cutter. The chatter frequency is directly related to the most dominant natural frequency of the machine tool system (hence, the validity of Scott Smith's HSM definition noted earlier). For example, if the chatter frequency of a particular process is found to be 2000 Hz (cycles/second), then the goal is to match this chatter frequency with the tooth passing frequency of the tool. Given a two-tooth cutter, the optimum spindle speed would be 60,000 RPM (2000 cycles/second times 60 seconds/minute divided by 2 teeth equates to 60,000 revolutions/minute).

If your machine is not capable of running at 60,000 RPM, then choose an integer fraction of this spindle speed. That is, if you only have a 40,000 RPM spindle, then you want to operate at 30,000 RPM (60,000 RPM divided by 2). By matching the spindle speed to the chatter frequency, regions of higher stability are achieved. These regions of high stability are referred to as stability zones. Once a stability zone or optimum cutting speed is found for a particular tool and machine, then the depth-of-cut can be increased until a new chatter limit is encountered. At that time, the process of measuring the chatter frequency and determining the best spindle speed can be repeated again. The goal is to determine the optimum spindle speed that results in the highest material removal rate possible.

Several commercially available software packages are available to aid in the determination of these optimum spindle speeds. Many of these packages are easy to operate and can be used to quickly improve the metal removal rates of the majority of machining processes. These systems typically utilize a microphone to measure the chatter frequency during a cutting process combined with "chatter algorithms" that output the optimum spindle speed. The drawback to this method is that cutting tests must be performed on the actual production machines with the actual production tools in order to find these optimum cutting parameters. Although some production time may be lost in determining these cutting parameters, the lost time will soon be made up for with the improvements in material removal rates.

There are other tools that can be used to optimize HSM processes, such as modal analysis equipment and analytical chatter prediction models. While these tools help to eliminate the lost production time in individually testing machine tools on a production machine, they also require a much greater knowledge of the cutting mechanics and dynamics of the cutting process.

It should be noted that the techniques described above are for the elimination of machine or tool chatter. Often times chatter can result due to actual part vibrations. In this case the technique of matching the spindle speed to the chatter frequency can be difficult since the natural frequency (or chatter frequency) of the part is constantly changing with the removal of material. In these instances, the cutter paths of the tool must be chosen in such a way as to maintain good part stiffness (rigidity) at the cutting tool. This requires that part programmers understand the HSM process and that "smart" cutting paths are chosen for a particular cutting operation.

HIGH-SPEED MACHINING AND THE FUTURE OF AEROSPACE MANUFACTURING

In the aerospace industry, high-speed machining promises pumped-up production to meet the estimated 13,000 new aircraft that will be required over the next twenty years. Leading manufacturers like Boeing have changed their manufacturing processes to significantly reduce part cycle times and meet the rising demands of the industry.

The future of high-speed machining is very bright. It is hopeful that all part manufactures--from giants like Boeing to the small job shops--will be implementing good HSM techniques. This improved understanding throughout the industry will help to drive the machine tool builders and tool manufactures to improve their products and produce faster, more reliable, more accurate, and longer lasting machines and tools. With these improvements come bigger, more accurate, monolithic machined parts, and ultimately faster, better, and less costly aircraft.

In summary, high-speed machining can be implemented by understanding the basic machining process and knowing how to control the tool/machine dynamics (vibrations). The underlying physics of HSM make it comprehensible, as well as relatively easy to implement into a shop floor environment. We say "relatively easy" because HSM does require a philosophy change from conventional machining techniques. One of the big first steps in implementing high-speed machining techniques is accepting change. Once HSM processes are implemented onto a shop floor, and once the philosophies of HSM are respected, then the benefits can be tremendous.

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